apogeeweb title
Home arrow Thyristor

arrow left

arrow right

Dec 31 2019

Selection, Replacement and Detection of Thyristor

Introduction

Thyristor is a four semiconductor layers or three PN junctions devicea solid-state semiconductor device with four layers of alternating P- and N-type materials. It is also known as “SCR” (Silicon Control Rectifier). The term “Thyristor” is dervid from the words of thyratron (a gas fluid tube which work as SCR) and Transistor. And It acts exclusively as a bistable switch in electronic circuit.

What is a Thyristor?

Catalog

Introduction

Ⅰ Types of Thyristors

Ⅱ Thyristor Selection

2.1 Specific Requirements of Applying Circuit

2.2 Main Parameters of the Thyristor

Ⅲ Replacement of Thyristor

Ⅳ Detection of Thyristor

4.1 Detection of Unidirectional Thyristors

4.2 Detection of TRIAC


Ⅰ Types of Thyristors

Commonly used thyristors include unidirectional thyristors, TRIAC, and turn-off thyristors, etc., which should be selected reasonably according to the needs of the circuit.

Unidirectional Thyristor

The unidirectional thyristor is characterized in that the current can only flow from the anode A to the cathode k, and is mainly used in the control of DC power supply or pulsating direct current, AC power rectification, and DC power inverter.

Unidirectional thyristors can be divided into ordinary thyristors and high-frequency thyristors (the working frequency is above 110kHz). Commonly used unidirectional thyristors are 3CT series, 3DT series, KP series and KK series (high frequency thyristors), and imported MCR series, SF series, BST series etc.

TRIAC

TRIAC was developed on the basis of the unidirectional thyristor and is an AC power control device. TRIAC can not only replace two unidirectional thyristors in anti-parallel, but also requires only one trigger circuit, which is more convenient to use.

The characteristic of TRIAC is that alternating current can pass through it, which is mainly used in the control of AC power supply and the adjustment of AC voltage. Commonly used TRIAC include 3CTS series and KS series, as well as imported MAC series, SM series, BCR series, etc.

— Gate Turn-off Thyristor

The characteristic of the gate turn-off thyristor is that it can be switched off by the control electrode. It is mainly used in gate turn-off contactless switches, DC inverters, dimmer, speed regulation and other occasions.

Gate turn-off thyristors are power-type control devices developed on the basis of ordinary thyristors. After the ordinary thyristor is triggered to be turned on, its control electrode does not work. To turn off the thyristor, the power must be cut off, or the forward current flowing through the thyristor must be less than the holding current. Gate turn-off thyristor overcomes the above drawbacks. When the control electrode G is added with a positive pulse voltage, the thyristor is turned on, and when the control electrode G is added with a negative pulse voltage, the thyristor is turned off.

Gate turn-off thyristors are ideal high-voltage, high-current switching devices. For example, the DG series high-power gate turn-off thyristors can reach a maximum voltage of 4500V and a maximum current of 3000A.

 

Thyristor Selection

2.1 Specific Requirements of Applying Circuit

There are many types of thyristors, which should be selected reasonably according to the specific requirements of the application circuit.

For AC/DC voltage control, controllable rectification, AC voltage regulation, power inverter, switching power supply protection circuit, etc., ordinary thyristors can be selected.

For AC switch, AC voltage regulation, AC motor linear speed regulation, lamp linear dimming, solid state relay, solid state contactor, etc., a TRIAC should be selected.

For AC motor variable frequency speed regulation, chopper, power inverter and various electronic switch circuits, you can choose gate turn-off thyristor.

For sawtooth wave generator, long time delay, over voltage protector and trigger circuit with power transistor, etc., BTG thyristor can be selected.

In electromagnetic cookers, electronic ballasts, ultrasonic circuits, superconducting magnetic energy storage systems, switching power supplies and other circuits, reverse conducting thyristors can be selected.

In the photocoupler, light detector, light alarm, light counter, photoelectric logic circuit and operation monitoring circuit of automatic production line, the light-control thyristor can be selected.

2.2 Main Parameters of the Thyristor

The main parameters of the thyristor should be determined according to the specific requirements of the application circuit.

The selected thyristor should have a certain power margin, and its rated peak voltage and rated current (on-state average current) should be higher than the maximum operating voltage and maximum working current of the controlled circuit by 1.5 to 2 times.

The parameters of the thyristor's forward voltage drop, gate trigger current, and trigger voltage should meet the requirements of the application circuit (this refers to the control circuit of the gate), and should not be high or low, otherwise it will affect the normal operation of the thyristor.

