How to test a thyristor?

How to test a thyristor (SCR):

  1. Obtain a bench DC power supply with adjustable output voltage and current limit. For example 0–30Vdc 0–5A
  2. Set the power supply to around 30 Vdc and short-circuit the output wires and adjust the current limit to 5 A or 50% of the nominal value of the thyristor, whichever is less.
  3. Now reduce the power supply output voltage to zero and connect the thyristor anode to the positive and the cathode to the negative.
  4. Increase the output voltage of the power supply to approx. 24Vdc. No current should flow. This proves that the thyristor blocks properly when it is forward biased.
  5. Now connect the thyristor gate to the positive supply via a 100 ohm 1W resistor. The thyristor must light up and the power supply output current must reach the preset current limit level. The output voltage of the power supply must be reduced to 1.5 Vdc. This proves that the thyristor trips correctly in conduction.
  6. Now remove the connection to the door. The thyristor should continue to drive. This proves that the thyristor locks correctly.
  7. Now reverse the connections (anode to negative, cathode to positive) and increase the output voltage of the power supply to approx. 24Vdc.
  8. Now connect the thyristor grid to the positive terminal of the power supply via a resistance of 100 ohms 1 W. The thyristor must NOT start to drive. This proves that the thyristor locks correctly in reverse.

If the thyristor is actually a triac, the tests are similar. But the terminals are different – for the read anode MT2, for the read cathode MT1. However, step 8 is different – do the following:

  • Connect the triac door to the power supply negative via the 100 ohm 1W resistor. The triac should start driving and the supply current should reach the preset limit level. The supply voltage should drop to around 1.5 Vdc. This confirms the correct triggering of quadrant 3.
  • Now remove the connection to the door. The triac should continue to drive. This confirms the correct locking of quadrant 3.

 

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