×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

dc coil surge supression
11

dc coil surge supression

dc coil surge supression

(OP)
I was asked to provide a "quick"  (i.e. not my real job...helping someone else out) general recommendation for dc coil suppression for relay coils powered from 125vdc.  .  There are about 30 different  coils in the cabinet, all powered by 125VDC and all drawing about 0.25A or less.   Various problems have been experienced that are believed attributable to voltage spikes from coil switching.  Response time is not critical in this application... 1 sec delay would not hurt anything.  It is important for relays to change state reliably and even more important not to short out the dc power supply if the surge suppression device fails.

I did a quick search and it has been discussed many times on eng-tips.  
Also I found:
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf
http://relays.tycoelectronics.com/kilovac/appnotes/fig48.asp
http://www.sprecherschuh.com/library/techdocs/get/TECH_Surge_Suppression_109.pdf

I've read through the above and formed my own conclusions, submitted for your comments.

I think the 2 most common discussed options are:
1 – flyback diode
2 – varistor.

The first link especially seems to push the option of varistors.   They highlight a concern that flyback diode can make the coil be so sluggish that it might not even operate.  Also apparently when it operates slowly, it's output contacts can be degraded.    I have to admit I have not heard much about these concerns before (other than time response).

I really don't like varistors in this application.  I think every time the coil switches open there can be fairly high current at high breakdown voltage drop and these things degrade.  Sure there is a rating, but they use a little life every time they cycle. If they ever short circuit, life is not good.

So I like the flyback diode. Diodes can short, but then again I have an easy solution: put two diodes in series.... Makes me feel a lot better.  Either diode can fail short and not a problem.    Also I'm thinking I would put a resistance about equal to the coil resistance (R = 125V/0.25A = 500ohms) in series.   That should tend to minimize concerns about effect of slow field collapse upon the relay discussed in the first link.   I picked 1 times coil resistance since when the full coil current switches into flyback loop the voltage is limited to the original 25VDC voltage (if I had double the resistance I could have double the voltage).

In summary I am thinking about two reverse-biased diodes (during normal operation) and a resistor 1x coil resistance... all connected directly in parallel with the relay.

What do you think? Am I grossly overlooking anything?

By the way, any tips for diode selection?

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)
The other engineer seems to be interested in pursuing my idea but is stuck on the idea of using directional TVS unidirectional diodes (Littlefuse 5KP series).  They are really being used as diode and not expected to break down.  I'm not sure what it buys us, but he really wants to do it. Do you think that would that hurt anything that you can see?

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

If (?) I understand how the diodes work, they provide a path for the coils' 0.25A current to continue to flow (preventing high voltage from developing in the first place). As you noted, the release of the contacts will be delayed by the time it takes for the current to decay.

But I don't think that the diodes are 'tortured' in this sort of duty. All they see is the 0.25A current suddenly appearing and dying away. They should last 'forever'.

Stand by for others...
 

RE: dc coil surge supression

(OP)
That TVS unidirectional diode is (obviously) only intended to conduct in reverse.  They give power ratings related to a short pulse... nothing for continuous.  Even though a 1-sec or so decay is not continuous, I'm thinking I personally would rather have a continuous rated diode (at least then I can look at the data sheet and verify it is more than 0.25A). What would your diode suggestion be?

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

3
I think you can forget about all expensive and exotic devices in this case. Your diode+resistor thinking is right on.

1N4007 plus resistor equal to coil resistance can never go wrong. Delay will be minimal and the diode's reverse voltage is more than adequate. Resistor wattage need not be high, but you may need to use resistors with same gague leads as the diode, for practical reasons.

Momentary power dissipation will be half the coil wattage for a very short time, around 30 W during the first milliseconds and then quickly dropping to zero. I would use a 2 W resistor and put resistor and diode in shrink tubing. Make sure the polarity can be seen on the package.

For high switching frequency, you may need to use a resistor with more watts.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

Given that diodes are almost free, I'd splash out for 1A and maybe 400 PIV (the old 'double and round-up' rule). 1N4004 or better. Switching speed might be something to watch for. If the diode is slow, then 'bad things' might happen in that duration. The 1N4000-series are not known for being fast. Almost certainly a better choice out there.

 

RE: dc coil surge supression

Sorry, half the coil wattage is not 30 W, but 15 W. So 2 W resistor will be fine even with high cycling rate.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

With relay drop times between 100 and 300 milliseconds, even 1 ms in the diode doesn't matter. And the 1N400X are definitely much faster than that.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

My concern about the diode switching speed is that the coil current will try to continue even while the diode makes up its mind to forward conduct. During that time, the voltage may spike and possibly stress the diode. Peak voltage would be limited by the junction capacitance. And the diodes are rugged anyway. I doubt it's a real world problem.

But nothing directly to do with the "relay drop time".

