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transient voltage suppressor protecting a relay coil

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Sep 10, 2009
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diagram of transient voltage supressors

I have a question in terms of protecting a microprocessor output that drives a relay from back emf.

I have attached a diagram of the circuit, which I have come across.

I understand why the TVS (transient voltage suppressor) is used, but why is the resistor connected in parallel over the coil.

Is it acting as a filter or is the resistor absorbing the back emf spike?

Is the resistor not just providing another path for the back emf to damage the output card? What are the risks involved with connecting the resistor in this configuration?

Hopefully this makes sense?

The output card does have enough current to pick the coil.

Any comments / suggestions would be appreciated.


relay voltage suppressors

What is point "N" connected to?

transient suppressor relay

Sorry just my nomenclature.

The output card would supply the positive voltage e.g. +12V and N is the negative or OV.

Basically 12V is put over the coil to pick it up.

protecting coils from back emf

Looks like serious overkill to me. All you need is a decent diode like a 1N4004 across the relay coil to snub the back emf. Dump the resistor and don't spend a lot of bucks on a transient suppression diode. I'm assuming that the relay is not as big as a house; more like something that draws only 100 ma or so. No resistor is needed because the resistance of the relay coil will dissipate the energy of the snubbed transient.

But, yes, you do need transient suppression across every inductive load you drive. Simple diode will do the trick. By the way, put it at the relay itself. Otherwise the transient will radiate in the wiring and you will have an EMI problem.

Since you didn't include exact details about what device is driving the output, I would be worried that it can't supply enough current to drive a relay and would also be worried that if the diode dies then it would take out the microprocessor.

I recommend that you put a transistor or high-current driver IC between your processor and the relay. Transistors are very cheap and it is crazy to not use one in this case.

I did some quick searching on the internet and found the following links:

**broken link removed**

**broken link removed**

**broken link removed**

Good luck!

Although it is not common today, in years gone by when small diodes were not available and wiring vacuum tube diodes across relays was not practical, resistors were used like this. The idea is not to dampen the reverse voltage at switch off, it is to reduce the 'Q' factor of the relay coil so the kick is made smaller.

There is another reason why *sometimes* a diode is not the best solution, they slightly slow down the operation of the relay. Allowing a small amount of back EMF, more than 0.6V but not enough to damage the driver circuit, will allow the relay to operate at higher speed. The usual solution it critical applications is to use a TVS or a normal diode in series with a Zener diode so it only clamps at Zener + 0.6V.



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