Need Help!- Relay coil Question

[Bird_E]

Can I power 5 Volt relay coil with 24 volts with the proper series resistor?

I assume this will work. Should i be looking for a resistor that will reduce the current to what the realy coil is looking for? Or should I be looking at reducing the over all power (watts)

Example. 5volt relay coil @ 120 Ohms = 42ma or 210 milliwatts

If I was to drive the same coil from 24 volts should I be looking to limit the current to 42ma or the total power to about 200 milliwatts?

Current limited to .042 @ 24Volts
24 Volts / .042amps = .571 ohms - (120 coil resistance) = about 450 ohm resistor. But the power draw will be about 1 watt!!

Power limited to about 200 milliwatts:

.200 (watts) / 24vdc = about .008 milliamps (is that enough current??)

24/.008 = 3000 Ohms - (120 Ohm Coil Res) = 2.7K resistor.

How should I be looking at this?
Thanks!
Bob

[Al_The_Man]

Only if it is a DC relay, you would need a 450ohm 1 watt resistor.
You are dropping 19v not 24v.
Not very efficient, how large a relay do you need, 24vdc relays are 10 a penny (well almost!).