# What ohm resistor to use?

1. Aug 23, 2011

This will (maybe) be an easy question to answer. Here is my situation:

I have a 12v 200mA capacity transformer (wall wart) that I am using to power up a relay which takes about 5mA and has an input voltage range of 5-24v. Basically I need to burn off the other 185mA or so of the transformer's capacity, that way the relay shuts off immediately when the transformer is unplugged instead of staying on while it drains the residual power in the transformer. I am not exactly sure what ohm of resistor I should use to do this or if that is even what i should use. Thanks for any help!

2. Aug 23, 2011

### MATLABdude

This is a really easy question to answer: the coil only provides as much current as drawn, there is no 'reserve' to draw down. Because of the inductive nature of the coil, it will sustain the current for a little bit (L / R) but this shouldn't last very long.

3. Aug 23, 2011

Sorry, I was using the wrong terminology. It's a DC power supply and there's a filter cap inside that I would like to discharge faster when it's unplugged. Initially I tried a 68 ohm 1/2 watt resistor but it nearly caught on fire (oops). Someone suggested a 100 ohm 2 watt resistor instead. Would that become as hot?

4. Aug 23, 2011

### phinds

Across a 12V supply, a 100ohm resistor will dissipate 1.4watts so a 2 watt resistor will get hot but be within tolerance.

Your 68ohm resistor was trying to dissipate 2 watts so no surprise that it fried.

5. Aug 23, 2011

Thanks so much for the help!
Would you mind explaining, just so I understand, why the resistor can be 100 ohm and not 68 ohm (like the math says)?

6. Aug 23, 2011

### phinds

I have not idea why 68 ohms was selected to discharge the capacitor but RC discharge takes an amount of time proportional to the R value so a 100ohm will discharge it more slowly than a 68ohm.

7. Aug 23, 2011

### Jiggy-Ninja

You can have it 68 ohms, you just need much more than .5 Watts. Go with 3-4 Watts for safety.

8. Aug 24, 2011

### dillonjerry

I say if you want it to discharge faster, go with a higher wattage wire coil resistor. Something like a 10 W 50 Ohm.

9. Aug 24, 2011

### Carl Pugh

Why don't you just put the switch between the wall wart and the relay?

10. Aug 24, 2011

The situation is already rigged up like crazy. Basically I've created a sequencer to power up audio amplifiers one by one

11. Aug 25, 2011