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Relay Coil Suppression with single Zener Diode across BJT4

Relay Coil Suppression with single Zener Diode across BJT

(OP)
In terms of relay coil voltage spike suppression, the following thread focuses on using a 1N4007 diode in series with a Zener across the relay coil:

But there are times when you need to suppress coil spikes but cannot put components across the relay coil. In this case, a single Zener diode across the Collector and Emitter of a common-emitter BJT (which drives the low side of the relay coil), should be adequate to suppress the coil's voltage spike to the Zener voltage. And using a Vz = 2 x Vcc (with Vcc being the coil's positive voltage) would ensure the presence of the Zener does not slow the switching mechanism in any significant way.

My question is how to properly calculate the wattage / power rating of the Zener. While we could just use a multi-Watt Zener "for safety," that is a rather "shot-in-the-dark" method that ignores size and cost. A 0.5W 1N5252B 24V Zener costs much less than a 5W 1N5359B 24V Zener, and there is a large size difference as well. As such, it is advantageous to know if the smaller and lower cost 0.5W Zener would be a long-term reliable choice to suppress relay coil spikes.

Consider this circuit:

The OMRON G8NW-2 relay shown is a single device that houses 2 coils and 2 switches. Here is the datasheet:
https://components.omron.com/components/web/pdflib...

In the above circuit we are using an NPN BJT to switch the low side of both coils, with their high side connected together at an automotive +12V. (That 12V is filtered, so don't mind about Load Dumps.) The Zener voltage was chosen to be twice Vcc, which is 24V. The resistance of one relay coil is 225Ω at 20°C, but the datasheet shows a worst case of 180Ω at -40°C. Low resistance means more current flow. At 225Ω we have 0.64W and 53mA current, but at 180Ω we have 0.8W and 64mA current; and again, that is for only 1 of the 2 coils.

I built a test circuit that matches follows the above schematic, first without any suppression and then with the Zener across the collector and emitter. On a scope I measured relay coil spikes at the low side of the relay coil (at the Collector of the BJT) to be from 100V up to about 143V:

The transistor used has a Vcc=50Vmax specification. Vce (voltage across the Collector and Emitter) will obviously be lower than Vcc with the relay in the circuit, low side connected to the Collector. But even if Vce was spec'd at 50V, that's still only half of the measured voltage spikes coming off the relay coils. As such, we need to protect the transistor, and that's where our Zener across the Collector and Emitter comes in.

Assuming a safe maximum Vce of 40V, it takes between 150us and 175us for the spike to decay to that level (varies by Vcc voltage level):

And with the 0.5W 24V Zener connected across the transistor, we see this on the scope:

But what is the minimum reliable wattage rating we should choose for the 24V Zener? That's my question. These are short-term voltage spikes, not continuous high voltage. And again, there are times when you need to reduce both cost and size, making a random selection of a high-wattage Zener impractical. I used a 1N5252B 24V 0.5W Zener across the transistor with the OMRON relay shown in the schematic for my measurements above. I've even used that same 0.5W Zener in a bench-top test circuit with 2 larger automotive relays which draw 122mA each. I only tested for a matter of minutes, not hours or days or weeks. But the 0.5W 1N5252B did not blow. (And actually, even without any suppression at all, for my short-term bench testing, the high voltage did not fry the transistor.) So I am curious if a 0.5W rated 24V Zener across the transistor would be a long-term reliable choice for relay coil spike suppression in this particular application.

I would appreciate hearing your thoughts.

RE: Relay Coil Suppression with single Zener Diode across BJT

Best way to tell is to put a low value resistor in series with Mr. Zener and then scope the voltage across the resistor during a transient which will give you the current pulse thru the zener. Then don't go to any clapped-out crap data sheets like the defective ones ON Sloppyconductor company has screwed up from the good Fairchild data sheets but instead go to someplace like Vishay and look up the zener. In their data sheet they will have a graph that shows the maximum current at the zener voltage. See where your current spikes land on that graph.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Relay Coil Suppression with single Zener Diode across BJT

Howdy JDWO,
Why not simply put a free-wheeling diode across the relay coil(s)? This is done all of the time. You can even buy (some) Omron relays with the FWD already built-in.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Relay Coil Suppression with single Zener Diode across BJT

Groovy guy, the straight diode works, but can cause very slow dropout of the relay, and possibly bounce of the contacts, which can degrade contacts due to repetitive arcing.

RE: Relay Coil Suppression with single Zener Diode across BJT

What about high speed TVS diodes?

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
analogkid2digitalman, a TVS diodes would be more expensive than a single Zener, and as mentioned in my opening post, the aim here is good long-term suppression at the lowest possible cost.

GroovyGuy, btrueblood already posted an answer for you, but I wish to further answer it by repeating what I said in my opening post. Some circuits are designed to have a BJT output that is used to drive an external relay. You don't know if that relay is suppressed and in most cases it is not. That means in that kind of circuit, it is IMPOSSIBLE to put any suppression "across the relay coil" and expecting people to do that external to the circuit, on the relay itself, is not something we ought to assume. Therefore, my circuit is sound in that the suppression component is placed across the Collector and Emitter of the BJT transistor. The benefit of the Zener is that it doesn't slow down the relay operation like a simple diode across the coil. A diode across the coil may result in tack-welding of the relay contacts, but a Zener across the BJT would not. So why even start this thread? To determine the best "wattage rating" for the Zener, keeping cost concerns paramount.

I used my Fluke 8845A to measure the exact value of a 10Ω, 5%, 0.5W resistor, after 1 hour of warm-up, and the 2x4W probe, with 6-digit, 100PLC precision. After settling time, the reading was 9.8651Ω. The rightmost digit was fluctuating a bit, but it's safe to say the resistor is 9.865Ω at room temperature, the temperature at which I conducted the scope test.

