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Will a Zener Diode work for me here?
4

Will a Zener Diode work for me here?

Will a Zener Diode work for me here?

(OP)
I need to insert a component or a VERY simple circuit intoa project that will limit DC voltage to 18 volts (I could use 16 or 17 volts too, but nothing more than 18 volts).

I don't want a voltage regulator because the voltage wil be running small motors and must vary from 0V to 18V, but if it goes over 18V it will damage some delicate circuitry I am using.

I am designing the circuit to handle up to 10 amps under load.

I was thinking of using a zener diode with a 17V or 18V rating but I am unsure if this will work.

Will an 18V  Zener allow 0-18V to pass and then clamp the voltage at 18V?

If this is true, then anything over 18V would be disipated as heat, right?

If the likely maximum voltage that could ever ba applied to this cicuit is 20.5 volts from batteries, then it would be dissipating 2.5 volts at 10 amps (25 watts) right?

If this is all feaible, then I have more questions about what end of the zer gets connected to the battery and what end goes to my protected circuit device.

RE: Will a Zener Diode work for me here?

It is a common mistake to think of a zener diode as an ideal device. In reality the characteristic is a bit soggy. An 18V zener will start conducting well before 18V and at 10A even a huge zener would be up over 20V. The power in the zener is the current into the zener times the voltage across the zener.

10A * 18V = 180W.

Everyone would like a clamp that is either on or off. They don’t exist, and certainly not at 10 amps. You absolutely have to have a range over which the clamp has to work.  Suppose you have a diode clamp to an ideal battery. When the diode is reverse biased the current will be pretty low. When the diode starts being forward biased the current will rise pretty quickly. At 10A even a fat silicon diode would be over 0.8V.

You need to get a more realistic expectation of what is achievable. For 10A you will probably need to use a comparator/opamp and a power MOSFET to shunt the excess current elsewhere, but allow several hundred millivolts between the on and off states. The tighter the transition spec, the harder it will be to design.

RE: Will a Zener Diode work for me here?

Why is the load current related to the clamp current?  

To the battery, an 18V clamp is essentially a short circuit, so you could easily get 100 Amps, until the battery drains itself down to the clamp voltage.  A single, fresh  AA battery will dump 10 Amps into a short until it drains itself and can sustain a 1-Amp load for at least a few minutes.

You really, really need to have a REGULATOR to regulate the voltage to the load, particularly if the battery is also recharged while online.

TTFN

RE: Will a Zener Diode work for me here?

Can you put a regulator on your control circuitry, and have the higher voltage go only to the motors?  It would protect your circuitry and avoid eliminating the extra voltage as heat instead of more torque on the motor.

RE: Will a Zener Diode work for me here?

What are the specs of the "delicate circuitry"? The
motor may be pulse modulated and you need regulation only
for the additional circuits.

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Will a Zener Diode work for me here?

You don't necessarily have to have a single regulator.  You can use a fixed regulator for the circuitry and a variable regulator for the motors.  This will also improve your circuit's noise immunity by isolating the motor noise from the circuit power.

TTFN

RE: Will a Zener Diode work for me here?

Used a voltage clamp to protect a system comprising of a zener a relay and a fuse. The way it worked was that when the zener conducted it blew the fuse and dropped out the relay, crude I know but did the purpose. Id go with the using avoltage regulator to supply the delicate circuits and then have some filtering on the dc side for the motors. that way you isolate the system and provide good stable voltage for the delicate circuits.

Rugged

RE: Will a Zener Diode work for me here?

(OP)
Hey everyone....THANK YOU for all your replies.

Here is a bit more information.

The "delicate circuit" I am trying to protect is in fact a "radio control throttle receiver".  In other words, the full 18+V is fed into this device and, by means of a handheld radio control transmitter, I can vary the voltage to the model locomotive motors to increase or decrease the speed.

The receiver has a specification of maximum 18V input at 10 Amps.  In reality, I have inserted an 8 amp fuse in the circuit so that fuse would certainly blow well before we hit 10 amps.

