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What ohm resistor to use?

  1. Aug 23, 2011 #1
    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. jcsd
  3. Aug 23, 2011 #2


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    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.
  4. Aug 23, 2011 #3
    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?
  5. Aug 23, 2011 #4


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    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.
  6. Aug 23, 2011 #5
    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)?
  7. Aug 23, 2011 #6


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    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.
  8. Aug 23, 2011 #7
    You can have it 68 ohms, you just need much more than .5 Watts. Go with 3-4 Watts for safety.
  9. Aug 24, 2011 #8
    I say if you want it to discharge faster, go with a higher wattage wire coil resistor. Something like a 10 W 50 Ohm.
  10. Aug 24, 2011 #9
    Why don't you just put the switch between the wall wart and the relay?
  11. Aug 24, 2011 #10
    The situation is already rigged up like crazy. Basically I've created a sequencer to power up audio amplifiers one by one
  12. Aug 25, 2011 #11
    Thanks for all the help guys, I ended up going with a 50 ohm 10 watt resistor and even fashioned a homemade heatsink complete with thermal compound to keep it cool. It works wonderfully and now my relay doesn't take 20 extra seconds to power off. I have been to some forums with some unpleasant people and I must say everyone here seems friendly and ready to help.
  13. Aug 25, 2011 #12
    Are you permanently placing a 50 ohm resister across the Power-Supply output?
    Thats crazy way of solving your problem. I would have just opened the power supply and used a much lower capacity capacitor or not at all. Relays work fine with rippling DC.
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