How to set the value of bleeder resistor to dicharge 10kV capacitor, which has capacitance value of \$5\mu\$F?

# Electronic – How to select value of resistor to dicharge a capacitor?

capacitorchargingpowerpower supply

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## Best Answer

The relation between momentary voltage across the cap \$V(t)\$, the resistance used for discharge \$R\$, the initial voltage \$V_0\$ and the capacity \$C\$ is as follows:

$$V(t) = V_0e^{-\frac{t}{RC}}$$

Lower \$R\$ will give you a faster time to reach a safe voltage, say below 100V. The problem with this is, that the bleeder resistor will constantly drain current from your power source and turn it into heat. Assuming we want to reach 100V within the first 10 seconds of power-off. Solving the above equation for \$R\$ yields:

$$R=\frac{t}{\ln(V_0/V)C}$$

Then we'll need a resistor equal to or

~~higher~~less than 440kOhm. It will dissipate a power equal to $$(10\,\text{kV})^2/440\,\text{k}\Omega \approx 230\,\text{W} $$ Oops, that's alot. Can your power supply give 230W just for the bleeder? From this simple calculation you can see that making this PS safe after power-off won't be easy (let alone during power-on, but that's off-topic).Thus my suggestions:

Using yourself as a bleeder for 10kV in 5µF

willkill you.