My batteries (Qty 3 6V 7Ah sealed lead acid bateries in series)put out 18.65 to 19.0 volts when fully charged and I am trying to make sure that the extra .65 to 1.00 volts doesn't hurt the receiver.

It is my understadning that a "voltage regulator" will peg the voltage so that it will neither go below 18V nor above 18V.  Is that correct?

If so then I can't use a "regulator".

What I need is a "limiter".  I have heard the term "clamp" used but I am unsure what it means.  What I need is something that will pass all voltage from zero up to 18 volts anmd if the voltage goes over 18 volts, then only allow 18V to pass to the protected receiver.

Am I off base thinking that a zener will do that?

RE: Will a Zener Diode work for me here?

(OP)
By the way...an additional note to Logbook....

I don't really need all 18V to run my trains.  I could live with 16 or 17 volts.  When combined with the new lower restriction that the 8 amp fuses enforces, could I find a zener that would do the job for the short time that these batteries will be over 18V?

It is my understanding that this over voltage condition is very temporary and that after a few minutes under load the bateries will settle down to their rated maximum voltage.

I appreciate all youtr help.

RE: Will a Zener Diode work for me here?

A zener will only do part of that.  While the voltage is less than 18V, it will, more or less, behave as you describe.  As you go above 18V, the zener will simply drain your battery down to 18V.  If that's acceptable, then you've got a solution in hand.

TTFN

RE: Will a Zener Diode work for me here?

The radio don't need 10A -- perhaps 10 mA.

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Will a Zener Diode work for me here?

(OP)
Dear nbuska,

You are quite correct.....

The radio receiver is operating off of a fraction of that current but it is controlling the variable voltage circuitry that changes the motor's speed.

The radio receiver circuit (or decoder) has only one voltage input to the board......up to 18V and 10 Amps, then it has various outputs, one of which is the variable voltage for the motors that is responsible for the majority of the load on the circuitry.

Does that help?

RE: Will a Zener Diode work for me here?

Can you give me the type of the radio and the WEB-site where I can find its data ?

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Will a Zener Diode work for me here?

(OP)
It is called the airwire 900 and it is manufactured by CVP products.  Their web site is http://www.cvpusa.com/

RE: Will a Zener Diode work for me here?

If you don't really need the voltage, power the electronics with two diodes in series from the battery.  This will drop the battery voltage approximately 1.2 volts.  Franky, I don't think the max voltage specification is real.  Rather, it represents the nominal battery voltage.

RE: Will a Zener Diode work for me here?

RailKing,

I think you have misunderstood something about the Airwire 900. …That is a nice bit of kit by the way.

Anyway, it is expecting a fixed input voltage from a battery. It does not require any sort of variable voltage. If the controller wants to deliver more power, it will draw more current from the battery. Ideally the battery voltage would be constant. So there is no question of varying the input voltage for any useful purpose. Of course the battery voltage will change with discharge which is probably what you meant. Ok, the battery voltage may go above 18V and the data sheet is not very clear if this would be acceptable. On reading the data sheet, it seems as if it is really designed for 12V, and 18V represents the over-voltage limit.

A standard (series) voltage regulator would do nicely. You would probably use an "adjustable" regulator with the output set to 17V. Additional applied voltage would be dropped across the regulator. This is exactly the function you need and want. If the battery was at 19V and the regulator output was at 17V then you would lose 2V across the regulator, which would be 20W at 10A load current. Now that is a pretty larger regulator. You do not want a zener diode as it is a shunt regulator.

Operahouse’s idea of using two silicon diodes in series with the power rail is an easier idea. Although the voltage out will not be as constant, you will get the required protection. It is also easier to get 10A rated diodes rather than a 10A rated regulator. The dissipation in the diodes will then be 10A times 1.4V = 14W. Not a trivial amount of heat to get rid of.

RE: Will a Zener Diode work for me here?

(OP)
Logbook!

That is the most authoratative and relevant response yet to date.  Can you tell me if the standard series voltage regulator you are refering to, adjusted down to a peek of 17 Volts, would pass 12 volts Ok when the batteries drained down to that level?

RE: Will a Zener Diode work for me here?

(OP)
Oh, and by the way......could you help me to identify a purchasable part that would provide me with the voltage regulation capability you describe?

