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
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
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
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
November 2014 29
■ FIGURE 8. Bottom of assembled PCB. ■ FIGURE 7. Top of assembled PCB.
■ FIGURE 9.