Robin said:

I bought a relay to make a simple motor controller (w/o the overload

protection). The relay's coil is rated for 100VAC and I want to run it on

120VAC. Is this a big issue? I've been told that because a coil is a highly

inductive load, one cannot simply find the correct value for the resistor

(guess and test would be a good idea).

This is for a hobby machine tool, not an industrial installation.

Thoughts?

Regards,

Robin

This is actually trivial. First, a 100 VAC relay coil is not going to burn

out quickly if it "sees" 120 VAC. It will happily take an overvoltage for

quite some time before it dies. A simple power resistor will work

fine. There

is NO need to worry about inrush current harming the relay. You DO want

to reduce the voltage to reduce the heat in the coil, not because it

will burn

out quickly, but to make the thing last for a long time. Next, your 100

volt

relay will probably pull in as low as 80 volts, maybe lower, so precise

computation of the resistor's value is not needed.

Try and test, as you mentioned, will work fine. Put the resistor in series

with the coil and measure the voltage at the coil. You want it under 100

volts.

You DO need to use power resistors, say 10 watts or higher. And once

you have determined the resistance, you want to make sure the resistor

won't burn out. In this case, use the formula 400/R (where R is the

resistance in ohms) and double that figure to come up with the wattage.

Go to the next higher standard wattage value if the figure computed is not

standard.

Example: Say a 100 ohm resistor is selected. 400/R would be 4, double

that would be 8, and the next higher standard wattage would be 10 watts.

By the way, you could buy 4 40 ohm, 10 watt resistors for $2.00 from

All electronics. In series, or series/parallel or parallel you could get

the following useable combinations: 40 ohms at 40 watts, 60 ohms at

30 watts, 80 ohms at 20 watts, 120 ohms at 30 watts and 160 ohms

at 40 watts.