I put that 9.865Ω resistor ("Rtest") in series with my 24V 0.5W Zener as shown in this schematic:

I used both scope probes, putting them on either side of the resistor. I then toggled the relay with my MCU. I tested repeatedly. The maximum voltage I could measure on the scope, on the relay side of Rtest was 26.0V (CH2):

At that CH2=26.0V, the other probe (CH1) consistently measured 25.2V. So the voltage drop across Rtest is:

26.0V - 25.2V = 0.8V

Therefore:

i ("current pulse") = e/r = 0.8V/9.865 = 81.1mA

The Vishay datasheet for the 1N5252 (24V, 0.5W) Zener is here:

http://www.vishay.com/docs/85588/1n5221.pdf

Note Fig. 9 at the top right of page 4, which shows Iz vs Vz. It's below 20mA for the 24V Zener, and none of the Zeners shown are above 30mA. So the datasheet gives the 1N5252 a thumbs down for relay coil spike suppression. Even so, in my repeated testing, the 1N5252 does not blow. Why not? Because the spike duration is very short. When the 24V Zener suppression is used, my scope measurements show that the total time the spike is surpassed, including decay time, is almost exactly 2.0ms. Ignoring the slow decay (which is at 26V or lower, safe for the BJT), the actual spike suppression time is more on the order of 900us (and the highest voltage part of the spike has a duration of about 150us (142V to 40V, shown in the first scope photo in my opening post).

Now consider that relay activation is performed by an MCU. There's a push button switch that allows the user to energize or de-energize the relay. And there is a timer for switch debouncing that prevents the user from energizing the relay more than once every couple seconds. In other words, even if one energized the relay as often as possible, the OFF time (no spikes) is measured in seconds whereas the ON time (triggering spikes) is 900us. That ensures the Zener is not being heated.

Despite all this, I am really at the same place at which I originally started this thread. The 0.5W 24V Zener across the BJT works in practical bench testing and logically I see why it is not blowing. But I am simply curious about long term reliability (several years of use).

When checking the datasheets of 1W rated 24V Zeners (which are twice the price of the 0.5W 24V Zeners), even the glorious datasheets of Vishay, there is no Iz vs. Vz graphs at all. However, many datasheets for the 1W Zeners have a "Non-repetitive Peak Reverse Current" or "Maximum Surge Current" column in their specifications tables. The 1N4749A is a 1W 24V Zener that can withstand 190mA for 8ms, which would seem perfectly adequate for coil spike suppression.

I would appreciate hearing your thoughts.

RE: Relay Coil Suppression with single Zener Diode across BJT

"The 0.5W 24V Zener across the BJT works in practical bench testing and logically I see why it is not blowing. But I am simply curious about long term reliability (several years of use)."

It will likely die. Repeated overstressing is one common method for running accelerated life tests.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Relay Coil Suppression with single Zener Diode across BJT

Use a TVS diode (Transient Voltage Suppression) like a Little Fuse 1.5KE series.

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
A LittleFuse 1.5KE27A 27V TVS diode costs 6x more than a 1N4749A 1W 24V Zener at in quantities ranging from 1000pcs to 20,000pcs. It makes absolutely no sense to use a part costing 600% more when the suppression result and protection of the BJT and relay is the same. And while a 1N4007 across the relay coil is cheaper still, it potentially harms the relay AND you are forced to use it across the coil.

As mentioned in this thread, there are times when you need to provide an open collector output that will be used with a an external relay concerning which you do not know if external suppression is used. That's why a 24V Zener diode across the transistor is really the best suppression choice, because you then need not worry about a suppressor across the coil. It would also appear that a 0.5W 24V would not be up to the task in long term use, hence the decision to go with a moderately more expensive 1W 24V Zener instead.

RE: Relay Coil Suppression with single Zener Diode across BJT

When I read this thread, I couldn't help but think it would be an interesting exercise for a Spice simulation.

Please Note: I am wayyy out of my discipline in electronics, it's just a hobby for me, and I have used Spice (LTSpice from Linear Technologies) for a total of 5 hours. This is worth what you pay for it.
I'm doing this as much for my own education as for the possiiblity that it might help you.

Here's the circuit I drew:

I selected the components from the list of available elements in LTSpice, NOT the components JDW0 is using - those simply aren't available without manual configuration of the individual element properties.
That said, I tried to select components relatively close:
Transistor 2SC5876 has a Vceo of 60V and collector current of 500mA
The Zener diode is a Rohm TDZ24B whose breakdown voltage is 24V
The plain diode is a 1N4148 - but doesn't seem to matter what I pick
There are no relays, as far as I can see, in LTSpice (I must be missing something) so I placed an inductor coil with approximately the right properties.
For the inductance of the coils, I measured the 28VDC coil of a relay I happened to have on my desk. It was 2.6 Henrys, so I assume a 1.0 Henry inductance on the 12V coil in the model.
I will comment on the coil resistance below...
To measure the voltage spikes, I created a Node called Collector which I attached to the collector of the transistor.
This node is plotted on the graphs that follow.

The circuit as you see it is in "no suppression" mode. The plain diode across the coil is not connected, and the zener is not connected either.
Spice can run the simulation with these disconnected components and they have no effect.
To engage either suppression method, I just have to draw the wire that connects the component.

Here's the voltage plot at the Collector with NO suppression:

Here's the voltage plot at the Collector with Diode suppression across the relay terminals:

Here's the voltage plot at the Collector with Zener suppression across the transistor:

Footnote: When I set the resistance of coils 1+2 both to 225 Ohm the simulation gave me a huge hash that I assume is resonance or ringing on the Collector that never damped out. When the value of coil 1 or 2 was set to a slightly different value the ringing disappeared quickly. The plots above are at 225/220 ohms, respectively, and have no more ringing.

Hope this is helpful or interesting!

Attached below is a copy of the circuit file.

STF

RE: Relay Coil Suppression with single Zener Diode across BJT

Sorry, forgot to mention, I controlled switching of the transistor by supplying a pulsed 5V source.
The 5V source is ON for 1 second, OFF for 1 second, ON for 1 second, then OFF and stays off.
The rise/fall time on each pulse is 1 msec.