 

RE: dc coil surge supression

No, it is not a problem. We use this type of protection in lots of installations in paper mills, steel works, power plants. Done it at least 40 years. Never had a problem.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

(OP)
Outstanding! That is fantastic and timely info.

This is not a high cycle rate application. I think we are going with 500ohm resistor fairly low wattage. And 1N4007 diode selection.

The other engineer pointed out that we don't really need those 2 series diodes to protect the power supply. If one diode should short circuit, the power supply is protected by the resistor which will limit currrent to 0.25A....

...but now the followup question: what happens when if the diode shorts and the resistor is now carrying 0.25A and protecting my power supply... and heating up since it is not sized for that 0.25A  (we don't really want to use a big resistor rated for the full 0.25A / 30 watts since the physical weight will be supported by the electrical connections).  Can we count on the resistor to fail gracefully and open circuit?  Are there certain resistors that are expected to behave this way?  We were thinking wirewound resistor might tend to open circuit easily.

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

You could use a small 7W 120V incandescent lamp for your resistor. Then it would light-up if the diode failed. A dim blink may be visible when the relay switches off. One drawback may be that lamp filaments are more fragile than some resistors.  

RE: dc coil surge supression

(OP)
I think "fusible resistors" are designed to open-circuit in a controlled fashion.  For example this:
http://www.vishay.com/docs/31031/cmffuse.pdf

Maybe that would address the concern. Anyone know anything about these things?

 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

It is not likely that a 1N4007 ever breaks down. And if it would, most resistors just open and do not catch fire. So it is only in the most critical applications that one needs to consider that failure mode.

Interesting to see that the fusible resistors withstand around 1 second at 15 W (the .25 W types) and the .5 W types six seconds before fusing. Lots of margin there.

It is not a bad idea to use the fusible resistors. Not that they are needed, but it doesn't hurt to use them.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

In some applications it can he better to have an 'obvious' failure that draws attention to itself, instead of a silent failure that quietly causes subtle problems.
 

RE: dc coil surge supression

(OP)
We will go with the 0.5 watt resistor to for added comfort.

Good point about silent failures.  If diode shorts and resistor then opens, I expect there would be no visual evidence.  (We have seen visual evidence of diode short before, but only when accompanied by high current).  The light-bulb has an advantage in that regard, but poses too much difficulty to install the way we were planning (supported by the leads from the device terminals).  In the worst case, we end up missing some surge protection... the same as we are now... at least our modification has made the situation no worse.   We will think about what strategy is viable for spot check of these circuits.
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

A TVS would be fine, but pricey.
The 1N4007/resistor is the better option.
Use a flame-proof resistor.  Don't cover it up.
Makes a great visual indicator. :)
We had a whole cabinet full of these (>200 pcs.)
and they worked fine.
<als>
 

RE: dc coil surge supression

Re: Don't cover it up

There is transparent shrink tubing. Those little components need some physical protection in the sometimes harsh environment you get in a cabinet (pulling wires and removing/putting new ones).

But, as I said before, there is no real need to worry about the diode break-down. If you were using 1N4148 - yes. But never with 1N4007. Never.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

(OP)

Quote:

Use a flame-proof resistor.  Don't cover it up.
Makes a great visual indicator. :)
Can you explain what you mean by that 2nd sentence?
I haven't seen one of these but I was imagining it would melt open inside with no external discoloration.  Have you ever seen one burnt open?  (I might try that once we get them).

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

FWIW,

I have spoken to people using high-performance (fast, high current, DC) relays, and they were adamant that any reduction in the transfer speed of the contacts would result in and increase in arcing across the contacts.  I suspect they were also concerned about control responses within the system if relays were delayed, but that's a more difficult problem to quantify.  I also know of cases (solenoid valve coils on small spacecraft thrusters) where any variability in coil drop-out delay would cause unacceptable variations in the controlled variable (impulse of the spacecraft thrusters in that case).  The suggestions by Skogs of course are a good solution for epete's posted problem, but recall that resistors have a typically uncontrolled/variable TCR, and the drop out time can thus vary if the duty cycle of the resistor changes (i.e. it heats up and its resistance changes).  

In those cases, the use of a backwards-facing zener of sufficiently high voltage is preferred, as the transfer times are much more repeatable and less susceptible to variation.  Have to agree that the use of MOV's is not a good idea due to breakdown over time/number of pulses.

RE: dc coil surge supression

(OP)

Quote (Gunnar):

there is no real need to worry about the diode break-down. If you were using 1N4148 - yes. But never with 1N4007. Never.
That's valuable input, but can you explain why you say that?   I'm not very familiar with various diode types and their differences.

