How to Test an SCR

A silicon-controlled rectifier (SCR) is a conditional diode. When forward-biased (i.e., positive charge to the anode and negative to the cathode), it will operate like a normal diode, passing current with a small voltage drop. When back-biased (i.e., negative charge to the anode and positive to the cathode), it will block current flow until it receives a pulse of adequate voltage at its trigger (i.e., trigger voltage) or its peak inverse voltage is exceeded. Here's how to test an SCR for proper operation.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Variable DC voltage power supply: 2 supplies or one dual output with independently controlled outputs (Note: Power supply(s) must show both voltage and current outputs.)
• 4 leads with clips
• Test board

1. Mount the SCR to your test board to allow for room to attach the power supply leads and DMM leads.

2. Connect leads from one DC power supply, with positive to the SCR anode and negative to the SCR cathode.

3. Turn on the power supply to 6VDC. When forward-biased in this way, a current flow should be evident. If it is, the device is working properly, so go to Step 4. If there is no current flow through the forward-biased device, the SCR has failed (opened) and the test is over.

4. Turn off the power supply and reverse the leads: positive to the cathode and negative to the anode.

5. Turn on the power supply and check the current reading. When reverse-biased, there should be no current flow in normal operation. Turn the voltage up to the maximum of the power supply or roughly half of the SCR's peak inverse voltage rating. If there is no current flow, the device is working properly, so go to Step 6. If there is current flow when back-biased, the SCR has failed (shorted) and test is over.

6. Turn power supply down to the voltage you expect the SCR to see in normal operation and then turn it off. Set the second power supply so it will turn on at a 0 VDC and turn it off. Attach the leads from the second power supply: positive to the SCR trigger and negative to the negative of the first power supply.

7. Turn on the first power supply. There should be no current flow while the SCR is still back-biased. Turn on the second power supply and carefully increase its output until you reach the SCR's trigger level as defined by its specifications. When the SCR trigger level is reached, current flow should be evident in the first power supply. This flow should continue as you turn the trigger voltage back to 0 VDC. If current in the first power supply goes back to zero when the trigger voltage is dropped, the SCR has failed. Current should flow in the first power supply after the trigger voltage is dropped back to 0 VDC and should continue until voltage on the first power supply is turned off.

8. If the SCR passed Step 7, turn the first power supply on again. When back-biased and with 0 VDC on the trigger, there should be no current showing on the first power supply. If the SCR has passed all of these steps, it is operating properly.

Tips & Warnings

• If your power supplies lack current displays, put a digital multimeter (DMM) in series with the first power supply's positive output. In normal operation, the back-biased SCR may read in microamps.