Build Your Own Test Equip111ent Project
Jean-Guy Hardy
Service Coin Ltd.
New Brunswick, Canada
IN-CIRCUIT DIODE TESTER
The diodes placed across the coils of elec-
tronic flipper games are very important in
protecting the driver circuit that control
them. These diodes absorb the current and
voltage that is generated from the collapsing
magnetic field of the coil whenever power is
removed. This voltage can damage the driver
circuits of the coils, therefore it is important
that these diodes function properly.
I have built such a device to do this simply
and quickly. The unit was built for about
$40.00 (Canadian). The diode tester was
assembled in a logic probe type case, it has
it own power supply, a DC adaptor that was
converted to AC output for checking the
diodes in circuit.
Rl
T1
R2
,10J I
Voe
D1
D2
THEORY OF OPERATION
The diodes are easily checked with a meter if
they are shorted, but, what if they are open?
This poses a problem. The coil resistance
being in parallel with the diode is too low to
give an accurate reading of the diodes back
to front resistance ratio, you must lift one
end of the diode to test it. This procedure is
time consuming, thus we need a way to test
them in circuit.
The unit operates on the principle that if you
apply an AC voltage across a diode, one half
of the AC cycle will be passed through,
leaving the other half cycle to produce a
measurable voltage output. If two light emit-
ting diodes (LEDs) are installed in reverse~
polarity, in parallel, across the coil using the
above principle -one of the LEDs will not
light if the diode is functioning properly.