 SCR Discrete Thyristor

Ⅲ Replacement of Thyristor

After the thyristor is damaged, if no thyristor of the same type is replaced, another type of thyristor with similar performance parameters can be used instead.

When designing an application circuit, a large margin is generally left. When replacing the thyristor, just pay attention to its rated peak voltage (repeated peak voltage), rated current (on-state average current), gate trigger voltage and gate trigger current, especially the two indicators of rated peak voltage and rated current.

The switching speed of the thyristor used for replacement should be consistent with the switching speed of the damaged thyristor. For example: After the high-speed thyristor used in the pulse circuit and high-speed inverter circuit is damaged, only the same type of fast thyristor can be used instead of the ordinary thyristor.

When selecting a thyristor to be used for replacement, it is not necessary to leave too much margin for any parameter, and the parameter of it should be as close as possible to the parameter of the replaced thyristor, because an excessively large margin is not only a waste, but also sometimes has side effects, such as non-triggering or insensitive triggering.

In addition, the appearance of the two thyristors should be the same, otherwise it will cause inconvenience to the installation.

 

Ⅳ Detection of Thyristor

Thyristors are usually represented by the letters "SCR" in circuit schematic diagrams. For example, SCR2 refers to the thyristor numbered 2. The symbol of the thyristor in the schematic diagram is shown in figure 1.

 Figure 1. Symbol of Thyristor

Figure 1. Symbols of Thyristor

4.1 Detection of Unidirectional Thyristors

(1) Discrimination of each electrode: According to the structure of an ordinary thyristor, it can be seen that there is a PN junction between the gate G and the cathode K, which has unidirectional conductive characteristics, while there are two PN junctions of opposite polarities connected in series between the anode A and the gate. Therefore, by measuring the resistance between the pins of an ordinary thyristor with the R × 100 or R × 1 k Q level of the multimeter, three electrodes can be determined.

The specific method is: use the black probe of the multimeter to connect one electrode of the thyristor, and use the red probe to touch the other two electrodes in turn. If the measurement result has a resistance value of several thousand ohms (kΩ) and another resistance value of several hundred ohms(Ω), it can be determined that the black probe is connected to gate G. In the measurement with a resistance value of several hundred ohms, the red probe was connected to the cathode K, and in the measurement with a resistance value of several thousand ohms, the red probe was connected to the anode A. If the measured resistance values are both very large, it means that the black probe is not connected to gate G. Apply the same method to test other electrodes until three electrodes are found.

You can also measure the forward and reverse resistance between any two pins. If the forward and reverse resistance are close to infinity, the two electrodes are anode A and cathode K, and the other pin is gate G.

Each electrode of the ordinary thyristors can also be judged according to its packaging form.

For example, the bolt end of the bolt-type ordinary thyristor is anode A, the thinner lead end is gate G, and the thicker lead end is cathode K.

The lead end of the flat thyristor is gate G, the flat end is anode A, and the other end is cathode K.

A thyristor of metal package (T0-3) is a common thyristor and its shell is anode A.

The middle pin of the plastic thyristor (T0-220) is anode A, and it is mostly connected with its own heat sink.

 Pin Arrangement of Several Common Thyristors

Figure 2. Pin Arrangement of Several Common Thyristors

(2) Judging whether it is good or bad: Use the R×1 kΩ level of a multimeter to measure the forward and reverse resistance values between anode A and cathode K of ordinary thyristor, which should normally be infinite (∞) ; If the forward and reverse resistance values are zero or the resistance values are both small, it indicates that a breakdown short circuit or leakage occurs inside the thyristor.

Measure the forward and reverse resistance values between gate G and cathode K. Normally, there should be forward and reverse resistance values similar to diodes (the actual measurement results are smaller than those of ordinary diodes), that is, the forward resistance value is small (less than 2 kΩ) and the reverse resistance value is large (greater than 80 kΩ). If the resistance values of the two measurements are both large or small, it means that the thyristor is open or short-circuited between electrode G and K. If the forward and reverse resistance values are equal or close, it indicates that the thyristor has failed, and the PN junction between its electrodes G and K has lost its unidirectional conduction effect.

Measure the forward and reverse resistance value between anode A and gate G. In normal conditions, both resistances should be several hundred kiloohms (kΩ) or infinite. If the forward and reverse resistance values are not the same (there is unidirectional conduction like a diode). One of the two PN junctions connected in reverse series between gate G and electrode A has been short-circuited.