Quote (btrueblood ):

I have spoken to people using high-performance (fast, high current, DC) relays, and they were adamant that any reduction in the transfer speed of the contacts would result in and increase in arcing across the [relay output] contacts
That seems to be the main message of the first article linked above (tyco), repeated here:
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf

I take a small comfort from the fact that we already have a bunch of applications where someone has put in simple diode with NO resistor... at least we are doing better than that in trying to bring the current down quickly with the 500ohm resistor.  

Quote (btrueblood ):

recall that resistors have a typically uncontrolled/variable TCR, and the drop out time can thus vary if the duty cycle of the resistor changes
The linked resistors have 100ppm/C spec and we don't be cycling that often.

Quote (btrueblood ):

In those cases, the use of a backwards-facing zener of sufficiently high voltage is preferred, as the transfer times are much more repeatable and less susceptible to variation.  Have to agree that the use of MOV's is not a good idea due to breakdown over time/number of pulses.  
I'm not sure I follow.  Is the "backward facing zener" configuration you're talking about the same as Figure 3 in link above?  In that one it seems like the zener takes the place of my resistor and must break down every time the coil is interrupted, which I think would be an aging mechanism.
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

Pete and BTB. I will be back later with comments. Have to do one of my lab things to find out about contact separating speed. I do not think that the diode makes a big difference. It delays the drop-out. But since the armature doesn't start moving until flux has reached the drop-out value, I see no reason why the separation speed would be affected. I will set up a few relays and do some 'high speed' recordings. Stand by.

Re the 1N4148. It is a small signal diode that I have seen used across relay coils. I do not like it. They break down easily and have caused paper machines to stop. The 1N4007 is a rectifier diode with 1000 V reverse voltage and 1A continuos forward current and 30 A half-wave 60 Hz current. It will not break down if the system isn't hit by lightning. And then, you have a few other problems.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

We had a problem with contacts releasing too slowly when using just a zener diode across the coil.  The contacts would burn and stick together.  We changed to the Tyco method using a zener and a regular diode and haven't had any problems with the contacts since.

RE: dc coil surge supression

A small capacitor is often used to reduce contact sparking. It allows the coil current to keep flowing for a millisecond after the contacts open so they can separate far enough so the voltage cannot arc across.

RE: dc coil surge supression

(OP)
Thanks Gunnar.   So we have a beefy diode in a relatively mild application so should not suspect many problems.  It will be interesting to see what your testing shows about effects of slower dropouts.  Although the linked article included traces on the input side of the coil they didn't include anything related to the output contacts to prove their point.

Brian – thanks that's a good datapoint.  

Again at least I have a rectifier diode plus resistor in there, which I think is better than rectifier diode only... and the resistor roughly plays the same role as their zener (with the advantage that my resistor behaves much better during the hypothetical possibly-unlikely scenario of rectifier diode short).

Let me think about that comparison (my resister vs their zener) a little closer... in figure 3 at moment of switching the coil voltage changes from +100% to -200%.  So I assume the zener breakdown voltage much be around twice the supply voltage.  Is it too simplistic to think that quates to an initial resistance of Rzener_eq = 200%voltage/100%current = twice coil resistance?  In that case my resistor is at least in the same ballpark, right?  Maybe 150-200% coil resistance (1000ohm) would be better since I doubt 150-200% overvoltage would hurt anything.

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

"The linked resistors have 100ppm/C spec and we don't be cycling that often. "

Should be fine, and as stated, the variability in transfer time with even cheap carbon film resistors (where the spec. can look like +/-1000 ppm/C) doesn't sound like it would give you any real worries.  I agree with Skogs, by the way, that (some) of the issues about transfer times are overblown.  Certainly when switching AC relays, the AC current itself acts to snuff arcs.  But with even 12 volt DC relays, again at current levels across the contacts approaching the ratings for the contacts, I have had experiences similar to Brian's, with contacts welding with only a diode for snubbing.  But adding the resistor as Skogs suggests eliminates the worry about transfer speed, and is just as good if not more robust than a zener diode, and the transfer time variability is really only a concern to persnickety system engineer types (like those who send spacecraft to Jupiter and beyond...)

Yes, the backwards zener is as shown in your posted links, sorry.  Zeners won't degrade any faster than the diodes will, i.e. the breakdown voltage won't change dramatically over time, at least not the way a MOV will.  Assuming you get a zener that can take the heat pulse without releasing its magic smoke, which is not always possible.

RE: dc coil surge supression

(OP)

Quote:

Zeners won't degrade any faster than the diodes will, i.e. the breakdown voltage won't change dramatically over time, at least not the way a MOV will.  
I am still struggling with this.  I think of varistor as roughly a pair of back-to-back zeners.  The varistor breaks down whenever it conducts.  The zener in figure 3 will break down whenever coil is deenergized causing it to conduct. Why wouldn't this cause similar aging in the zener as we see in varistors?  I can see that the shorting failure mode of the figure 3 zener (loss of damping during the deenergization) is not as severe as the failure mode of a typical varistor (shorting of the power supply voltage), but I think they are both seeing similar stresses, right?
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)
....Maybe it's the continuous voltage that gets the mov (?)

