Nuts and Volts - November 2014

inductance of the multimeter doesn’t affect the readout accuracy.

When measuring an unknown zener diode and the yellow LED lights up, check to see if the diode is open by checking its forward voltage drop with a multimeter, or just reverse it in the tester. If the yellow LED goes out with the zener in the forward direction, then the diode is likely good but has a breakdown voltage above 55V. If you wish to measure the breakdown voltage in that case, insert an external variable power supply across the yellow and black terminals, with the minus terminal of the power supply connected to the yellow terminal. Increase the value of the supply slowly until the yellow LED is just extinguished, then measure the voltage across the diode.

I’ve measured zener diodes with breakdown voltages near 200V in this manner, as well as MOVs and other high voltage protective devices without concern for excessive power dissipation since the diode current when the yellow LED turns off is quite low.

Circuit Modifications and Improvements

The circuit configuration back in Figure 1 is robust and will operate with a variety of modifications. The three variable elements you can experiment with are: the secondary winding of T1; the resistor R6, which determines peak current in Q1; and the breakdown voltage of D2.

If you want the output voltage to have lower maximum voltage, you can eliminate one winding in the secondary or reduce the breakdown voltage of D2. If you want a higher maximum voltage at the output, you can put a third winding (there are two unused windings on T1) in series with the two shown, or simply replace D2 with a higher voltage zener diode. If you take this latter route, then you’ll need to pick a higher breakdown voltage transistor, like the MPSA06 (VCEO = 80V vs. 40V for the 2N3904).

Capacitors C4 and C5 are rated at 520V, and D3 has a reverse breakdown voltage of 600 volts, so there’s room to play ... but be careful if you go to higher voltages. Although C4 is a small value (0.22 µF), energy storage goes up as the square of voltage, so higher voltages can deliver a very dangerous and potentially fatal jolt! Be careful!!