How to measure the quality of a thyristor with a multimeter?
1. Discrimination between Unidirectional and Bidirectional Thyristors
First, randomly measure any two electrodes. If the pointer of the multimeter remains stationary in both forward and reverse measurements (using the R×1 range), it could be the A and K electrodes or the G and A electrodes for a unidirectional thyristor, or the T2 and T1 electrodes or the T2 and G electrodes for a bidirectional thyristor.
If one of the measurements shows a resistance value ranging from several tens to several hundreds of ohms, it must be a unidirectional thyristor. In this case, the electrode connected to the red pen is the K electrode, the one connected to the black pen is the G electrode, and the remaining one is the A electrode.
If both forward and reverse measurements show a resistance value ranging from several tens to several hundreds of ohms, it must be a bidirectional thyristor. Then, switch the knob to the R×1 or R×10 range for re - measurement. There must be one measurement with a slightly larger resistance value. In this measurement, the electrode connected to the red pen is the G electrode, the one connected to the black pen is the T1 electrode, and the remaining one is the T2 electrode.
2. Performance Differentiation
For 1 - 6A Unidirectional Thyristors: Switch the knob to the R×1 range. Connect the red pen to the K electrode and the black pen to both the G and A electrodes simultaneously. While keeping the black pen connected to the A electrode, disconnect the G electrode. The pointer should indicate a resistance value ranging from several tens to one hundred ohms. At this time, the thyristor has been triggered, and the triggering voltage is low (or the triggering current is small). Then, momentarily disconnect and then reconnect the A electrode. The pointer should return to the ∞ position, indicating that the thyristor is in good condition.
For 1 - 6A Bidirectional Thyristors: Connect the red pen to the T1 electrode and the black pen to both the G and T2 electrodes simultaneously. On the premise of keeping the black pen connected to the T2 electrode, disconnect the G electrode. The pointer should indicate a resistance value ranging from several tens to over one hundred ohms (the specific value varies depending on the thyristor's current and the manufacturer). Then, reverse the connection of the two pens and repeat the above steps. The pointer indication should be about ten to several tens of ohms larger than the previous measurement, indicating that the thyristor is in good condition and has a low triggering voltage (or current).
If the pointer immediately returns to the ∞ position when the G electrode is disconnected while keeping the A electrode or T2 electrode connected, it means that the triggering current of the thyristor is too large or the thyristor is damaged. You can further measure it according to the method shown in Figure 2. For a unidirectional thyristor, when the switch K is closed, the lamp should light up, and when K is opened, the lamp should remain lit. Otherwise, it indicates that the thyristor is damaged.
For a bidirectional thyristor, when the switch K is closed, the lamp should light up, and when K is opened, the lamp should remain lit. Then, reverse the battery connection and repeat the above steps. The same results should be obtained in both cases, indicating that the thyristor is good. Otherwise, it means that the device is damaged.