Anyway that's actually not a big factor in our currrent thought process. The bigger problem with the zener in our mind would be that it doesn't handle shorting of the rectifier well which is the main reason I prefer the resistor. Gunnar may be very right about the rectifier diode being buletproof, but we just don't want to take any extra chances of shorting out that power.

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)

Quote (electricpete):

Let me think about that comparison (my resister vs their zener) a little closer... in figure 3 at moment of switching the coil voltage changes from +100% to -200%.  So I assume the zener breakdown voltage much be around twice the supply voltage.  Is it too simplistic to think that quates to an initial resistance of Rzener_eq = 200%voltage/100%current = twice coil resistance?
I'll answer that. Too simplistic. A a closer look at the graph shows that for the 2 millisecond period after interruption the voltage is relatively flat at -200% while the current is dramatically decreasing from 100% toward zero.  So I guess the equivalent resistance of the zener averages much higher than suggested by my 200% number above.... and if quick deenergization is the goal it does much better in that particular respect than suggested by my earlier discussion.

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

There are two entirely different physical effects at play in zeners and varistors.

The zener breakdown is an avalanche effect and there is nothing being 'worn'. The varistor works with grain boundaries that conduct when the voltage between them excedds a rather poorly defined, but still characteristic, limit. Each time that happens, some of the existing grain boundaries 'wear out' and after a number of pulses, there will be a (mostly) short. The number of pulses depends on the energy contained in every pulse. Not linearly, but monotone.

Doing mesurements. There seems to be a dependency between current rate-of-fall and contact opening speed. My first measurements are quite crude. I have set up a relay where the diode can be switched in and out. There is also a resistor that can be switched from 0 ohms to megohms. I measure the contact speed using the time it takes for the C to move from NO to NC.

They delay is, of course, very dependent on diode/resistor and there is a small variation (around 1 ms) between diode withouut resistor and no diode. The difference in travelling time (equals speed) is difficult to see when diode+500 ohms and no diode are used.

More tomorrow.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

(OP)
Thanks Gunnar.  I will try to erase from my brain the  idea that a varistor is like a back-to-back zener.

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

Good description of zeners vs. MOV, and even gooder data on the transfer speed, Gunnar.  

Oh, and I forgot one other reason that probably explains why some like zeners:  if there are spike-sensitive electronics attached somewhere in the path of the coil (e.g. driver FET's instead of relay contacts), the negative-going pulse from the coil flyback could zot them; the zeners give a limited voltage spike, whereas a resistor controls the spike magnitude less closely.  If that makes sense.  Some people add blocking diodes to prevent that from occurring, but that adds a voltage drop too.

RE: dc coil surge supression

Gunnar,

On the oscillograms you've labelled them:

Oscillogram 1. Free wheeling diode without series resistor.

Oscillogram 2. Free wheeling diode with 4.7K series resistor.

Oscillogram 3. Free wheeling diode with 4.7K series resistor.

Should that be:

1. No snubber
2. Diode plus 4.7k
3. Diode only

?

BTW, very interesting :)

RE: dc coil surge supression

(OP)
I think it's
1. Diode only
2. Diode plus 4.7k
3. No snubber
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

There were a few more errors in the comments. Also, the discussion presented the recordings in reverse order, which may have added to the confusion. I have corrected it in the attached pdf and hope that it is more clear now.

For those that think that the word 'snubber' should be reserved for an RC unit, I agree, but have used it in a broader sense. In this case for a single free-wheeling diode as well as for a diode/resistor combination. Anything that 'snubs' is a 'snubber' - isn't it?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

Sorry was busy correcting the text. Yes, you are right, Pete.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

(OP)
That's great data Gunnar.   A good idea to use the change of 2 contact states that way as indication of speed.

I can imagine there is a wiggle as the magnetic circuit breaks and there is a reaction trying to keep the flux constant.  It is basically the same principle as the voltage that lights the LED when I break the hinge off of the magnet in the video below (except that instead of me supplying the force to break the hinge open, the spring is supplying the force):
http://www.youtube.com/watch?v=HI5ERdvroec&amp;playnext=1&list=PLA7FF4251D29DB01B

I'm not sure I understand if and why we'd expect the electrical contact to break exactly at the local extreme value of that wiggle.. or maybe just a coincidence of timing. Not a big deal, just a curiosity.

