Capacitor Leak Tester 3

I built up the circuit on the bread board but I am having some Bias issues. So I went back to the drawing board and pulled out some books and did some hunting around on the web. After much head scratching it hit me. There is a very simple variable voltage circuit that is used in modern power amps. Nothing more than a N-Channel MOSFET, A POT, some resistors and a Zener Diode.

new-schematic-3

Works great and pretty much any N-Channel MOSFET will do as long as the voltage threshold is with in spec of the input voltage. If anything just toss on a 600v rated MOSFET.

To make things even better I came across a four-part YouTube video series from M Caldeira that uses the exact circuit for his Capacitor Leak Tester. So since I know his circuit works I am going to use it.

new-schematic-2

I’ve changed a few values to work for a maximum of 50V instead of his 338V. Since I don’t play with high voltage stuff like Tube equipment.
I’m still using a huge 7 watt 1.2 ohm resistor for the discharge since just shorting it out could harm the cap. I might have to change the bleed resistor to a lower value on the power supply side.

A few nifty points on the power supply side of the circuit. The 100ohm resistor after the Rectifier is to protect the whole circuit from inrush current. The 1.6K Resistor and 1n4007 Diode are there for protection. So if the Capacitor under test is shorted it will only let the transformer push out 30mA and the Diode will block any voltage going back into the power supply causing the Zener to push voltage/current into the Gate of the MOSFET.

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