Introduction to the Method of Measuring Thyristors with a Multimeter
Thyristors are divided into two types: unidirectional thyristors and bidirectional thyristors, both with three electrodes. A unidirectional thyristor has a cathode (K), an anode (A) and a gate (G). A bidirectional thyristor is equivalent to two unidirectional thyristors connected in anti-parallel. That is, the anode of one unidirectional thyristor is connected to the cathode of the other, and their common terminal is designated as electrode T2; the cathode of one unidirectional thyristor is connected to the anode of the other, and this common terminal is also designated as electrode T2. The remaining terminal is the gate (G).
① Identification of Unidirectional and Bidirectional Thyristors Measure any two electrodes arbitrarily. If the pointer does not deflect in both forward and reverse measurements (R×1 range), the measured terminals may be A & K or G & A for a unidirectional thyristor, or T2 & T1 or T2 & G for a bidirectional thyristor. If one reading shows tens to hundreds of ohms, the component must be a unidirectional thyristor. The red test lead is connected to the K electrode, the black test lead to the G electrode, and the remaining terminal is the A electrode. If both forward and reverse readings show tens to hundreds of ohms, it must be a bidirectional thyristor. Switch the knob to the R×1 or R×10 range for re-measurement. One measurement will yield a slightly higher resistance value. In this measurement with the higher reading, the red lead is on the G electrode, the black lead is on the T1 electrode, and the leftover terminal is T2.
② Performance Differentiation Set the knob to the R×1 range. For 1~6A unidirectional thyristors, connect the red lead to K, and the black lead to both G and A simultaneously. Keep the black lead in contact with A while disconnecting G. The pointer should indicate tens to one hundred ohms, meaning the thyristor has been triggered with low trigger voltage (or low trigger current). Then momentarily disconnect and reconnect the A terminal. The pointer should return to infinity (∞), which indicates that the thyristor is in good condition.
For 1~6A bidirectional thyristors, connect the red lead to T1 and the black lead to both G and T2 at the same time. Keep the black lead touching T2 and disconnect G. The pointer should read tens to more than one hundred ohms (values vary with thyristor current rating and manufacturer). Then swap the two test leads and repeat the above procedure. The new reading should be about ten to several tens of ohms higher than the previous one. This means the bidirectional thyristor is good with low trigger voltage (or trigger current). If the pointer immediately returns to ∞ after disconnecting G while keeping A or T2 connected, the thyristor has excessively high trigger current or is damaged. Further testing can be performed as shown in Figure 2. For a unidirectional thyristor, close switch K; the lamp should light up and remain lit after opening K. If not, the thyristor is defective.
For a bidirectional thyristor, close switch K; the lamp shall light up and stay on after opening K. Reverse the battery polarity and repeat the procedure. Identical results in both tests confirm the device is intact. Otherwise, the component is damaged.