* It is interesting to compare graph 3 (2 megaohm resistor) to graph 2 (4k resistor plus diode) after graph 3 settles down.  At first glance, we'd expect the L/R time constant graph 3 to be on the order of 500 (2 Meg/4k) times shorter (faster) than graph 2, but it's not.. it looks like not even twice as fast.     I can't explain that.   Now another question... if we look for a "dropout voltage" we see it is roughly the same on slides 2 and 3.... but that means the current in slide 3 (2 meg resistor) was roughly 500 times lower than slide 2 at time of dropout.  That's a weird thing... suggests maybe there are some complicated dynamic effects to consider.... or... not sure what it means.
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

Gunnar,

I'm quite surprised by the amount of inductive kickback shown by the coil when the resistor is included in the circuit.

Is it enough to damage any relay driving device?

RE: dc coil surge supression

Pete and sibeen
The measurements were made with one objective, to see what a diode/resistor combination does to contact separating speed. So, the resistor value could have been chosen better. I have one measurement with a 470 ohm resistor (twice the coil resistance). I shall use that instead of the 4700 in a revised PM. That will reduce kickback to more reasonable values.

Pete
Most of the energy in the collapsing field is consumed during the arcing period. That's why the usual L/R relation doesn't hold.

This seems to be the start of an FAQ where AC and DC inductive loads and suitable snubber techniques will be treated. I got plenty of time (will not go to El Salvador) so this is the right time to do it. Stand by!

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

Gunnar, looking foward to it.

Can we expect a book? :)

RE: dc coil surge supression

Very cool stuff, Gunnar.

Yes, the little wiggles in the curve do show the opening and closing times of the armature.  It's just like DC motor theory - you have a field, however transient it is, and a moving magnet.  Speed of motion of the magnet drives a back-emf thru the coil, thus generating the blips in a voltage trace across the coil.  In the spacecraft biz, for measuring solenoid valve opening and closing transients, I used to measure the drop across a low-ohm resistor in series with the coil to see those blips, and we would make the assumption (quite well correlated by actual pressure/flow measurements) that they corresponded to the start and end of solenoid armature motion.

Gunnar writes:

"So, the slow flux decay does actually hold the armature back a little. If that difference has any effect on contact destruction or not is an open question. Any thoughts?"

I'd have thought the answer to this would be "it's negligible"; delay in dropout yes, and measurably so, but transfer speed of the contacts should behave from first principles of force balance independently from a dissipator linked to the coil.  But experience has shown me to be wrong, for certain types of contacts and high-amperage DC.  And after think about it more, there could be an effect.  Yes, the first motion of the armature should occur at a fixed value of the coil current (all other variables equal).  But, if a dissipative device is continuing to cause the coil current (and any back-emf current from the armature motion) to fall, one could argue that this dissipation lowers magnetic forces during the armature motion, and thus elimates the hold-back force, ie. more dissipation means more acceleration of the armature, equals higher transfer speed.  Somewhere in an Omron data sheet (of which there are many thousands) I think I read a description of what we are all talking about, and it had recommendations against a simple freewheeling diode (and for something with more back-emf effective resistance) to reduce relay transfer times and subsequent arcing.  Will search for it...wish me luck.

RE: dc coil surge supression

Well...

an appnote from Tyco:

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

has this to say on page 1:  

"Even though the use of coil suppression is becoming more significant, relays are normally designed without taking the dynamic impact of suppressors into account. The optimum switching life (for normally-open contacts) is therefore obtained with a totally unsuppressed relay and statements of rated electrical life are usually based on this premise. The successful "breaking" of a DC load requires that the relay contacts move to open with a reasonably high speed."

A typical relay will have an accelerating motion of its armature toward the unenergized rest position during drop-out. The velocity of the armature at the instant of contact opening will play a significant role in the relay's ability to avoid "tack welding" by providing adequate force to break any light welds made during the "make" of a high current resistive load (or one with a high in-rush current). It is the velocity of the armature that is most affected by coil suppression. If the suppressor provides a conducting path, thus allowing the stored energy in the relay's magnetic circuit to decay
slowly, the armature motion will be retarded and the armature may even
temporarily reverse direction. The reversing of direction and re-closing of the contacts (particularly when combined with inductive loads) often leads to random, intermittent "tack welding" of the contacts such that the relay may free itself if operated again or even jarred slightly
"

a little later it says:

"The use of a reversed-biased rectifier diode in series with a zener diode will provide the best solution when the relay can be polarized. This suppression is often recommended by Siemens Electromechanical Components (SEC) for use in automotive circuits. The impact on release dynamics is minimal and poses no loss of reliability"

and

"A reversed-biased rectifier in series with a resistor may be used
successfully with some relays when maximum load switching capacity is
not required. Care must be taken to use a resistor large enough in value to quickly dissipate the relay's stored energy but yet stay within the desired peak voltage transient. The required resistor value may be approximated from the following equation:
R = Vpeak/Icoil
where;
R = resistor value in Ohms
Vpeak = peak transient voltage permitted
Icoil = steady-state relay coil current


I had to think about that statement for awhile, but am now understanding why they say it after re-reading the whole paragraph, and thinking about Gunnar's results.