STF

RE: Relay Coil Suppression with single Zener Diode across BJT

Hi SparWeb,
It is interesting that your model seems to indicate that the free-wheeling diode (FWD) is the most effective at suppressing the voltage peak. I was also surprised to see the peak voltage of > 300V when no suppression is provided. Perhaps I shouldn't be surprised though, knowing the impact that di/dT can generate when dT->0.
I have always used FWDs on (thousands of) DC relay coils, and MOVs or snubbers on all AC coils, so far have not blown any output transistors.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Relay Coil Suppression with single Zener Diode across BJT

One would have thought that an FWD's lower forward voltage and, therefore, lower, required power rating, would have a big advantage. When you compare running even a 1V forward drop vs. a 24V Zener breakdown, that's got to be a significant advantage.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Relay Coil Suppression with single Zener Diode across BJT

My 2 (maybe 3) cents:

I hooked up two 12 V relay coils in parallel and tested with different combinations of 1N4118 (no zeners) and resistors in series with it.

The recordings have been augmented with comments. First - no series resistor:

The kick-back voltage is, as expected, very low (one diode drop) and the off delay is 12.6 ms. There is bounce.

Next is with the 390 ohms resistor:

Kick-back is ariund 25 V, also expected, and off delay is now shorter with 6.2 ms. Not much influence on bounce.

I also ran a test with 2x390 ohms. The delay is somewhat reduced and the kick-back is higher. So high that the measurement range was exceeded. So no further comments on that one.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay Coil Suppression with single Zener Diode across BJT

Hi Gunnar,
It's interesting to compare our results. Can you measure the inductance of the coil on your relay?
My measurement of 2.6 Henrys (on the relay I have on my desk) makes me doubt my multimeter... It seems too high.

Inductance of the coil was the strongest determinant of the spike voltage (perhaps obvious, to some) so I'd like to have a better "gut feel" for realistic values.

Relay datasheets don't typically publish a value for coil inductance.
And good luck trying to google-search the subject because all you get is the contact rating when switching "inductive" loads... :\

STF

RE: Relay Coil Suppression with single Zener Diode across BJT

"seductive"? perhaps?
will measure later - midnight here

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
SparWeb, thank you for the SPICE simulation.

GroovyGuy (and everyone else), although the 300V may appear on the simulation, the actual benchtop circuit measurements I provided earlier in this thread were the peak voltages detected on a scope in my testing over the course of 1 hour. In other words, I saw only half that 300V. But there are variances that a simulation cannot account for, hence my measured values being lower.

As to a cheap diode across the relay coil, there are zero surprises about it's effectiveness in "spike suppression. In fact, in my benchtop circuit test with a scope, I don't even see that little blip of a spike on the left side of the pulses shown in the spike simulation. Hands down, if you need to suppress relay coil kickback, a cheap diode (1N4007) across the coil will do it. BUT, and this is a big BUT and the very foundation of this thread, a cheap diode across the coil will slow the relay movement speed and thereby potentially cause tack welding of the relay contacts over time, which is why many datasheets now say "don't do that!" Again, that is the entire premise of this thread.

As to why the Zener doesn't suppress the kickback as well as the 1N4007, that too is obvious because the Zener voltage is what determines the clipping voltage level of the kickback spike. And the very reason to use a 24V or 27V Zener is to ensure there is no slow-down in relay contact movement, thereby allowing the relay to be used within the confines of the datasheet, which should ensure you won't have a tack-weld problem down the line.

But more than that, the reason to even suppress the kickback in the first place is to protect the transistor we all will inevitably use with an MCU to control the relay in the first place, since an MCU lacks the power output to drive many relays, and even if an MCU did have the power output, good design dictates you'd want to protect the pricier MCU with a cheap-yet-powerful transistor to drive the really. And since the Vce (voltage across Collector and Emitter) of most BJT's is about 40V max, a Zener of 24V or 27V would be well within that limit, thereby protecting the transistor from harm by relay coil kickback.

And lastly, as I have repeatedly said throughout this thread, yet another disadvantage of using a 1N4007 across the relay coil is that you cannot always use it across the coil! Think about a design where you need to provide an open collector output (from a transistor) and you know that the user will attach an external relay in most cases, but you have no idea if the kickback will be suppressed externally (e.g., with a diode across the coil, etc.) To protect the transistor, you would simply add the Zener as I did and then there is no worry. Now if one of you have a much better solution, I would certainly love to hear it, but I doubt it would be more cost effective than the Zener across the BJT (from the Collector to Ground, in the case where the BJT is controlling relay coil ground).

By the way, the Zener across the BJT idea did not originate with me. Other engineers have been chatting about this for a while now:

https://electronics.stackexchange.com/questions/17...

When pondering this further, please keep "COST" in mind as much as "EFFECTIVENESS" (suppressing the spike, protecting your circuit).

RE: Relay Coil Suppression with single Zener Diode across BJT

Hi JDW0,

Attached a document that seems to cover the subject well, and discusses alternatives.
The zener across the switch/transistor is one of the solutions recommended for printed circuit board installations.
It also discusses the downside of the reverse-biased diode across the coil, for the same reasons.

I think I should zoom-in closer on the plots of the switch-off transitions in my simulation, to see more closely the rate of coil discharge in each scenario.
I believe you could be right, and both Gunnar and I have come up with ways to see if the relay opening will be retarded by the zener less than by the diode.

Note: the 300V peak in the sim is arbitrary. The coil inductance value which determines it is arbitrary without any data.
Change it by 50% and the voltage spike will also change by 50%.

STF

RE: Relay Coil Suppression with single Zener Diode across BJT

OK
Changed the inductance values to 0.6Henry to produce the 143V spike you measure.
I also zoomed in the plot to show just a 100msec time frame around the switch-off:

With the same diode suppression:

With the same zener suppression:

I'm interested in finding out if the "ringing" is just an artifact of the simulation, or something that would be found in a real circuit.