(3) Detection of triggering capability: For ordinary thyristors with low power (working current is below 5A), it can be measured with R×1 level of the multimeter . During the measurement, the black probe is connected to anode A and the red probe is connected to cathode K. At this time, the watch hand does not move, and the resistance value is displayed as infinite (∞). Use tweezers or wires to make anode A and gate G of the thyristor be short-circuited(see figure 3), which is equivalent to applying a forward trigger voltage to gate G. At this time, if the resistance value is several ohms to tens of ohms (the specific resistance value will vary according to the part number of the thyristor), it indicates that the thyristor is conducting due to the forward trigger. Then disconnect electrode A and gate G(the probes on electrode A and K do not move, only the trigger voltage of gate G is cut off). If the value indicated by the watch hand is still in the position of several ohms to tens of ohms, it indicates that the triggering performance of the thyristor is good.

 Detection of Triggering Capability

Figure 3. Detection of Triggering Capability

For medium and high power ordinary thyristors with a working current above 5 A, the on-state voltage drop VT, holding current IH and the gate trigger voltage Vo are relatively large. The current provided by the R × 1 kΩ level of the multimeter is low, and the thyristor cannot be completely turned on, so a 200Ω adjustable resistor and one to three 1.5 V dry batteries can be connected in series at the end of the black probe (depending on the capacity of the thyristor being tested, if its working current is greater than 100 A, three 1.5 V dry batteries are applied), as shown in figure 4.

 Detection of Trigger Voltage

Figure 4. Detection of Trigger Voltage

You can also use the test circuit in figure 5 to test the triggering capability of an ordinary thyristor. In the circuit, vT is the thyristor under test, HL is a 6.3 V indicator (small electric beads in a flashlight), GB is a 6 V power supply (four 1.5 V dry batteries or 6 V regulated power supply can be used), and S is the button, R is the current limiting resistor.

 Test Circuit to Test the Triggering Capability

Figure 5. Test Circuit to Test the Triggering Capability

When the button S is not connected, the thyristor VT is in a blocking state, and the indicator light HL is not on (if HL is on at this time, there may be breakdown of vT or leakage damage). After pressing the button S once (turn S on for a moment to provide the trigger voltage for gate G of the thyristor VT), if the indicator HL is always on, it means that the thyristor has a good triggering capability. If the brightness of the indicator is low, it indicates that the thyristor has poor performance and a large conduction voltage drop (the conduction voltage drop should be about 1 V under normal conditions). If button S is on, the indicator light is on, and when button S is off, the indicator light is off, indicating that the thyristor is damaged and the triggering performance is poor.

4.2 Detection of TRIAC

(1) Discrimination of each electrode: Use the R×1 or R×10 level of the multimeter  to measure the forward and reverse resistance values between three pins of the TRIAC. If it is measured that one pin is not connected with the other two pins, then this pin is the main electrode T2.

After finding the electrode T2, the remaining two pins are the main electrode T1 and the gate G3. Measuring the forward and reverse resistance values between these two pins will gain two smaller resistance values. In a measurement with a small resistance value (about tens of ohms), the black probe is connected to the main electrode T1, and the red probe is connected to gate G.

One end of the bolt of the bolt-shaped TRIAC is the main electrode T2, the thinner lead end is gate G, and the thicker lead end is the main electrode T1.    

The shell of the metal-encapsulated (TO-3) TRIAC is the main electrode T2.

The middle pin of the plastic-encapsulated (TO-220) TRIAC is the main electrode T2, which is usually connected to its own small heat sink.

 Pin Arrangement of Several TRIAC

Figure 6. Pin Arrangement of Several TRIAC

(2) Judging whether it is good or bad: Use the R×1 or R×10 level of a multimeter to measure the forward and reverse resistance values between the main electrode T1 and the main electrode T2 and between the main electrode T2 and gate G of the TRIAC. Normally it should be close to infinity. If the measured resistance values are all very small, it means that the electrodes of the TRIAC have been broken down or are short-circuited.

Measure the forward and reverse resistance of the main electrode T1 and gate G. Normally, it should be between tens of ohms (Ω) and one hundred ohms (Ω) (when the black probe is connected to electrode T1 and the red probe is connected to gate G, the measured forward resistance value is slightly smaller than the reverse resistance value). If the forward and reverse resistance values between electrode T1 and gate G are measured to be infinite, it indicates that the thyristor has been damaged by an open circuit.

(3) Detection of triggering capability: For small power TRIAC with working current below 8A, it can be measured directly with R×1 level of the multimeter. When measuring, first connect the black probe to the main electrode T2 and the red probe to the main electrode T1, then use tweezers to make electrode T2 and gate G be short-circuited, and add a positive polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T2 → T1.