They then have this to say about a diode only:

Many engineers use a rectifier diode alone to provide the transient
suppression for relay coils. While this is cost effective and fully eliminates the transient voltage, its impact on relay performance can be devastating. Problems of unexplained, random "tack welding" frequently occur in these systems. In some applications, this problem is merely a minor nuisance or inconvenience and the controller or operator will cycle the relay until the proper response is obtained. In many applications; however, the first occurrence may cause a complete system failure or even present a hazardous situation. It is important that these systems be designed with another method of relay suppression.


And I think I agree with that too.

They then give a little table to show the measured effects of various suppression methods.  Note the difference in dropout times for a 24v reverse-biased zener, versus a 100 ohm resistor (both giving transient suppression of about the same voltage peak values).

RE: dc coil surge supression

(OP)
A few questions:
1 – What type of coil?  Clapper type telephone coil?
2 – What is coil resistance?

I agree smaller resistor will be more representative of intended application.  Ratio of peak overvoltage to nominal voltage is roughly the same as ratio of snubber resistance to coil resistance.

For all cases except for the diode-only case, we can infer the current from the voltage trace.   For the diode-only case, it would be interesting to provide some trace that could be used to infer the current... like voltage accross a very small shunt resistor.  

Looking at previous traces, I still say they raise some unanswered questions that we would like to understand to really know what the heck is going on in the circuit.

Regarding the difference in L/R time constant, look at the interval where voltage decays down from 60 to 20 volts.  From the qualitative shape of each individual curve, my guess would have been that the arcing is already completed and the gap in the magnetic circuit has not yet opened. What we expect and what we appear to see in that region is an exponential decay  following the rule exp(-t*R/L)

For slide 2 it takes about 0.002 seconds to decay from 60 to 20 volts
exp(-t*R/L) = exp(-0.002 * 4,000/L) = 20/60.
L = -0.002*4000 /  ln(20/60) = 7.28H (!)  

For slide 3 it takes about 0.0011 seconds to decay from 60 to 20 volts
L = -0.002*2E6 /  ln(20/60) = 3640H (!)  

Have I done this calc wrong?  Not only does the same L act vastly different, but the inductance seems unrealistically high.    Also if we consider magnetic non-linearity, the permeability is highest at normal operating point presumed slightly below saturation and monotonically decreases as we decrease from there which suggests slide 3 should have lower L (opposite of what we saw).    

And again, the dropout "voltages" are the same which suggests vastly different dropout currents.  That suggests that magnetic force as function of current doesn't tell the story and paints a picture of a more complicated dynamic system that I can't quite fathom.  I think I can piece together a simple mental model to explain why the coil moves slower with lower snubbing resistance, but I can't get anywhere close to piecing together an explanation for these measurements.  

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)
Going to figure 3 of the very first linked Tyco article, they show the rectifier diode and zener used in place of our resistance.

The reason for the big advantage of the zener is now clear.  For resistance, we are prevented from using a high resistance because it gives a voltage spike at the moment just after input switch opening. Let's say we use Rsnubber = 150% Rdc, then we get roughly peak voltage of 150%.  In contrast, let's say we use zener of 150% voltage.  The effective resistance of the zener just after switch opening (when we want it low) is also 150% which is in accordance with the voltage limit.  But in the time after that when voltage spike is no longer a concern, the effective resistance of the zener continues to increase, drawing maximum power out of the coil for that voltage.   So the zener is the best characteristic we can achieve that satisfies maximum energy removal rate at voltage limited to 150%

Sorry if this is already obvious

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

BTB

..which proves that also simple systems can be quite complex and that there are no trolls in technology, but sometimes badly understood mechanisms. I shall now put an inductive load on that relay contact and measure voltage and current and then multiply the two to get instantaneous power and also integrate the two over time to see how much energy is developped in the arc and how it depends on contact seoarating speed. A nice project for a rainy winter day.

Pete
I have added one recording with a 'better' resistor. This one is twice the coil resistance. Does it make more sense?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

"So the zener is the best characteristic we can achieve that satisfies maximum energy removal rate at voltage limited to 150%"

I think that is right, and it was obvious (to me) only after thinking about it for several years.  Good that those several years were several years ago, now I can sound smart...

"I shall now put an inductive load on that relay contact and measure voltage and current and then multiply the two to get instantaneous power and also integrate the two over time to see how much energy is developped in the arc and how it depends on contact seoarating speed. "

That sound like fun...or maybe the short winter days are getting to me too! ;)

RE: dc coil surge supression

(OP)
Thanks Gunnar. I will take a look.

