Will a Zener Diode work for me here? 4

[railking]

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.

 

[logbook]

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.

 

[IRstuff]

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

 

[felixc]

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.

 

[nbucska]

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

 

[IRstuff]

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

 

[ruggedscot]

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

 

[railking]

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?

 

[railking]

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.

 

[IRstuff]

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

 

[nbucska]

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


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

 

[railking]

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?

 

[nbucska]

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

 

[railking]

It is called the airwire 900 and it is manufactured by CVP products. Their web site is

http://www.cvpusa.com/

 

[OperaHouse]

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.

 

[logbook]

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.

 

[railking]

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?

 

[railking]

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?

 

[OperaHouse]

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.

 

[nbucska]

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