The continuity tester is a handy adjunct to an ohmmeter. The unit or
component whose continuity is to be checked is connected between
terminals E1 and E2 (which may be probes or croc clips). The test
current then flowing through the unit/component on test causes a
potential drop across resistor R2, which is applied to the non-inverting
input of buffer IC2. The output of the op amp is applied to transistor
T1, in the emitter circuit of which there are a number of
parallel-connected light-emitting diodes. Each LED is in series with a
zener diodes and a resistor. The zener diodes have dissimilar zener
voltages as shown in the diagram. When the drop across R2 exceeds the
sum of base-emitter voltage of T1, a zener voltage, and the threshold
voltage of the LED in series with that zener diode, the relevant LED
lights.
The
diagram shows at which resistance value of the unit/component on test a
particular LED lights. Bear in mind, however, that these values depend
to some extent on the type of LED, and also that the zener voltages are
subject to tolerances. Serious deviations may be corrected by the
addition of a standard diode or a Schottky diode. It is also possible to
add branches to individual requirements, or to use a bar display
instead of LEDs. It is important that the op amp used has a rail-to-rail
output since the input voltages as well as the output may rise to the
peak supply voltage. This requirement is met by the MAX4322 as used in
the prototype.
The
diagram shows at which resistance value of the unit/component on test a
particular LED lights. Bear in mind, however, that these values depend
to some extent on the type of LED, and also that the zener voltages are
subject to tolerances. Serious deviations may be corrected by the
addition of a standard diode or a Schottky diode. It is also possible to
add branches to individual requirements, or to use a bar display
instead of LEDs. It is important that the op amp used has a rail-to-rail
output since the input voltages as well as the output may rise to the
peak supply voltage. This requirement is met by the MAX4322 as used in
the prototype.
