Complete detection methods of unidirectional/bidirectional thyristors

　　A multimeter can be used to determine whether a bidirectional thyristor is good or bad, but the specific parameters cannot be measured. The method of measuring with a multimeter is as follows.

　　Determination of T2 pole: Use the multimeter R*1 or R*100 to measure the reverse resistance of each pin respectively. If the forward and reverse resistances of the two pins are very small (about 100 ohms), they are T1 and G poles, and the remaining pin is T2 pole.

　　Distinguishing between T1 and G poles: Assume any one of the two poles as T1 pole and the other pole as G pole, set the multimeter to R*1, use two test leads (regardless of positive and negative poles) to touch the determined T2 pole and the assumed T1 pole respectively, and touch the test lead that touches T1 to the assumed G pole at the same time. Disconnect the assumed G pole while ensuring that the assumed T1 pole is not disconnected, and the multimeter still displays the on state. Swap the test leads and measure in the same way. If the multimeter still displays the same result, then the assumed T1 pole and G pole are correct. If the assumed G pole is disconnected while ensuring that the assumed T1 pole is not disconnected, and the multimeter displays the disconnected state, it means that the assumed T1 and G poles are opposite. If you make a new assumption and measure again, the result must be correct.

　　If the above results cannot be measured, it means that the bidirectional thyristor is bad. Although this method cannot measure specific parameters, it is still feasible to determine whether it is usable.

　　1. Silicon is divided into unidirectional thyristor and bidirectional thyristor.

　　The unidirectional thyristor has three lead pins: anode A, cathode K, and control electrode G. The bidirectional thyristor has three lead pins: the first anode A1 (T1), the second anode A2 (T2), and the control electrode G. Only when a positive voltage is applied between the anode A and cathode K of the unidirectional thyristor, and the required positive trigger voltage is applied between the control electrode G and the cathode, can it be triggered to conduct. At this time, A and K are in a low-resistance conduction state, and the voltage drop between the anode A and the cathode K is about 1V. After the unidirectional thyristor is turned on, even if the controller G loses the trigger voltage, as long as the positive voltage is maintained between the anode A and the cathode K, the unidirectional thyristor continues to be in a low-resistance conduction state. Only when the voltage of the anode A is removed or the voltage polarity between the anode A and the cathode K changes (AC zero crossing), the unidirectional thyristor will be converted from a low-resistance conduction state to a high-resistance cut-off state. Once the unidirectional thyristor is cut off, even if the positive voltage is reapplied between the anode A and the cathode K, the positive trigger voltage must be reapplied between the control electrode G and the cathode K before it can be turned on. The on and off states of the unidirectional thyristor are equivalent to the closed and open states of the switch, and it can be used to make a contactless switch. Between the first anode A1 and the second anode A2 of the bidirectional thyristor, no matter whether the voltage polarity is forward or reverse, as long as a trigger voltage with different positive and negative polarities is applied between the control electrode G and the first anode A1, it can be triggered to turn on and present a low resistance state. At this time, the voltage drop between A1 and A2 is also about 1V. Once the bidirectional thyristor is turned on, it can continue to remain in the on state even if the trigger voltage is lost. Only when the current of the first anode A1 and the second anode A2 decreases and is less than the holding current or when the voltage polarity between A1 and A2 changes and there is no trigger voltage, the bidirectional thyristor is cut off, and it can only be turned on by reapplying the trigger voltage.

　　2. Detection of unidirectional thyristor.

　　Select the resistance R*1Ω position on the multimeter, and use the red and black test leads to measure the forward and reverse resistance between any two pins until a pair of pins with a reading of tens of ohms is found. At this time, the pin of the black test lead is the control electrode G, the pin of the red test lead is the cathode K, and the other empty pin is the anode A. At this time, connect the black test lead to the determined anode A, and the red test lead is still connected to the cathode K. At this time, the multimeter pointer should not move. Use a short wire to short-circuit the anode A and the control electrode G. At this time, the multimeter resistance block pointer should deflect to the right, and the resistance reading is about 10 ohms. If the anode A is connected to the black test lead and the cathode K is connected to the red test lead, the multimeter pointer deflects, indicating that the unidirectional thyristor has been broken down and damaged.

