The circuit in the
diagram is a very handy tool for rapidly checking all kinds of thyristor
(SCR, triac, …). In case of a triac, all four
quadrants are tested, which is done with S3, while in case of a
standard thyristor, a positive power supply and trigger current need to
be set, which is done with S1. The value of resistors R1 and R2 is
chosen to obtain a current of about 28 mA, which is more than sufficient
for most thyristors. The hold current is determined by R3, and is 125
mA, which is more than adequate to keep the thyristor in conduction
after it has been triggered. Since D1 is a red, low-current LED, and D2 a green, low-current LED, it can be seen in a wink in which quadrant the thyristor conducts.
Testing is started with S2, and the circuit is reset with S4 after
the test has been concluded. Three short lengths of circuit wire
terminated into insulated crocodile clips on connector K1 will be found
very convenient for linking any kind of thyristor to the circuit. Mind
correct connections, though: in the case of a triac, MT1/A1 is linked to
earth, the gate to S2 and MT2/A2 to R3; in the case of a standard
thyristor, the anode is linked to R3, the cathode to earth, and the gate
to S2. If, in a rare case the trigger current needs to be altered, this
can be done by changing the value of resistors R1–R3 as appropriate.
The trigger current may also be made variable by the use of a variable
power supply. If that is done, make sure that the dissipation in the
resistors is not exceeded.