Estimated relay speed comparison:
no suppression: 1 msec
diode suppression: 8 msec
zener suppression: 2 msec

STF

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
SparWeb,

I'd actually seen that PDF (or something similar) before. Interestingly, your PDF is date stamped Nov. 1998. But despite that good advice from 19 years ago, many engineers continue to use a 1N400x across the relay coils, unconcerned about possible tack-welding implications. I believe that it's one part ignorance (which also means the engineer isn't fully reading the relay datasheet), one part cost-cutting (a single 1N4007 is dirt cheap), one part a lack of concern for long-term product durability, and one part "doing it like everyone else." Honestly, most people suppress the coil with a 1N400x across the coil. But that doesn't make it a proper suppression solution.

For now, a Zener rated at 2*Vcc (or higher, so long as Vce is not exceeded) placed across the transistor seems to be the best overall solution, even for circuits where the relay is on the same PCB. It's good, cost-effective suppression.

But if anyone wishes to add something new to this discussion, I am truly all ears.

Best wishes to all who have kindly contributed to the discuss thus far.

P.S. I just saw your new post with additional simulations. Thank you! If you scroll back up this thread and review my scope measurements, you won't see any ringing. Note my scope images showing the circuit with the Zener being used and note the scope is set to 1ms/division. Also note in my scope measurements that when 14.4V is used the Zener suppressed spike is just under 2.0ms, and at 12V it's closer to 1.5ms.

RE: Relay Coil Suppression with single Zener Diode across BJT

JDWD, when you say that "Other engineers have been chatting about this for a while now" I understand that you are rather new to electronics design. Kick back reduction in relays and contactors has been used ever since relays first appeared back in thirties-fourties and many different methods have been used. Selenium stacks were popular in the fifties and there were also varistors (MOV) that are still being used. A varistor may be what you are looking for, BTW. There are also snubbers, usually RC combinations, gas discharge devices and still some.

A subject like this is not something that one shall "discuss". Like all engineering, it is about setting up criteria that need to be filled and then either use your own knowledge or the collected knowledge of manufacturers and other engineers. Even text-books can be a valuable source of information. "Discussions" and chats where the level of the participants is unknown usually leads nowhere. This thread is partly showing that.

If the delay as such can be tolerated, then use a simple diode. Delay will typically go from less than 5 ms to between 10 and 20 ms.

If the delay cannot be tolerated, then add a resistor in series with the diode. As was done in my
18 Jun 17 19:11 answer, second picture. Delay is then reduced from (in this case) 12 to 6 ms. Doubling the resistor doesn't help much (a reduction of delay with another millisecond).

If the bounce and/or contact dynamics is a problem, then you will need to use another relay. One that doesn't bounce. Diode or not - bounce will be there - and not very much dependent on delay.

Zeners, transient voltage suppressor diodes and other "expensive" components bring a reliability problem with them. If they short out, you can get other, more difficult problems. A simple diode + resistor doesn't short out and that can be a good thing in itself.

So, I seldom use anything but a parallel diode (1N4148 or 1N400x) or the diode inherent in driver circuits like ULN2803 and similar ones. If there is a delay problem, I add a resistor that has roughly the same value as the (combined) coil resistance so that peak voltage gets up to around twice the nominal coil voltage.

If that isn't good enough, I have to search for other means of doing the job. SSR, opto-coupler with high CTR or any other component. Or I may need to rethink the design goals - do I really need that speed? Or will I ever switch those 10 A? and so on.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
Skogsgurra, I appreciate your input. Thank you for performing a practical test and posting the results.

As to the "level" of all participants, I think that is only partly relevant. I've met a lot of PhD's in EE who had no practical experience in electronic design. They could talk theory but they lacked experience getting their hands dirty. What matters is if the solutions proposed can be tested and properly measured data is exchanged in an understandable way. That is why I connected an actual circuit on the bench and provided scope waveforms. SparWeb supplemented my measurements with SPICE simulations. I think this is all useful information, despite it being a "discussion," for any EE to contemplate, along side their textbooks. I believe your earlier post with measured data contributes positively to this "discussion."

Speaking of your earlier post with the series resistor, such that I could put your data into perspective, what is the coil resistance of the relay you used? And if you don't mind revealing it, what is the brand and model of relay that you used?

Regarding varistor use for relay coil spike suppression, I am inclined to believe that a Zener would yield longer life when it comes to suppression of relay coil spikes. MOVs have their place -- for example, in the power supply of a circuit used in an automobile to handle Load Dumps. You won't typically have a load dump daily or even weekly. But a relay might be switched ON/OFF 20 times or more per day.

As to the "delay problem," it is not so much that 2ms or 8ms or 10ms is "too long" in human terms, but rather is a matter of what may happen over time to the relay contacts. Tack-welding is a serious problem the basically renders the relay useless, and that is why, as has been mentioned in this thread, that data sheets of major relay brands warn against use of a 1N4xxx diode across the relay coils. For example, OMRON datasheets will often say this:

Note: External coil suppression will cause a measurable increase in release times and may cause the relay’s characteristics to fall out of the specifications given here.

SONG CHUAN relays often come with this warning:

The use of a single diode in parallel with the coil for transient suppression causes longer contact release time. On power relays, longer release time may reduce relay life.

But so long as that kick-back voltage is below Vce for a transistor driving the ground side of the said relay, and so long as the kick-back is suppressed not much lower than 2*Vcc (Vcc being the coil voltage), the above datasheet warnings become moot in that there is no risk of tack-welding the relay contacts due to slow operation.

In my experience, an opto-coupler is usually not a good choice when you are trying to control costs. Optos are expensive, even in high volume. And as has been discussed in this thread, cost is a major concern when speaking about the best approach to suppression of relay coil spikes. Furthermore, optocouplers draw a noticeable amount of current, which in and of itself may exclude them from many low-current designs.

As has been discussed in this thread, the design premise presented in this thread is that some circuits may need to use an open-collector transistor to drive an external relay which may or may not have suppression. We cannot necessarily expect the end user to add coil suppression external to the circuit, and therefore we should assume no external suppression and consider how best to protect the transistor. That is precisely why a Zener across the Collector and Emitter makes good design sense, and a Zener is a cost effective solution as well.

As to your 0.25W Zener (1N4118) suppression solution with in-series resistor, would you mind posting a schematic so it is more clear how you have everything connected? (A quick hand-drawn schematic will be fine if creating an electronic one is troublesome for you.)

