Simple ways to test triacs and thyristors

In practice, there are different semiconductor switches. They are used to switch the load supply or to smoothly adjust the voltage and current. One of such devices is a triac. It is used in dimmers of lighting, in household appliances and industrial converters. In this article we will tell you how to check the triac for serviceability with a multimeter or on a homemade stand.

Purpose and device

Triacs are semiconductor semi-controlled keys that are opened by a current pulse through a control electrode. To close it, you need to interrupt the current in the circuit or apply a reverse voltage.

By the principle of action, they are similar to those of thyristors. They differ only in that the triac consists of two thyristors connected counter-parallel. The designation in the diagram you see below.

UGO in the drawing

By definition, they are often used in relay mode - in simple words they work on “on” and “off”, by the way such relays are called semiconductor.

The differences from the electromechanical are as follows - the speed is orders of magnitude higher, there are no contacts, and therefore a great durability. The main condition for long-term operation is to provide the rated thermal regime and load.

Verification Methods

To diagnose malfunctions of an electronic circuit, it is necessary to consistently check its elements. First of all, they pay attention to power circuits, namely to all semiconductor switches. To check them, you can use one of the methods:

  • multimeter (ohmmeter or continuity);
  • battery with LED or bulb;
  • at the stand.

For diagnostics, you should drop out the element, because when checking any components of electronic circuits for serviceability, without evaporating from the board, there is the possibility of incorrect measurements. For example, you will find short circuit not the element being tested, but connected in parallel with it in a circuit.

In any case, you can check the triac and thyristor for serviceability without soldering, and if you find a possible malfunction - solder and take measurements again.

Using a multimeter

To check the triac for breakdown with the help of a tester, you need to put the device into sound ringing mode.

In most cases, the continuity is combined with the verification of diodes.

Diode Test Mode

A typical arrangement of conclusions or, as it is also called, a pin, is shown in the figure below. A1 and A2 (sometimes T1, T2) are power leads, a larger current flows through them to the load, and G (gate) is the control electrode. The pinout may vary, so check it out on your triac

Pinout Options

In the diode test mode, the voltage drop between the probes in millivolts is displayed on the screen. At the same time, there is voltage on the tester probes, which ensures the flow of current in the measured circuit (as in Ohmmeter mode).

To check the element for breakdown, touch the probes A1 and A2 with the probes; if the element is OK, “1” or 0L will appear on the screen, and if it is broken, the value will be close to 0. If there is no short circuit between terminals A1 and A2, check the control electrode. To do this, touch the probes to one of the power terminals and the control electrode; the screen should have a low value of 80-200.

Validation algorithm

To check whether the triac opens or not, you can short-circuit its control electrode with one of the terminals of the multimeter, so you apply a control voltage (current) to it. The verification algorithm for the thyristor example you see below.

Another way to check

After you remove the voltage from the control electrode - the triac can close back. This is due to the fact that some minimum current must flow through it to hold the conductive state. The same phenomenon can be observed in the following verification methods.

The same can be done with an ohmmeter: if the element is broken, the resistance will be low, and if it is not broken, it will tend to infinity.

This verification method is discussed in detail in the next video, but keep in mind that the author made a mistake in the wording, calling the voltage drop resistance. Otherwise, it is very clear.

Using a battery with a light bulb or LED

If you do not have a multimeter, you can easily check the triac with a simple circuit, for this you need a light bulb or an LED and a battery, the circuit you see below.

LED circuit

If instead of using an LED a small-sized incandescent lamp from a flashlight is used, then the resistor R1 needs to be removed from the circuit, if you use a battery with a low voltage, remove the resistor R2 or reduce its resistance. You can use 3 consecutive finger batteries (3x1.5 = 4.5V) or even a crown (9V). If you assemble a portable tester according to this scheme, you can install a button without fixing with normally open contacts, as shown in the diagram.

If you will not assemble such a device, then just briefly touch the control electrode with a wire, as was shown in the method with a multimeter.

Other verification methods

Perhaps the most convenient way to test electronic components is a universal tester of radio components, it is often called a transistor tester. He "knows" to measure capacitance, resistance, inductance, determine the pin and type of unfamiliar components, while working from the crown.

Universal tester

Such a device costs about 4-10 dollars for aliexpress depending on the delivery set (with or without a case) and model (even the cheapest one is a quite functional tool of a home master).

To check the health of an element, you just need to insert it into the terminal block and press a single button. If the component is detected correctly, then it is healthy. If you see that the image of a deliberately different part (a resistor instead of a thyristor, for example) appeared on the display, then it means it’s burned out

Check triac tester

The network has a lot of circuits of small stands or devices for testing triacs. Their principle of operation is no different from the methods described above. Let's consider some of them.

To check the triacs on the control unit in the washing machine, experts advise using a circuit with a light bulb, without evaporating the part from the board.

By the way, repairmen are faced with the replacement of keys in a washing machine automatically. In this case, they are responsible for engine control and speed control, as in a vacuum cleaner, and in an electric kettle - in the control circuit of the heating element.

Triac verification circuit

Another test bench scheme was published in one of the issues of Radio magazine and a similar one from a foreign forum. When checking at the stand according to this scheme, you can check whether the triac opens in both directions, for this there are switches SA1, SA2 on the first circuit and S1 on the second.

Schemes of stands

We recommend also to see:

We examined the main methods for diagnosing circuits with thyristors and triacs.They are suitable for all cases, no matter where it was installed in a vacuum cleaner, dimmer, washing machine or other device. Please note that during verification, the key may spontaneously close after removing the control pulse - this is due to the peculiarity of their internal device and nominal operating parameters.

Related materials:

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One comment

  • Igor

    It is written very competently and clearly, i.e. in an amateur radio, because many, without even understanding what they are writing about, simply print the online translation curve from foreign sites and from there call themselves experts.

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