We contrasted 2 approaches:
1 - I liked the rectifier diode + resistor because it was robust to failure of the rectifier diode.   
2 - I like the rectifier diode + zener because it seems better at quickly deenergizing the coil, which might help it switch faster, which would be easier on any output NO contacts (which is an unknown risk...probably depends on the load fed from the contacts, whether it's inductive, whether it has its own surge suression etc... better to be safe in absence of details on that).

We could probably combine the advantage of 1 and 2 above each design just by inserting an additional rectifier diode into (1), which will give us two rectifier diodes plus zener all in series... gives us advantages of zener (fastest deenergization) plus robust against single failure of a rectifier diode shorting out the power supply.

I shy away from proclaiming this is an "ideal" configuration because although I don't know much about it, the field of "snubbing" winky smile seems to be something well studied with quite a variety of approaches... perhaps motivated by subtle differences in specific application requirements that I am unaware of.  FWIW, yet a few more or listed in the attached table excerpted from "Electrical Relays: Principles and Applications" by Gurevic (note some are for dc and some are for ac as indicated by circuit symbol on left of diagram).

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)
Sorry. I need a correction in bold:

"We could probably combine the advantage of 1 and 2 above each design just by inserting an additional rectifier diode into (1), which will give us two rectifier diodes plus zener all in series... gives us advantages of zener (fastest deenergization) plus robust against single failure of a rectifier diode shorting out the power supply."

should have been:

"We could probably combine the advantage of 1 and 2 above each design just by inserting an additional rectifier diode into (2), which will give us two rectifier diodes plus zener all in series... gives us advantages of zener (fastest deenergization) plus robust against single failure of a rectifier diode shorting out the power supply."
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

250 Volt Zener in series with 1n4007 diode.

The varistor is not reliable long term and the clamping voltage is important for turn off time.

wire so that the inductive kick current flows through the zener in its breakover mode through the diode in its normal conduction mode.

 

RE: dc coil surge supression

2
Besides overworking the solution, I think you guys have veered away from the actual problem. In a suppression solution there are three areas of concern:  1. Protecting the source from over voltage, either arcing of contacts or spikes to a semiconductor.  2 Limiting the speed of the device,  3. Controlling radiated emissions.   You have focused on the first two but the primary problem of the customer is the last.

 

RE: dc coil surge supression

(OP)
Thanks OH.

All I know about the original problem was that there was mis-operation in a 24 vdc circuit during switching of a 125vdc system coil.  These two  systems are very independent electrically (125vdc is rectified outside the panel and supplied to the panel, whereas the 24vdc is produced from ac inside the panel). So I suspect you are right, the interference is probably not conducted, but coupled through either radiative coupling or capacitive coupling.

Aside from general EMI procedures in other parts of the circuit/wiring, how specifically should we incorporate this concern into our coil input surge suppression design?  I can imagine perhaps it might be important to know how fast the zener transitions to reverse conducting?... or perhaps this is an advantage to the resistor (vs zener) ? .... or completely other design suggested?

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

That doesn't sound like a very sensitive application.  I am always concerned about contacts when switching 125V DC. An "open" contact continuing to conduct and causing two exclusive functions to operate at the same time overloading the line. That may have caused the 24V supply to drop out.  I remember one manufacturer that used a 25A analog supply just to power one relay because of short period line dropouts.  A large capacitor (20,000uF) on the 24V line might be a solution.  I would be examining all the relay contacts for some more evidence of a logic malfunction.  Just get the feeling your solution may not come easily. Is there any other electronics in there besides that 24V power supply?  I assume this has worked for a while without a problem or is the 24V section a recent addition?  

 

RE: dc coil surge supression

Adding to the other discussion.  I used to work for a company that manufactured all forms of suppression devices MOV, RC, diode/resistor, and combinations.  Low tech but it was the profit end of the company vs the custom electronics which barely broke even. I got involved with a lot of exotic customer problems.  Business got even better with the increase in variable speed drives.  Few manufacturers cared about what noise they dumped onto the mains. Like rivers, it was thought they could handle whatever you threw in.  That was true back when there were lots of incandecent light bulbs lamps to dampen the noise.  Now I recommend RC networks on the power lines to absorb noise.  One customer had a photo cell module that had a simple phase angle power supply.  Normally that would work fine except he had 50 of them on the power line and would fire at the same point on the sine wave.  A bunch of RC networks on the power line solved that.  A machine tool manufacturer had noise in the cabinet from a couple of VFDs.  I mention this because the line noise was so bad that we had to use real polypropylene 2uF snubber caps that were rated at 5A at 10KHZ.  The polyester caps they tried before were just cooking.  The only way to get rid of noise on a power line is to dissipate it in a resistor.  Otherwise it will just bounce around the entire factory on the power lines.

