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Yesterday I had laptop charger opened and take a closer look at it. And went for capacitor because it looks damaged. Then the new capacitor is replaced, plugged in and doesn't work. I don't know if the MOSFET is dead but I saw smoke coming out. The capacitor is charged like almost full and it's getting messy dealing with the circuit. Can I discharge the capacitor safely with a resistor? Ff so, what value/wattage of resistor should I choose for discharging 450V 47uF capacitor?

winny
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Saji
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  • 10 kohm? But if you leave it in circuit for a while, it will most likely discharge itself. Measure the voltage with a multimerer and report back! – winny May 15 '20 at 14:44
  • I have 10kohm. Though I have only 1/4 wattage of them. Won't they burn if I leave it in the circuit ? – Saji May 15 '20 at 14:50
  • Oh, that's a bit too low. P=U^2/R. For 0.25 W and 450 V, the limit would be 810 kohm, but in practice there is not enough energy in the capacitor to smoke the resistor as analogsystemsrf stated below. 100 k is probably fine. – winny May 15 '20 at 14:57
  • What about the wattage ? won't it smoke the resistor ? It's only 0.25W :3 – Saji May 15 '20 at 15:04
  • Should be fine with 100kohm and 0.25W rating for your capacitor. Measure the voltage first with your multimeter! – winny May 15 '20 at 15:06
  • somewhat related: [Do capacitors automatically release their energy over time?](https://electronics.stackexchange.com/q/32529/7036) – Nick Alexeev May 18 '20 at 03:39

1 Answers1

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The capacitor stores 0.5 * V * V * C = 200v * 200v * 0.000047

Energy = 40,000 / 20,000 = 2 watt seconds. (oops forgot the 0.5 === ONE watt second)

Thus a 1 watt resistor should easily survive, of any value.

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Consider a 10 ohm. The time constant will be C * R = 47 * 10 = 470 microsecond.

The current will be 200 volts / 10 ohms = 20 amps peak.

The power (peak, at start of the discharge) will be 200v * 20a = 4,000 watts.

But the discharge tails off very rapidly.

You can write the integral of the energy, time = 0 to time = infinity.

analogsystemsrf
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  • Great, I have one. Though it is only 10 ohm, will that work ? – Saji May 15 '20 at 14:29
  • @Saji Do you have anything higher? – winny May 15 '20 at 14:45
  • I have higher value resistor but all of them 1/4 wattage. Though I can parallel the resistor for wattage boost maybe ? Currently have 100k ohm, 47k.. – Saji May 15 '20 at 14:48
  • What would happen if you short it with a screwdriver besides the screwdriver heating up? – Swedgin May 15 '20 at 14:51
  • It is a bad idea. You may end up damaging the capacitor. That is what they say xD – Saji May 15 '20 at 14:53
  • @Saji who is `they`? I'm genuinely interested in this, since I would just short them – Swedgin May 15 '20 at 14:56
  • It can also be dangerous to short with a screw driver at this voltage. Beside damaging and creating sparks. You can solder 4 x 10 ohm resistors in serie. Or several 47K in parallel (in order to have more than 1W). – Fredled May 16 '20 at 07:05