An SSR (Solid State Relay) is much more expensive than a normal relay, and again, this thread is about suppression while also keeping costs as low as possible.

As to what happens when a Zener fails, that is an important consideration seeing many of the Zener with wire leads fail shorted (which would keep the relay locked in its switching position -- powered). SMD Zeners could fail open (not affecting the circuit but eliminating the protection for the transistor controlling the relay). But to keep this in perspective, you mentioned use of MOVs which can be argued as having a shorter usable life than Zeners when it comes to absorbing repeated kick back energy. Cumulative degradation from repeated surge absorption is a legitimate concern when considering varistors, and while thermally Protected MOVs may yield longer life and perform more safely, they are more expensive as well. Again, low-cost is a key consideration in this discussion.

RE: Relay Coil Suppression with single Zener Diode across BJT

I agree with Skogs on this. For decades I've used the FWD across the relay and have never ever seen a relay weld because of it. Maybe it's because I never see how close I can get a relay's rating to the actual load. I read all of the data sheets typically and can't recall ever seeing a FWD prohibition though I've seen notes that said the relay will open more slowly.

The 5 most commonly used relays do not mention not using FWDs.

That said, I totally understand your issue of user supplied relays possibly not having FWDs.

Spar; I think a henry is way too big for a typical relay.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Relay Coil Suppression with single Zener Diode across BJT

Now, this is turning into a valuable exchange of points-of-views.

First, the MOV doesn't degrade if you don't overtax it. The degradation when subject to surge currents does not happen in relay coil applications. At least not at current levels seen in normal control and signal relay circuits where coil currents above one or two hundreds mA are very rare. I have, in my 50+ years of field experience, never seen a MOV fail due to repeated surge currents. Yes, they fail in other applications like being subject to high input voltage with "unlimited" current or repeated switching of highly inductive loads like lifting magnets, magnetic couplings or DC machine excitation. But never in a relay application.

Second, the fact that there CAN be a problem with material migration does not mean that it will happen. That, again, is something that I haven't seen. Never. But I have seen it in circuits where long cables (capacitance), low resistance filaments (incandescent lamps) or transformer inrush can destroy contacts after only a few hundred cycles. I may have been lucky, but I have the feeling that those warnings are a little like "Never dry your Poodle in the micro-wave oven" or "Do not wash your baby in the Whirlpool". It has been put there because it doesn't cost anything and - as the saying goes - Better Safe than Sorry.

Third, how do you measure contact speed? What I have seen so far is the duration of the kick-back voltage. That is not delay or contact speed. You need to measure time from de-energization to opening of contact if you want to measure delay. And then the difference between opening of the NO and closing of the NC contact if you want to compare contact speed. I think that you have inspired me to do so. I will test a few different relay models there.

If you wonder why I do this, I must mention that it is part of a book that I have been working with for several years. It is named "The Automation Engineer and the Reality" and contains lots of things just like this. So I also get something out of the subject.

I'll be back with more results soon.

The resistor is put here:

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
Skogsgurra, thank you for the clarifications and for the schematic. Curious as to why you prefer the resistor there instead of a Zener (with cathode facing the transistor's Collector)?

Regardless, you are putting the suppression across the relay coil. As I've been saying, there are designs where one must merely provide an open-collector output that drives an external relay, regarding which you know nothing (if suppression is used externally or not). As such, to protect that transistor, a Zener placed from the Collector to Ground, with Zener voltage 2*Vcc should be an adequate suppressor, if the wattage spec of the Zener is properly chosen (which was a big reason I opened this thread). I demonstrated the Zener's effectiveness in the scope measurements I posted earlier in this thread. So when you reply back with "more results" it would be appreciated if you would comment on this particular case, where you have an open-collector transistor driving an external relay that may or may not be suppressed.