Many years ago we built an automated inspection machine where the 24V solenoid lines had to run unshielded within the signal cable.  The lowest noise resulted when a resistor is added across the load along with a diode. Remember ringing happens in both polarities. I understand the the automotive industry is now using resistors instead of diodes for high reliability.  These draw power but quickly dampen ringing.  A RC network gets around the constant power draw.  Another advantage the RC is since often many relays are on at one time the networks are also absorbing power line spikes from other sources.

If there are no solid state devices driving the relay, a RC network is another valid solution.  Most use X2 rated capacitors which are internally two capacitors in series for high withstand voltage and reduce internal corona effects.  The foil is self healing and almost all failures (the weld to the foil) result in an open.  If building your own, 1/2W carbon composition are suitable but getting harder to find.. These can handle repeated surges over time.  Never use carbon film which will open over time even though there is no heating.  Many metal film resistors have surge capability.

 

RE: dc coil surge supression

I do not know if this is of any help. But I made a presentation back in 2002 to describe some of the problems with relay coils to people that never really understood what was going on in their factory. I did a crude translation to English. It can be found here: http://www.gke.org/presentationer/files/The_AC_DC_coil_arcing_EMI.pdf
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

We had one of those interference blasters. You forgot the most important part..... It has to be glued onto a big piece of foam rubber!   Even then it would hop off a table.

RE: dc coil surge supression

Yes, they walked away. Not exactly silently..

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: dc coil surge supression

(OP)

Quote:

I am always concerned about contacts when switching 125V DC. An "open" contact continuing to conduct and causing two exclusive functions to operate at the same time overloading the line. That may have caused the 24V supply to drop out
That possibility was examined during troubleshooting, the output contacts from the 125vdc coil were not associated with any 24vdc loads.

Interference blasters looks like an interesting toy.  The whole idea of doing a survey for emi is interesting.  I know Doble has a lot of fancy equipment they use for that (developed by Jim Timperly).

======

As an aside, attached is a spreadsheet that I put together as a followup to of Gunnar's relay test graphs.  Results were not earthshaking... pretty much as expected (although the parameters were SWAG'd).

Tab "model" is textbook model of a simple coil with slug, including a speed voltage term. I pretty much stuck to that model, except I omitted damping.

Tab "main" has inputs (green cells) and controls (grey buttons) if you want to run a simulation for yourself.

Tab "plotsheet"  is updated to show the results each time a new simulation is run. You can adjust the appearance using green cells in plotsheet.

The simulation starts after switch has opened and we have an "initial" current that will decay.
The simulation continues until time Tstop (set to 0.05 sec).  However any data after the displacement reaches top of the graph (corresponding to 0.005m) should be disregarded... I did not try to model the mechanical interaction with the open contact...so the simulation just keeps on going and eventually overshoots the anchor position of the main spring, then oscillates back.

Tab "comparison" shows comparison of results (cut/paste graphic from plotsheet into comparison) using total resistance (external plus coil) of 4000 ohm, 1500 ohm, 500 ohm.  The wiggle is there, and the initially-decaying current begins to increase at the moment the magnetic contact begins to part (I presume electric contact opening is a few milliseconds later for Gunnar's graphs).    The duration of the hump following the  wiggle is highly dependent on the resistor as shown in tab comparison, and it is also easy to see the acceleration is much faster for the higher resistance and the contacts.

Again there is nothing new here, just decided to try it out and share for your info.
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)
There is a factor of 10 error in the constant mu0. Should be 4*pi()*1E-7 = 1.25664E-06.  You can adjust that for yourself in the green cell in the main tab.  Results remain qualitatively the same.
 

=====================================
(2B)+(2B)'  ?

RE: dc coil surge supression

(OP)
I have posted one more correction to my spreadsheet (attached)

The reason: There was actually a typographical error in Equation B of the textbook that I referenced and cut/pasted into my tab "model"  (a textbook typo, not my typo, although I should have noticed it)
The induced voltage terms of Equation B can be derived as follows:
Lambda = 2*w*d*mu0*N^2*i/(g+x)  (equation for ideal gapped inductor with gap g+x, area 2*w*d, and turns N)
L0 = 2*w*d*mu0*N^2/g (definition of L0)
Lambda = (L0 * i) /(1 + x/g)    (combines two previous equations)
dLambda / dt = dLambda/di * di/dt + dLambda/dx * dx/dt  (chain rule for differentiation, considering that x and i are both functions of t)
v = dLambda / dt = L0 /(1+x/g) * di/dt + L0*i / (g*(1+x/g)^2) * dx/dt
The factor of  i above was not included  in textbook equation B and therefore I omitted it from my previous spreadsheet.....
But I have now added it into my spreadsheet (bottom of model tab, and slope routine), and updated the comparison tab with new plots.

The general result of the change is that the hump seems smaller in proportion to the decay portion (compared to previous simulations using incorrect model).  Although the parameter values used are still SWAG'd.
 

=====================================
(2B)+(2B)'  ?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources