I build a circuit that charges a 400V 4.7µF capacitor to 307V.
I have been discharging this capacitor by shorting the terminals using a large piece of metal. The capacitor makes a small spark and then my multimeter reads 0-5V from the capacitor.
I have heard in various places that this is bad for the capacitor because it causes a large current spike that can permanently reduce the capacitance and might blow up the capacitor. People also recommend using a resistor to discharge the capacitor.
I found a listing for a resistor on Amazon that has a resistance of 1K ohms and can dissipate 100W of heat.
I don't know if it is necessary to use a resistor like this or if a regular 1/4W resistor would suffice.
I calculated that at 1000ohms and 300V there should be a current of 0.3A and a wattage of 90W. This made me think that I need this big resistor.
Best Answer
That resistor would be massive overkill. Yes, the instantaneous current and power dissipation is that high, but it only lasts for a very short time. After one R-C time constant (about 5 ms), the voltage and current have already decayed to 37% of their initial values, and the power is only 14% of its initial value.
The total energy stored in the capacitor is:
$$\frac{1}{2}C\cdot V^2 = 0.22 \text{ Joules}$$
Even if you discharged it once a second continuously, that would be less than 1/4 W average dissipation in the resistor.
However, you do need to pay attention to the voltage rating of the resistor as well as its power rating. Very small resistors cannot safely handle hundreds of volts.