Nuts and Volts - November 2014

to the PCB (Figure 9). Neither is necessary, although I find pin headers convenient for securing oscilloscope probes when evaluating the circuit.

Having the front panel separate and easily disconnected also facilitates board construction and modification. The only caveat in using pin headers is to make sure the switch terminals of S1 don’t contact the pin headers when the case top and bottom are put together.

All the components on the board’s topside should be mounted first and checked for continuity, and T1 soldered last on the underside. A final continuity check should be performed with the front panel connected.

Of particular importance is having D2 connected through LED2 to ground. If this connection is open, the voltage on the collector of Q1 can quickly rise to levels that will destroy the transistor.

When the continuity of the wiring checks out, connect a battery, leave the output terminals open (no DUT), and press S1. The yellow LED, LED2, should light, along with the flashing LED1. This is all you need to do to assure the circuit is operating. If the yellow LED doesn’t light, recheck your wiring.

Aside from the transformer, most of the circuit components aren’t critical but D3 must be a fast-recovery type rectifier — although any fast recovery diode with a breakdown voltage above 100 volts is acceptable. C4 and C5 should have a low equivalent series resistance (ESR) to avoid excessive ripple in the DUT. Film types are adequate in this regard, and the capacitors selected have an ESR under 0.1 ohms. If you’re not sure of the ESR of the capacitors you have, parallel several of lower value (e.g., two 0.1 µF) and make sure they have an adequate voltage rating. The low value (0.22 µF) of these capacitors is adequate for measurement accuracy, while limiting the stored energy for safety reasons.

Operating the Tester

Short the yellow to the black terminal with a jumper, connect a zener diode across the red and yellow terminals (banded side to the red terminal) along with a voltmeter, and read the voltage across the diode after pressing S1.

You’ll note that as you hold S1 down, the zener voltage will drift up as the diode heats, so make a quick measurement. Testing of LEDs and other low voltage diodes is also possible; just make sure the positive end of the LED or diode is connected to the red terminal to measure the forward voltage drop. Otherwise, the high voltage potential of the tester could destroy the LED or diode by exceeding its maximum reverse breakdown specification.

If you want to measure the current through a zener diode, remove the jumper between the yellow and black terminals and insert a millimeter. Capacitor C5 across these terminals provides a low impedance path to the pulsing current through the DUT so that the lead