FAQ: SCR Testing Circuit

The SCR-Silicon Controlled Rectifier is a semiconductor device that is a member of a family of Thyristors. It is a 3-leaded device and when a small current enters the Gate, the thyristor turns on, and stays on. It only conducts current between Anode and Cathode in one direction and it is mainly only used in DC circuits. In AC circuit it will only conduct for a maximum of half the cycle.

HOW TO TEST SCR:

1. 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.

2. Shorted SCRs can usually be detected with an ohmmeter check (SCRs usually fail shorted rather than open).

3. Measure the anode-to-cathode resistance in both the forward and reverse direction; a good SCR should measure near infinity in both directions.

4. 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).

4. 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.

6. 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.

7. 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.

BUILD YOUR OWN SCR TEST CIRCUIT: