How to judge the quality of high-power thyristors

How to judge the quality of high-power thyristors

Judgment of the quality of high-power thyristors

The working principle of unidirectional thyristor: The conduction condition of thyristor is that a forward voltage must be applied between the anode and cathode of the thyristor, and a forward voltage must also be applied to the control electrode. The working principle diagram of the unidirectional thyristor must meet both of the above conditions simultaneously in order for the thyristor to be in a conducting state. In addition, once the thyristor is turned on, even if the control electrode voltage is reduced or removed, the thyristor still conducts. Condition for thyristor shutdown: Reduce or remove the forward voltage applied between the anode and cathode of the thyristor, so that the anode current is below the minimum holding current.

Bidirectional thyristor:

Controllable silicon has the characteristic of alternately conducting and turning off in two directions. A bidirectional thyristor is essentially two anti parallel unidirectional thyristors, which are semiconductor devices composed of four PN structures formed by NPNPN five layer semiconductors and have three electrodes. Due to the symmetrical structure of the main electrode (both leading out from the N layer), its electrodes are not called the anode and cathode respectively like unidirectional thyristors. Instead, the electrode closest to the control electrode is called the first electrode A1 and the other is called the second electrode A2. The main disadvantage of bidirectional thyristors is their low ability to withstand voltage rise rates. This is because when the bidirectional thyristor completes conduction in one direction, the carriers in each layer of the silicon wafer have not yet returned to the cutoff state, and corresponding protective measures must be taken. Bidirectional thyristor components are mainly used in AC control circuits, such as temperature control, lighting control, explosion-proof AC switches, and DC motor speed regulation and commutation circuits.

Comparison between the two

Unidirectional thyristors and bidirectional thyristors both have three electrodes. Unidirectional thyristors have a cathode (K), an anode (A), and a control electrode (G). A bidirectional thyristor is equivalent to two single-phase thyristors connected in reverse parallel. One unidirectional silicon anode is connected to the other cathode, with its lead end called T1 pole. One unidirectional silicon cathode is connected to the other anode, with its lead end called T2 pole, and the rest is the control pole (G).

Discrimination between single and bidirectional thyristors

First, test two poles. If both the forward and reverse pointers do not move (R × 1 gear), it may be A, K, or G, and pole A (for unidirectional thyristors) or T2, T1, or T2, G (for bidirectional thyristors). If one of the measurements indicates tens to hundreds of ohms, it must be a unidirectional thyristor. And the red pen is connected to the K pole, the black pen is connected to the G pole, and the rest is the A pole. If both the forward and reverse test indications are tens to hundreds of ohms, it must be a bidirectional thyristor. Turn the knob to R × 1 or R × 10 and retest. If there is a slightly larger resistance value, connect the red pen to the G pole, connect the black pen to the T1 pole, and connect the remaining to the T2 pole.

Differences in performance

Turn the knob to R × 1 position. For 1-6A unidirectional thyristors, connect the red pen to the K pole and the black pen to both the G and A poles. While keeping the black pen in the A pole state, disconnect the G pole. The pointer should indicate several tens of ohms to one hundred ohms. At this point, the thyristor has been triggered and the triggering voltage (or current) is low. Then instantly disconnect the A pole and reconnect it. If the pointer should return to the ∞ position, it indicates that the thyristor is in good condition.

For 1-6A bidirectional thyristors, connect the red pen to the T1 pole and the black pen to both the G and T2 poles. Disconnect the G pole while ensuring that the black pen does not detach from the T2 pole. The pointer should indicate several tens to over a hundred ohms (depending on the thyristor current and manufacturer). Then swap the two pens and repeat the above steps to measure once. If the pointer reading is slightly larger than the previous time by more than ten to tens of ohms, it indicates that the thyristor is good and the trigger voltage (or current) is small.

If the G pole is disconnected while keeping the A or T2 pole connected, and the pointer immediately returns to the ∞ position, it indicates that the thyristor trigger current is too high or damaged. Further measurement can be carried out using the method shown in Figure 2. For a unidirectional thyristor, when the switch K is closed, the light should light up, and when the switch K is disconnected, the light should still not turn off. Otherwise, it indicates that the thyristor is damaged.

For bidirectional thyristors, when switch K is closed, the light should light up, and when switch K is disconnected, the light should not go out. Then reverse the battery and repeat the above steps. The same result should be obtained to indicate that it is good. Otherwise, it indicates that the device has been damaged.