RE: Will a Zener Diode work for me here?

While this is an interesting intellectual exercise, I suggest you email the manufacturer with your concerns.  If this design can't take 25 volts, the designer should be shot! (Not that a lot of them shouldn't) You will not find anything simple and small that will do what you want.  This item was designed to work with standard battery configurations.

RE: Will a Zener Diode work for me here?

I haven't been able to find the schematics of the receiver
but I think it was a design error to connect the power
input of the relay/load to the one of the receiver
proper. I suggest separate them by rework and regulate
the receiver's power only.

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Will a Zener Diode work for me here?

Using a distributor to find parts is a good way to go. I tried RS components

http://rswww.com/

searching on "regulator" and going to the table for adjustable, linear, positive regulators, the maximum current rating available is 7.5A. You must understand that these are high power levels for small-signal electronics guys. Probably nobody would use a linear regulator above these levels because the power loss would be so excessive.

There are two questions: one, how can you solve the problem, and two how would I solve the problem? Now me, being a "clever clogs" might use a power MOSFET in series with the power rail. When the input voltage was less than say 17V the power MOSFET would be on continuously. As the voltage increased the power MOSFET would be switched, reducing the overall output voltage. This is a pulse width modulated scheme, a buck regulator. There would need to be a 15A inductor (choke) in series with the power MOSFET and a 15A diode to provide the flyback for the inductor. There would be a switched mode controller chip to drive the power MOSFET. You will be able to find this sort of circuit in National Semiconductor application notes for their "simple switchers".

http://www.national.com/appinfo/power/

These 10A components , the MOSFET, choke and diode are all chunky components and the power MOSFET would fail within a few seconds if you failed to get enough heatsinking on it. Once the junction reaches 200degC the MOSFET has had it (fried; dead). This scheme will get the most out of the battery because you will only lose say 300mV across the MOSFET/choke when it is not switching. However, the two diodes in series approach, although it loses 1.6V continuously, is very easy for a non-specialist. The LT1083 data sheet (although it can only handle 7.5A) operates with 1V headroom, so with under-voltage conditions it will run with a little less than 1V drop, the amount being a bit uncertain because the manufacturer doesn’t characterise the regulator for this use. Maybe it is only half a volt drop; you would have to try one.

However, having talked around the problem, I have now come up with an even better solution for both of us. Why not use the double diode as the over-voltage protection. When it is not needed switch it out of circuit by short-circuiting across it. This shorting link can be done with a relay contact or with a P channel power MOSFET. All you now need to do is to switch the relay or MOSFET when the voltage is low. This requires a reference voltage, a resistive divider, and a comparator. That is not much circuitry and the efficiency will be excellent.

Suppose we pick a 5V reference. Two resistors form a divider chain from the input. At 17V input the output of the divider chain is 5V. A comparator detects the input is slightly too high and switches the relay off or the power MOSFET off.. Job done.  This should be enough to get you started.

Beware of one tricky point. When you switch in the extra diodes, the voltage will drop by say 1.6V. If the voltage was just too high it would then become 1.6v too low. The comparator would therefore keep switching and would be unable to "decide" if it should be on or off. The answer is to include a lot of hysteresis in the switching point. This consists of a resistor giving positive feedback to the comparator (making a schmitt trigger).

I have deliberately used some technical terms which you can look up to find the detailed information you will need.

RE: Will a Zener Diode work for me here?

Quite a few of the voltage regulator datasheets show basic circuits which use the regulator as a true regulator, and not the current carrying device.  This generally involves a handful of external components and one or more BJT/FET packages.  I would suggest this as a possibility... several FETs in parallel will be more than enough to handle the 10A necessary, with little voltage drop or wasted power.

RE: Will a Zener Diode work for me here?

MacGyver2000,
    the ones I have seen use an NPN pass transistor for a positive regulator. We need to use a PNP pass transistor (or P channel MOSFET) in order to get a low drop. Can you give us a specific example of such a regulator?

Using an emitter follower in the regulator is easy. Using a common emitter is much more difficult because the loop gain is poorly defined and getting stability in the loop is harder.

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