RE: Relay Coil Suppression with single Zener Diode across BJT

The 1.5KE was just an example. How about a Vishay SA22A transzorb, 500W peak Wattage.
$0.14@1000 ea. The Zener is spec'd at 190 ma Once. RE: Relay Coil Suppression with single Zener Diode across BJT (OP) 190mA surge "once forever more" on the 1N4749A 24V Zener? At this stage, we need to define datasheet terminology and review multiple datasheets. Consider this Vishay datasheet which says nothing other than 190mA for Tp=10ms: http://www.vishay.com/docs/85816/1n4728a.pdf Based on that datasheet alone, one would conclude "so long as I don't exceed 190mA for more than 10ms, this part will suffice." Now consider this Vishay datasheet, which says it's 190mA "non-repetitive and in accordance with IEC 60-1, section 8" -- a 1980's standard that surprising one cannot download for free to see what it has to say: http://www.vishay.com/docs/85153/bzd27series.pdf At least there's a graph that shows iRSM in a percentage, which indicates that the climb to the peak of 190mA needs to be 10us or less and the peak must fall to 50% within T2 - T1 = 0.9ms. Even so, how does one define "non-repetitive"? Is it really "once forever more"? Or rather is it that the peak must never be exceeded and if reached the current must decline in accordance with the graphic, followed by a cooling off period (which is perhaps defined in that IEC 60-1 standard), after which it can in fact repeat? This Vishay datasheet says something different still: Surge current is a non-repetitive, 8.3 ms pulse width square wave or equivalent sine-wave superimposed on IZT per JEDEC method But at least it references "the JEDEC method" which can be downloaded for free here: https://www.jedec.org/sites/default/files/docs/jes... And it would seem they calculate that 8.3ms using Figures 4.8 & 4.9 of the JEDEC method, 1/60s = 16.666ms / 2 = 8.3ms. Reading section "4.2.2.4 Nonrepetitive peak reverse voltage test" we see this: This is a nonrepetitive rating and it represents the maximum value of reverse voltage which may be applied nonrepetitively to a diode without causing permanent damage. Reading that alone seems to indicate "once and forevermore," but it still is not crystal clear, so let's read further down that page: 5) The time between voltage surges shall be long enough to permit the device virtual junction temperature to return to its original thermal equilibrium value. Wait... "between voltage surges"? I thought it was "one surge forevermore"? Now the waters have become even more muddy. Also in that same JEDEC pdf we read this: nonrepetitive peak reverse voltage (VRSM): The peak reverse voltage including all nonrepetitive transient voltages but excluding all repetitive voltages. and repetitive peak reverse current (IRRM): The peak reverse current including all repetitive transient currents but excluding all nonrepetitive transient currents. Figure 1.6 shows a graph of a sine wave with voltage multiple (repeated) voltage spikes on it, one being a higher voltage than the others (i.e., the "peak"). The presence of other lower voltage peaks indicates that voltage spikes can be repetitive so long as they are lower than the peak. But that is still vague because the question then becomes "how much lower"? Let's say one argues based on a given datasheet that an iZSM of 190mA (Non-repetitive Peak Reverse Current) is one and for all. In other words, if you surge at 190mA again, the part shall be destroyed in all likelihood. Fine. What if the current "peaks" at 180mA rather than 190mA? You see, it is rather vague. Specifics are lacking, even in that one datasheet which provides a graph. How do you gentlemen interpret iZSM, and based on what documentation do you base that conclusion? Thanks. RE: Relay Coil Suppression with single Zener Diode across BJT Quote (Skogsgurra) If you wonder why I do this, I must mention that it is part of a book that I have been working with for several years... And I'm trying to get an illustration credit! STF RE: Relay Coil Suppression with single Zener Diode across BJT (OP) sreid, Furthermore... Earlier in this thread I posted (14 Jun 17 02:22) a scope measurement and the following current-peak calculations: At that CH2=26.0V, the other probe (CH1) consistently measured 25.2V. So the voltage drop across Rtest is: 26.0V - 25.2V = 0.8V Therefore: i ("current pulse") = e/r = 0.8V/9.865 = 81.1mA The 1N4749A 24V 1W Zener datasheets mentioned in my previous post all mention the peak current is 190mA. But I have calculated the peak current in the actual bench-tested circuit to be 81.1mA peak. Even if we assume that current level to remain constant (which it actually doesn't -- it declines a bit), the entire duration of that 81.1mA would be about 1.2ms @Vcc=12V or 1.6ms @Vcc=14.4V. That again is why we must clarify the meaning of that "non-repetitive peak reverse current" specification in the datasheet. I would once again refer everyone to my previous post about that. One more consideration... If one argues that a US$0.14 (@1000pcs) Vishay SA22A transzorb would be best to suppress the relay coil spike, I could counter-argue that lower-cost (overall) Vce=350V 0.5A BST39 NPN transistor would allow one to forgo coil suppression entirely. For truly, the primary reason to even suppress the coil spike is to protect the transistor that switches the coil ON/OFF. With a transistor that can handle 350V (more than double my measured coil spike of 143V), coil spikes are no longer an issue for the circuit. Another very minor benefit of using a high-voltage NPN is that you can reduce the parts count (no suppressor diode required) and thereby gain more PCB space as a result.

Gentlemen, I look forward to hearing your thoughts.

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
Understood, but when reviewing my two previous posts, please ignore EMI considerations and let me know your thoughts regarding (a) your interpretation of "non-repetitive peak reverse current" on zener diode datasheets and (b) whether a 1N4749A zener would suffice in light of the current-pulse being only 81.1mA for a duration of 1.6ms or less (datasheet says the "non-repetitive peak reverse current" = 190mA).

Thanks.

P.S. SparWeb, I am unable to download your LTSPICE file for some reason (it results in a blank web page):

http://files.engineering.com/getfile.aspx?folder=6...

And my PartSim simulation (Transient Response) is way off with the spike being in the kV range:

https://www.partsim.com/simulator/#82014

RE: Relay Coil Suppression with single Zener Diode across BJT

The non-repetitive limit is 5x the working current, while this one is about 2.1x. I think you are simply risking premature failure from either junction spiking or metal migration. Death by a thousand cuts.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Relay Coil Suppression with single Zener Diode across BJT

(OP)
If the non-repetitive limit is indeed 5x the working current, and if one argues that 2.1x the working current is "death by a thousand cuts," then how many cuts will result in death for say 1.5x the working current? What about 1.2x? Does even a teensy but above 1x working current spell death?

You see, there's much vagueness here.

RE: Relay Coil Suppression with single Zener Diode across BJT

Quote (JDW0)

Does even a teensy but above 1x working current spell death?

Death is one of life's two certainties.

Might be worth having a look at Mil-Hdbk-217 - not necessarily with a view to doing the calculations (in fact, the only firm recommendation I'd make is that you don't get yourself buried in the arithmetic) - but just to see the way that, for most component and stress types, reliability/mean life is already on the slide well before you reach the rated values.

A.