　　3. Detection of bidirectional thyristor.

　　Use the multimeter resistance R*1Ω block, and use the red and black test pens to measure the forward and reverse resistance between any two pins. As a result, two groups of readings are infinite. If one group is tens of ohms, the two pins connected to the red and black meters in this group are the first anode A1 and the control electrode G, and the other empty pin is the second anode A2. After determining the A1 and G poles, carefully measure the forward and reverse resistance between the A1 and G poles. The pin connected to the black test pen in the measurement with a relatively small reading is the first anode A1, and the pin connected to the red test pen is the control electrode G. Connect the black test pen to the determined second anode A2, and the red test pen to the first anode A1. At this time, the multimeter pointer should not deflect, and the resistance value is infinite. Then use a short-circuit wire to short-circuit the A2 and G poles instantly, and apply a forward trigger voltage to the G pole. The resistance between A2 and A1 is about 10 ohms. Then disconnect the short-circuit wire between A2 and G, and the multimeter reading should remain around 10 ohms. Swap the red and black test leads, connect the red test lead to the second anode A2, and the black test lead to the first anode A1. Similarly, the multimeter pointer should not deflect, and the resistance should be infinite. Use a short-circuit wire to short-circuit the A2 and G poles again, and apply a negative trigger voltage to the G pole. The resistance between A1 and A2 is also about 10 ohms. Then disconnect the short-circuit wire between A2 and G poles, and the multimeter reading should remain unchanged, remaining at about 10 ohms. In line with the above rules, it means that the bidirectional thyristor being tested is not damaged and the polarity of the three pins is correctly judged. When testing a higher-power thyristor, it is necessary to connect a 1.5V dry battery in series in the black pen of the multimeter to increase the trigger voltage.

　　The following method can be used to distinguish the pins of thyristors: First, use the multimeter R*1K to measure the resistance between the three pins. The two pins with small resistance are the control electrode and the cathode, and the remaining pin is the anode. Then set the multimeter to R*10K, pinch the anode and the other pin with your fingers, and do not let the two pins touch. Connect the black test lead to the anode and the red test lead to the remaining pin. If the needle swings to the right, it means that the red test lead is connected to the cathode. If it does not swing, it is the control electrode.

Bidirectional thyristor is also called bidirectional thyristor
　　. Ordinary thyristor (VS) is essentially a DC control device. To control AC load, two thyristors must be connected in parallel with opposite polarity so that each SCR controls one half-wave. For this purpose, two independent trigger circuits are required, which is not convenient to use.

　　Bidirectional thyristor is developed on the basis of ordinary thyristor. It can not only replace two thyristors connected in parallel with opposite polarity, but also only needs one trigger circuit. It is currently an ideal AC switch device. Its English name TRIAC means three-terminal bidirectional AC switch.

Naming of bidirectional thyristors
　　Why are bidirectional thyristors called "TRIAC"?
　　Three-terminal: TRIode (take the first three letters)
　　AC semiconductor switch: AC semiconductor switch
　　(take the first two letters)
　　The above two groups of nouns are combined into "TRIAC"
　　which means "three-terminal bidirectional thyristor switch" in Chinese.
　　It can be seen that "TRIAC" is a general term for bidirectional thyristors.
　　-------------------------------------
　　Bidirectional: Bi-directional (take the first letter)
　　Controlled: Controlled (take the first letter)
　　Rectifier: Rectifier (take the first letter)
　　The first letters of these three groups of English nouns are combined to form: "BCR" which
　　means bidirectional thyristor in Chinese.
　　Typical manufacturers that name bidirectional thyristors with "BCR" include Mitsubishi of Japan,
　　such as: BCR1AM-12, BCR8KM, BCR08AM, etc.
　　--------------------------------------
　　Bi-directional: Bi-directional (take the first letter)
　　Triode: Triode (take the first letter)
　　The above two groups of words are combined into "BT", which is also the model name of bidirectional thyristor products. Typical manufacturers such as
　　ST and Philips of the Netherlands all use this to name bidirectional thyristors.
　　Representative models such as PHILIPS's BT131-600D, BT134-600E, BT136-600E, BT138-600E, BT139-600E, etc. These are four-quadrant/non-insulated/bidirectional thyristors;
　　Philips' product models are prefixed with "BTA", which usually refers to three-quadrant bidirectional thyristors.
　　ST, on the other hand, uses the prefix "BT" to name the component models and adds "A" or "B" after "BT" to indicate the combination of insulation and non-insulation: "BTA" and "BTB" series of bidirectional thyristor models, such as:
　　four-quadrant/insulated/bidirectional thyristor: BTA06-600C, BTA12-600B, BTA16-600B, BTA41-600B, etc.;
　　four-quadrant/non-insulated/bidirectional thyristor: BTB06-600C, BTB12-600B, BTB16-600B, BTB41-600B, etc.;
　　all ST product models with the suffix letter (the last letter of the model) containing "W" are "three-quadrant bidirectional thyristors".
　　Such as "BW", "CW", "SW", "TW";
　　representative models such as: BTB12-600BW, BTA26-700CW, BTA08-600SW, etc.
　　As for the trigger current of the model suffix letters, the representative meanings of each manufacturer are as follows:
　　PHILIPS: D=5mA, E=10mA, C=15mA, F=25mA, G=50mA, R=200uA or 5mA,
　　the trigger current of the model without suffix letters is usually 25-35mA;
　　PHILIPS's trigger current representative letters have no unified definition, and vary with the different product packages.
　　ST: TW=5mA, SW=10mA, CW=35mA, BW=50mA, C=25mA, B=50mA, H=15mA, T=15mA.
　　Note: The above trigger currents all have an upper and lower starting error range, which is described in detail in the product PDF file.
　　Generally divided into minimum value/typical value/maximum value, rather than "=" a parameter value.