The DUT`s(Device Under Test) cathode, anode, and gate are connected to the unit`s K, A, and G terminals, respectively. Pressing switch ·Sl feeds a gate current to the D UT, which triggers it on. Resistor Rl limits the gate current to the appropriate level. Resistor R3 limits the current through the LED to about 20 mA, which, with the current through R2, results in a latching current of about 110 mA. The LED is used to monitor the latching current. U the DUT is good, once the gate is triggered with Sl, the LED will remain lit, indicating that the device is conducting. To end the test, turn off the device by interrupting the latching current flow using switch S2.
Scr-tester - schematic

The LED should turn off and remain off. The preceding procedure will work with SCRs and triacs. Th check LEDs and other diodes, connect the anode and cathode leads to the anode and cathode of the diode; LED1 should light. When the leads are reversed, the LED should remain off.

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