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Capacitor discharger.

Before repairing electronic circuits, it is important to first discharge any charged capacitors in the circuit.
This "capacitor discharger" is designed for this purpose.
It also has a build in voltmeter, which measures the voltage in the capacitor.

Circuit diagram of the capacitor discharger.

Connector CON1 and CON2 are the inputs to be connected to the capacitor you want to discharge.
The capacitor is discharged by resistors R1 and R2, both 560 Ω / 50 watt, so together 1120 Ω / 100 watt.
This 100 watt dissipation is reached with a input voltage of 335 volt.
However the capacitor discharger can be used up to 500 volt, because the resistors can handle a higher power for short periods.
At 500 volt resistors R1 and R2 dissipate a total of 223 watt which is permissible for about 10 seconds.
The datasheet of these resistors can be viewed here: datasheet_HSA50 resistor.pdf
For instance a 1000 μF capacitor charged to 500 volt will in 1.12 seconds (one RC time) discharge to 184 volt.
So the overload for the resistors occurs only for a very short time, and is no problem for the resistors.

The rest of the components are for the voltmeter circuit.
Resistors R3, R4, R5, R6, R7 and R8 are 1 MΩ in total.
So with 50 volt across CON1 and CON2, meter M1 reads 50 μA.
In that case the voltage across D1 and D2 is about 40 volt.
When the voltage across CON1 and CON2 is higher then 50 volt, the voltage across zenerdiode D1 and D2 is limited to 40 volt, and meter M1 will never go higher then 50 μA.

I wanted the circuit to be able to withstand connecting to the 230 volt AC mains.
Capacitor C1 is added for this situation to reduce AC current to meter M1 to a low level.
Indeed I tested the circuit connected to the 230 volt mains for 30 seconds, and nothing special happens, meter M1 was standing still, and resistor R1 and R2 got handwarm.
 


The capacitor discharger.


Inside of the capacitor discharger.
Resistors R1 and R2 are mounted on a aluminium plate.
The enclosure was used before for another project, and has therefore some extra holes, which are welcome to remove the heat from the resistors.



The capacitor discharger with the test leads and probes.

I have tested the capacitor discharger with a big 6800 μF / 200 volt capacitor.
First the capacitor was charged up to 180 volt, and then discharged via the capacitor discharger.
After repeating this 10 times (that took about 10 minutes in total), the temperature of resistor R1 and R2 was measured and they were about 15 °C above ambient temperature.
So I expect not much problem with overheating R1 and R2.

Use test leads and probes rated for high voltage !
 

This is a safety test lead and safety test probe rated for 1000 volt.
The plug is shielded by a plastic tube.
Use these ones if the working voltage is higher the 60 volt DC.

This overview shows the safety probes and leads I use, other manufacturers might have similar products.



This is a low voltage lest lead and probe rated for maximum 60 volt DC.
Do not use these for higher voltages.

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