Test Procedure for Silicon Controlled Rectifiers
Posted 17 May 2002 - 06:54 AM
I am looking for detailed porocedure regarding the testing of 'block' or 'puck' style SCRs. Can anuyone help me?
Posted 01 June 2002 - 03:36 AM
There are a number of modes of failure that you can test for:
- Shorted SCR
- Leaky SCR (insulation breakdown)
- Shorted gate
- Open Gate
- Non Latching
1) Catastrophic SCR failure will generally result in a short circuit between the anode and cathode. This can be tested with an ohm meter. Measure the resistance between the Anode and Cathode of the SCR. The measurement should be checked on both directions. - posative to Anode and then negative to anode. If you get a reading less than 100K with an ohm meter, then you probably have a problem. If the resistance is less than 10K with an ohm meter, then you do have a problem. Zero ohms is a definite failure!!
2) A damaged SCR can go leaky at low voltage, but often the leakage will only show up when the device is under some voltage stress. Measure the resistance of the SCR with a high voltage insulation tester. Ensure that the voltage is less than the voltage rating of the SCR. Check the resistance in both directions. Typically, 1200V SCRs will have an insulation resistance well in excess of 1 meg ohm at 500VDC but can work OK with resistances as low as 100K at 500V. A low resistance generally would suggest that the device will fail soon, but may still work.
3) Measure the resistance between the gate and cathode in both directions. The resistance should be low, but not a short circuit. The actual resistance varies considerably between SCRs and also meters. Only use standard multimeters for this test and a high voltage could cause damage. Compare the results with a known good device. Typically, the resistance would be between 10 and 100 ohms.
4) Arrange a circuit with a lightbulb of a twelve volt suppy such that the light bulb is connected between the posative terminal of the supply and the anode of the SCR, and the cathode of the SCR is connected to the negative terminal. The power rating of the lightbulb must be such that the current exceeds the latching current of the SCR (typically les than one amp so a 12 watt bulb would be minimum size) Connect a resistor via a push button switch between the posative terminal of the supply and the gate of the SCR. The resistor should be sized to deliver sufficient gating current to the SCR. (Typically 150mA at 3 volts for medium current and 400mA at 3 volts for high current) The medium current resistor would be in the order of (12 - 3)/0.15 = 60 ohm for medium current SCRs or (12 - 3)/0.4 = 22 ohms for high current SCRs.
When the voltage is first applied, the lamp should not glow. When you press the button, the light should come on and stay on after the button is released. If the light does not glow, or the light extinguishes after the button is released, then the SCR is not latching and is probably faulty.
NB If you are using puck type SCRs, you must have the SCR under pressure to ensure that internal connections are made.
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