relay coil that heats.

Here is the diagram, the original and the change I made was with the 12v relay.



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The zener will not do what you think. The diode is there to catch the EMF kickback the relay coil produces when it is de-energised. You have 24V across a 12V relay, which is why the coil is overheating.

The easiest solution here is to add a resistor in series with the relay. Measure the resistance of your relay's coil, then add a resistor that will drop the required power using the Voltage Divider formula. For example, if your relay's coil is 400 ohms, you would do R = ((24 * 400)/12) -400.


2005-08-21 10:46 pm
that makes the relay pull in very susceptible to variation in mains supply voltage.

In principle yes, but I don't think this will be a problem, as most relays pull in at approx. 85% and, once activated, remain in the on position down to even lower voltages.

I would worry more about the Zener diode passing approx. 100mA while dropping 12V, making for 1.2W dissipation. Not every small Zener diode will take that in free air.

sofaspud, that would defeat the purpose of the catch diode, which is to prevent relay coil back-EMF destroying Q5.

I'd be surprised if the relay coil uses 100mA. I looked up a fairly typical relay for this job, a Finder 40.31 12V, which has a 220 ohm coil and that's 55mA. You could probably get away with 40mA of current and it'd pull in.

edit: Project16, what did you measure for the resistance of your relay's coil?
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OK, well because you are going from 24V to 12V that makes things easy - ideally, you want a 275 ohm resistor, but the closest are 270 ohms and 220 ohms.

If we go with 270 ohms, that is 270+275 = 545 ohms. At 24v this gives us 24/545 = 0.044A (44mA).

The power dissipated in our resistor will be P=I^2R = (0.044*0.044)*270 = 0.523W - as you can see, a half watt resistor will be rather hot ! A 2W resistor is best for safety.

A 220 ohm resistor will also work just as well, there will only be slightly more current flowing which will not damage the coil.
sofaspud, actually there's an interesting article on Rod Elliot's page about using a Zener to limit back EMF. The important thing is that it does not slow the opening speed of the contacts. However it's not used as a way to run a lower voltage coil on a higher voltage. I'm not sure what the implications of that are, but I wouldn't want to recommend a mod that might degrade the function of the DC protection.