Re: Which way is better, zener or resistor

Allen Bong wrote:
I have a 12V relay and I wanted to operate it with 18V supply. Could
I use a 6V 1W zener to drop the extra 6V from the supply or is it
better to use a 150 ohm 1/2W resistor. Which will generate more
heat? The relay operates with 20mA current.

In most cases, the resistor would be better.

As to specifics, you don't say what the coil resistance is.
But for any given coil resistance, a zener or a resistor that drop the same voltage in series with that coil will waste exactly the same power. The resistor is probably cheaper, and more reliable. It also has a big advantage if the input voltage has any variability. If value is chosen to drop 6 volts when the coil drops 12, it will drop this same fraction of the total voltage if the input voltage varies. So if the input voltage rises to, say, 21 volts, the resistor will drop 1/3rd of that, leaving 2/3rds of that for the coil, so the coil voltage will rise to 14 volts. Similarly, if the input voltage drops to 15 volts, the coil will get 10 volts.

If the 6 volt zener is used, it will always drop just about 6 volts, so a 21 volt input will put 15 volts across the coil and a 15 volt input will put 9 volts across the coil.

All this assumes that the coil resistance varies little as the coil current varies. The actual positive coil temperature coefficient will reduce the contrast between these two methods.

The really fine way to reduce the voltage would be to use a 12 volt integrated voltage regulator (LM7812) to throw away the excess voltage and regulate the coil voltage to a fixed 12 volts. Probably will cost about the same as a 1W zener.
.

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