(OP)
zeusfaber, thank you for the tip (and for not charging me $150k). More specifically, you are referring to this document from Dec. 1991: http://www.sre.org/pubs/Mil-Hdbk-217F.pdf Since we are specifically speaking about Zener diode reliability, the only mention of a Zener in that entire PDF is on page 6-2. The base failure rate is Lb = 0.0020 for a Zener, according to that page. Using the max junction temperature of a 1N4749A of 200°C, we can calculate the Temp. Factor to be 10.5. Taking all other Factors to be 1.0, we then have a Lp = 0.0020 x 10.5 = 0.0211. And Lp = Failures/1E6 hours = Failures/114 years. Hmmm... RE: Relay Coil Suppression with single Zener Diode across BJT I have a solution that's (nearly) free... Put a note in your user's manual that tells the end-user not to use the specific relay setup tat will cause you issues. Barring that, put in a reasonable solution (i.e., relatively inexpensive), put a disclaimer in the manual that usage beyond a certain limit may cause damage, and be done with it. Otherwise, it appears you're looking for a unicorn. You want guarantees for everything under the sun and you want it for a couple of pennies. As one engineer in here is fond of saying, TANSTAAFL (there ain't no such thing as a free lunch). Dan - Owner http://www.Hi-TecDesigns.com RE: Relay Coil Suppression with single Zener Diode across BJT The factors in MIL-STD-217 are for constant conditions. Otherwise, you'd have to have a weighted value. Moreover, the single, nonrepetitive pulse case kicks the acceleration factor substantially higher. That's free to you. TTFN (ta ta for now) I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm RE: Relay Coil Suppression with single Zener Diode across BJT Most circuit board that house an open collector output typically also have a 12V or 24V terminal making it possible to still put protection between 12V or 24V and the collector even with the relay coil being externally mounted. Just saying. IF turn-off was critical then I'd use a ~10V, 0.8W zener diode in series with a ultrafast diode connected between the 12V and the collector. This arrangement allows the collector side of the coil to rise to a similar 24V while halving the power in the zener. Otherwise, I'd just put a flyback diode in the circuit. We have a circuit board with a 15V, 1W zener and 1A, 1000V diode in series wired across a transformer coil. The circuit is 24V and driven by a NPN transistor at about 25kHz. I'm fairly certain the peak current in the zener must exceed the 1W rating. Knowing the circuit, I expect the current in the transformer coil is around 200-300mA when the transistor turns off. Regardless, the zeners have never been an issue, even on boards that have been in the field for 10+ years. So, I wouldn't be to concerned about putting the 10V, 0.8W zener in a circuit such as yours to protect a relay coil that is sees a low duty cycle. RE: Relay Coil Suppression with single Zener Diode across BJT JDWO re: your comment 19 Jun 17 09:39 to my post 19 Jun 17 08:59 I must ask; is this a personal wind-mill that you are fighting? I gave you a perfectly working alternative and you still think that you need to complicate things a lot more than necessary. A simple free-wheeling diode is what is used in most cases. And the delay it causes is very seldom a problem. If it is a problem, then a series resistor can be used to reduce delay to less than half of that caused by a single diode. And certainly as much as a zener does. If your application is so very special that you need to keep a discussion like this for a ridiculously period of time, then please tell us what the application is. And if it is as exotic as you seem to think that it is, then ten cents or a dollar can't be difficult to justify. But, actually, I don't think that your problem is real. Not even close to real. Gunnar Englund www.gke.org -------------------------------------- Half full - Half empty? I don't mind. It's what in it that counts. RE: Relay Coil Suppression with single Zener Diode across BJT (OP) MacGyverS2000, thank you for sharing your thoughts. Most people don't read product documentation, and while that is technically not the fault of a product manufacturer, it is prudent to give due consideration to that reality in a given design. And as to "wanting guarantees for everything under the sun," I would say that is a pretty extreme exaggeration. I am exploring information written in datasheets, basically. Some datasheets say not to use a single diode across the coil, as I've already stated earlier in this thread. That alone should lead any "thinker" among us to explore other options. And many have chimed in with their own preferred solutions. But I added something to the mix by saying there are times when an open-collector output dictates you cannot apply suppression across a relay coil. And that is why I have not merely said to Mr. Skogsgurra, "Yes, a diode + relay across the coil is perfect!" Because, again, I am pondering the case where I cannot put any suppression across a coil nor expect someone else to do so outside the circuit. Skogsgurra, it matters little to me if you "think the problem is real." It's actually rather amusing for you to say that in light of the fact we are exploring suppression options for kick back spikes -- something very real and confirmable on the bench. We are talking about datasheets because engineers use that information to design. If the interpretation is wrong or a datasheet lacks details, then any number of things could happen. I appreciate your advice and certainly think your diode and resistor in series across a relay coil is a very good solution IF AND ONLY IF one can put suppression across the coil, which again, as I have repeatedly said is NOT ALWAYS THE CASE. I'll give you an example... Vehicle Security Systems have existed for decades. Many of them have provided what is called a "GWA output" (Ground-When-Armed) which is nothing more than a 1A-capable open-collector output from a BJT, most of the time with no protection or perhaps a PTC fuse. That output is used by the security system installer to connect any many of things that can be controlled by Ground. They might connect an LED scanner on the dashboard to that GWA output, which would illuminate only when the alarm is Armed. They might alternatively or even at the same time, connect an external 12V relay to act as a starter kill switch, preventing a theft from entering the vehicle and starting the car even if they have the key, so long as the security system is Armed (and the GWA output is proving Ground to the relay). There might be other devices attached to that GWA line as well. That is just one example. With that said, it is interesting that the unprotected transistors driving unsuppressed relays on these security systems are not failing left and right. One could argue that few users are connecting relays externally, or perhaps they are adding a diode suppressor (I honestly don't think so though), or the transistor is merely robust enough to be overstressed beyond the Vce spec in its datasheet mainly because the kickback voltage spike from the relay coil is so short in duration and infrequent. But regardless of all that, it is still prudent to analyze datasheet information and consider the implications of relay coil voltage spikes and how to properly suppress those spikes for a give design. A 1N4007 across the coil does that job nicely, but only in cases where one has access to the relay coil AND on top of that some relay datasheets put notes about TACK-WELDING of the relay contacts, which is perhaps one reason why you yourself use a diode + resistor across the coil. But again (repeating myself countless times here), there are times when you want to protect your transistor but have no access to the relay to put suppression across it's coil, hence the discussion in this thread about a Vcc*2 Zener across the transistor, and other such suppression methods. LionelHutz, you basically are referring to the third circuit in this example (D1 & D2): Using something like a 1N4007 (1000V) for D2 and a part like a BZD27B10P-M (10V, 0.8W) Zener for D1 (cathode facing the Collector of the controlling transistor), going under the design assumption that Vcc (relay coil voltage) is 