Then connect the black probe to the main electrode T1, and the red probe to the main electrode T2. Use tweezers to make electrode T2 and gate G be short-circuited, and add a negative polarity trigger signal to gate G. If the resistance value measured at this time changes from infinity to more than ten ohms (Ω), it means that the thyristor has been triggered to conduct, and the conduction direction is T1 → T2.

If gate G is disconnected after the thyristor is triggered to be turned on, the low-resistance conduction state cannot be maintained between electrode T2 and T1 and the resistance value becomes infinite, it indicates that the TRIAC has poor performance or is damaged. If a positive (or negative) polarity trigger signal is added to gate G, the thyristor still does not conduct (the forward and reverse resistance values between T1 and T2 are still infinite), then the thyristor is damaged and has no trigger continuity.

For medium and high power TRIAC with a working current of 8A or more, when measuring their triggering capability, one to three 1.5V dry batteries can be connected in series to a probe of a multimeter, and then measure by using R×1 level as described above.

For a TRIAC with a withstand voltage of 400V or more, its trigger capability and performance can also be tested by using 220V AC voltage.

Figure 7 is a test circuit of a TRIAC. In the circuit, FL is a 60W /220V incandescent bulb, VT is the TRIAC under test, R is a 100Ω current limiting resistor, and S is a button.

TRIAC circuit 

Figure 7. TRIAC Circuit

After the power plug is connected to the working frequency AC, the TRIAC is in the off-state and the light bulb is off. (If the bulb is glowing normally at this time, it means that electrode T1 and T2 of the thyristor under test have been broken down and short-circuited; if the light bulb is slightly light, it means that the thyristor under test is damaged by leakage). Press the button S once to provide the trigger voltage signal for gate G of the thyristor. In normal conditions, the thyristor should be immediately triggered to turn on, and the light bulb will glow normally. If the bulb fails to emit light, the internal circuit of the tested thyristor is damaged. If the light bulb is turned on when the button S is pressed, and the light bulb is turned off when the button is released, it indicates that the triggering performance of the tested thyristor is poor.

When using a multimeter to detect low-power light-controlled thyristors, put the multimeter in R × 1 level, connect one to three 1.5V dry batteries in series to a black probe, and measure the forward and reverse resistance values between the two pins. Normally it should be infinite. Then use a small flashlight or laser pen to illuminate the light receiving window of the light controlled thyristor. At this time, a small forward resistance value can be measured, but the reverse resistance value is still infinite. In a measurement with a small resistance value, the black probe is connected to the anode A, and the red probe is connected to the cathode K.

The following method can also be used to measure light-controlled thyristors. Turn on the power switch S and illuminate the light receiving window of the thyristor VT with a flashlight. After adding a trigger light source (high-power light-controlled thyristor has its own light source, as long as the light-emitting diode or semiconductor laser in its optical cable is added with the working voltage, no external light source is required), the indicator EL should be on. After the light source is evacuated, the indicator light EL should remain illuminated. There is only one PN junction. Therefore, you just need to measure electrode A and G with a multimeter.

Put the multimeter in the R × 1 kΩ level, and the two probes can be connected to one of the two pins of the thyristor under test (measure their forward and reverse resistance values). If a pair of pins is measured with a low resistance value, the black probe is connected to the anode A, while the red probe is connected to gate G, and the other pin is the cathode K.

(2) Judging whether it is good or bad: Use the R×1 level of a multimeter to measure the forward and reverse resistance values between the electrodes of the BTG thyristor. Under normal conditions, the forward and reverse resistances between the anode A and the cathode K are infinite; the forward resistance between the anode A and gate G (when the black probe is connected to electrode A) is several hundred ohms to several thousand ohms and the reverse resistance value is infinite. If the forward and reverse resistance values between two electrodes are measured to be very small, it indicates that the thyristor has been short-circuited and damaged.

(3) Detection of triggering capability: Put the multimeter in the R × 1 Ω level, connect the black probe to anode A, and the red probe to cathode K. The measured resistance should be infinite. Then touch gate G with your finger and add a human body induction signal to it. If the resistance between electrodes A and K changes from infinity to low resistance (a few ohms) at this time, it indicates that the thyristor has a good triggering ability. Otherwise, the performance of the thyristor is poor.

Related Articles

pinglun 0 comment

Leave a Reply

Your email address will not be published.

 
 
   
code image
Rating: poor fair good very good excellent