Testing an SCR

An SCR can be tested with some multimeters but a minimum current Anode-to-
Cathode is needed to keep the device turned on. Some multimeters do not provide this
amount of current and the SCR Tester circuit above is the best way to test these
devices.
Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail
shorted rather than open).
Measure the anode-to-cathode resistance in both the forward and reverse direction; a
good SCR should measure near infinity in both directions.
Small and medium-size SCRs can also be gated ON with an ohmmeter (on a digital
meter use the Diode Check Function). Forward bias the SCR with the ohmmeter by
connecting the black ( - ) lead to the anode and the red ( + ) lead to the cathode
(because the + of the battery is connected to the negative lead, in most analogue
multimeters). Momentarily touch the gate lead to the anode while the probes are still
touching both leads; this will provide a small positive turn-on voltage to the gate and
the cathode-to-anode resistance reading will drop to a low value. Even after removing
the gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the
anode or cathode will cause the SCR to revert to its non-conducting state.
When making the above test, the meter impedance acts as the SCR load. On larger
SCRs, it may not latch ON because the test current is not above the SCR holding
current.
Using the SCR Tester
Connect an SCR and press Switch2. The lamp should not illuminate. If it illuminates,
the SCR is around the wrong way or it is faulty.
Keep Switch 2 PRESSED. Press Sw1 very briefly. The lamp or motor will turn ON and
remain ON. Release Sw 2 and press it again. The Lamp or motor will be OFF.