12V. And in your second example you provided a suggestion for a 24V system. I agree those are very reasonable suppression solutions when you have access to the relay coil. But in terms of your suggestion that I shouldn't be "concerned about putting the 10V, 0.8W zener in a circuit to protect a relay coil that sees a low duty cycle," someone will surely come along and remind us of that "death by a thousand cuts." And I don't mock them for having said that. However, I still feel that certain clarifications of diode datasheet information is prudent before making a design decision based on you "assume" it is actually saying. Like I said earlier in this thread, there is some vagueness as to the meaning of "repetitive" or "non-repetitive" in relation to actual current passed through a diode, the duration, and how warm the diode gets as a result. Again, if you look at my scope measurements earlier in this thread, you will see we are talking about a very short spike. The duration of the spike, until a safe Vce point of 40V is about 175us at 14.4V, and the duration of the portion of the spike above 100V is less than 25us in duration. (Again, one can glean all this from my scope measurements posted earlier.) So while I agree that death can happen by a thousand cuts, if the cuts are barely breaking the skin, death surely is not imminent. In my own testing, even a 0.5W 24V Zener across a BJT works works to suppress coil spikes in repeated tests (hours) without the Zener failing. And my decision to use a 1.0W 24V Zener adds more safety margin to what I already know. And while I could go berserk and just use a 3W Zener, as I said, there are cost considerations to some designs that may prohibit that added cost, even if some argue it is "but pennies." Anyway, I sincerely appreciate all of the helpful insight you gentlemen have shared in this thread. It has helped me to consider a wider set of variables. THANK YOU. RE: Relay Coil Suppression with single Zener Diode across BJT You wrote a lot, but nowhere in that writing did you actually say WHY you couldn't install an FWD across the relay. The fact that people haven't and the fact that the transistors have died isn't a reason why. If the OC transistor has a collector breakdown voltage of, say, 30V, that might be sufficient to kill the spikes anyway, particularly if the relay current is barely 100 mA. That's not necessarily going to immediately kill the transistor, and if the owner re-sells the system soon enough, they may get away with it. TTFN (ta ta for now) I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm RE: Relay Coil Suppression with single Zener Diode across BJT (OP) Actually, I have repeatedly said "why" throughout this entire thread. To repeat... There are some designs that provide only an open-collector output (a switchable ground) which may or may not be used to control an external relay. In the case where an external relay is attached (i.e., not mounted on the same PCB where you find the open-collector transistor), that relay may or may not have coil suppression. It is most prudent to assume (a) that external relay does NOT have suppression and (b) it won't be added by the person adding the relay. As such, the question then becomes, "How shall I best protect my NPN transistor, not knowing if someone will attach an unsuppressed relay to it?" As I have said repeatedly in this thread, one method I am rather warm to is to use a Vcc*2 Zener across the Collector and Emitter of the said transistor. The wattage rating of that Zener has been the heart of the discussion in my mind, but as I said, even with a 0.5W Zener (24V) I am not seeing the Zener fry in repeated testing. Yet another way to protect the transistor is to simply use a high voltage transistor such as a BST39 which has a Vce of 350V, more than adequate to handle coil kickback. But as I said in my previous post, a lot of products like vehicle security systems have for decades used an open collector transistor with no special protection, to which an unsuppressed relay is attached. Using a BST39 would just offer more of a "guarantee" (if I can dare use that term) that a coil spike would never harm the transistor. RE: Relay Coil Suppression with single Zener Diode across BJT In the end, the problem breaks down like this: 1) You provide a "reasonable" (to be defined by you) amount of circuit protection, state what that protection is in the manual, and it's up to the end-user to provide any necessary further protection. 2) Provide no extra circuit protection and let the chips fall where they may. If a user fails to read the manual, well, sucks to be them. This is particularly true for case #1 above. I'll state again... you appear to want a unicorn solution. You want to protect against all things a user might put on a specific line, and that's simply not reasonable/feasible. If you have an OC pin, then state in the manual the line is not to be used for relay control. Have a line specifically for relays if it makes you feel safer, but you're free to place limitations in the manual on that, as well. I have torn apart many of the alarm systems on the market (DEI makes the majority of them these days). They are pretty damn simple. There are some over-voltage protection clamp diodes on the majority of the input lines, but in the end they connect directly to the processor pins (which, BTW, is rarely more than a basic Microchip PIC or a bottom-of-the-barrel Atmel ARM). Only a few are connected to driver circuits for more power, and the outputs of those are generally left unprotected. But stop trying to prevent the circuit from the world... my laptop's manual says "Do not immerse in water", so I don't take it into the bath with me. Problem solved. Dan - Owner http://www.Hi-TecDesigns.com RE: Relay Coil Suppression with single Zener Diode across BJT Seriously? The car alarm is NOT an example. A car alarm has a 12V power connection to power it which means you certainly CAN put a protection device between the 12V and the collector of the transistor. We also have relays on the circuit board with simple diodes across the coils and the relays contacts survive quite fine while operating large inductive AC loads. RE: Relay Coil Suppression with single Zener Diode across BJT 3 JDW0 Were you trying to insult me when you said: "It's actually rather amusing for you to say that in light of the fact we are exploring suppression options for kick back spikes -- something very real and confirmable on the bench. We are talking about datasheets because engineers use that information to design. If the interpretation is wrong or a datasheet lacks details, then any number of things could happen"? Your problem with the diode and slow release is what I meant. That should be obvious to most engineers that has dealt with real engineering and not being overly cautious and mis-interpreting badly supported rumours on the web. Anyhow, I have been young and inexperienced myself. So I know how one can get a fixed idea in one's head and that such ideas can grow into real hinders. I therefore did a few tests with an OMRON 24 V DC relay (sorry, no 12 V). The comments in the recording should be clear enough. But I add some of them below: 1. The test shows coil voltage (brown traces) with a diode and also with a diode plus a 1200 ohms resistor (twice coil resistance) 2. The black traces are contact position. The movement is around 1 mm. A little less, actually. 3. The orange and yellow traces show NO and NC contacts and how they conduct. 4. The release speed is totally independent on if you use a single diode, diode plus resistors, zener, RC or MOV. 5. All that changes is delay. And that is of no concern in this application, if I understood you correctly. I would normally bill you$150,000 (like IR), but I didn't use that much time on this. So 15 kUSD is OK with me. Where shall I send the bill?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Relay Coil Suppression with single Zener Diode across BJT

As I said before, this is part of a book on The Automation engineer and Reality that I have been working on for quite a few years. If it is possible to attach a pdf, you can read it here:

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

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