The function and measurement method of unidirectional thyristor and bidirectional thyristor

SCR conduction conditions:
One is that a forward voltage must be applied between the SCR anode and the cathode,
The second is that the control electrode also needs to be applied with a forward voltage.
The above two conditions, the working principle diagram of the one-way SCR must be available at the same time, the SCR will be in the conducting state. In addition, once the thyristor is turned on, even if the gate voltage is reduced or the gate voltage is removed, the thyristor is still turned on. SCR turn-off conditions: reduce or remove the forward voltage between the SCR anode and the cathode, so that the anode current is less than the sustain current.

Triac:
Thyristor has the characteristics of turning on and off in turn in two directions. The bidirectional thyristor is essentially two anti-parallel unidirectional thyristors. It is a semiconductor device with four PN structures and three electrodes formed by NNPPN five-layer semiconductors. Since the structure of the main electrode is symmetrical (all derived from the N layer), its electrodes are not called anode and cathode respectively like unidirectional thyristors, but the one that is close to the control electrode is called electrode A1, and the other is called the first electrode. Two electrodes A2. The main disadvantage of triacs is their low ability to withstand the rate of voltage rise. This is because when the bidirectional thyristor is turned on in one direction, the carriers in each layer of the silicon chip have not returned to the position of the off 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 control and commutation circuits.

Comparison of the two
Unidirectional thyristor and bidirectional thyristor, both have three electrodes. One-way thyristors have cathode (K), anode (A), and control electrode (G). The bidirectional thyristor is equivalent to two single-phase thyristors in reverse parallel connection. That is, one of the unidirectional silicon anodes is connected to the other cathode, and its leading end is called T1 pole, and one of the unidirectional silicon cathodes is connected to the other anode, and its leading end is called T2 pole, and the rest is called control. Extremely (G).
Single and bidirectional thyristor discrimination
First measure two poles, if the positive and negative test pointers are not moving (R×1 gear), it may be A, K or G, A pole (for one-way SCR) or T2, T1 or T2, G Pole (for triac). If there is a measurement indication of tens to hundreds of ohms, it must be a one-way 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 the positive and negative test instructions are tens to hundreds of ohms, it must be a triac. Then turn the knob to R×1 or R×10 to repeat the test. One of the resistance values must be slightly larger. The larger red pen is connected to the G pole, the black pen is connected to the T1 pole, and the rest is the T2 pole.
The difference in performance
Turn the knob to R×1, for 1~6A one-way SCR, connect the red pen to the K pole, and the black pen to connect to the G and A poles at the same time. Keep the black pen from leaving the A pole to disconnect the G pole The pointer should indicate dozens of ohms to one hundred ohms, at this time the SCR has been triggered, and the trigger voltage is low (or the trigger current is small). Then turn off the A pole momentarily and turn it on again, and the pointer should return to the ∞ position, indicating that the thyristor is in good condition.
For the 1~6A triac, the red pen is connected to the T1 pole, and the black pen is connected to the G and T2 poles at the same time. The G pole should be disconnected under the premise of ensuring that the black pen does not separate from the T2 pole. The pointer should indicate dozens to more than one hundred Europe (depending on the size of the thyristor current and different manufacturers). Then reverse the two pens and repeat the above steps. The pointer indicates that it is a little more than ten to several tens of ohms, which indicates that the thyristor is good and the trigger voltage (or current) is small.
If the A pole or the T2 pole is kept on and the G pole is turned off, the pointer immediately returns to the ∞ position, indicating that the SCR trigger current is too large or damaged. It can be further measured according to the method in Figure 2. For the one-way SCR, close the switch K, the light should be on, and the K light will not stop after disconnecting it, otherwise the SCR is damaged.
For triacs, close switch K, the light should be on, and open K, the light should go out continuously. Then reverse the battery and repeat the above steps, all should be the same result, it is good. Otherwise, the device has been damaged.