This text is machine translated.
Triacs: Electronic component as a helper for switching operations
Anyone who has already regulated the brightness of a light bulb with a dimmer has unconsciously also already controlled a triac. Was a triac and how it works, we want to explain it in more detail with our guide.
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What was a triac?
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Where are triacs used?
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How is a triac structured?
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How does a triac work?
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What are the differences between triacs?
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What was to be taken into account when replacing a triac?
What was a triac?
A triac is one of the active components. Like a thyristor, it is an electronic component that can be used to switch on, switch off or control a bulb, for example.
Since the switching process takes place without mechanically moving contacts, these electronic switches are not subject to wear.
In contrast to a thyristor, which allows the current to flow only in one direction, a triac can switch the current in both directions. As such, Triacs are perfectly suitable for operation with alternating voltage.
The name Triac is derived from the English term "Triode Alternating Current Switch". The term bidirectional thyristor triode or symistor is also used in German language.
Where are triacs used?
Triacs are mainly used for switching operations or for power control in the alternating voltage range.
Areas of application include incandescent lamp dimmers, lighting consoles or the speed control of electric motors in kitchen appliances or power tools.
Since triacs are not able to cope with very large currents, individual thyristors are still used in the power electronics.
How is a triac structured?
To better understand the structure, one can imagine a triac like two antiparallel thyristors. One anode (A) and one cathode (K) of the two thyristors are combined.
The resulting main electrodes are labeled with H1 and H2 or according to the English term Main Terminal (MT1 and MT2). Alternatively, the terms anode 1 and anode 2 are also used.
As a rule, the main electrode H2 (MT2) is connected to the housing of the triac. This means that an insulated mounting must be carried out so that the mounting surface is not live.
The control inputs of the two thyristors (gates) are also connected.
In order to be able to clearly display the internal crystal structure, a cathode-controlled thyristor or also a p-gate thyristor (1) and an anode-controlled thyristor or also a n-gate thyristor (2) are connected in parallel according to sketch A.
The diagram B shows the crystal structure of the two semiconductors.
If you combine both semiconductor crystals into one block, this would look like sketch C.
In order to be able to control both thyristors with a common gate, additional N-doped zones have been inserted into the semiconductor crystal (see sketch D). These areas, which act as ignition or auxiliary thyristor lines (3), are responsible for ensuring that triacs require a higher control current for switching (ignition) than thyristors.
The main electrode H1 (MT1) has direct semiconductor contact to the gate and thus serves as a reference potential for the gate.
The gate can be controlled with a positive or negative pulse.
Depending on the triac type, a pulse of a few volts is sufficient for the activation, whereby a gate current of a few mA flows.
However, the sensitivity of the ignition depends on the polarity at H1 and H2 (see sketch of ignition type I + /I - and III + /III - ) and on the polarity of the gate pulse (see sketch of ignition type I + /III + and I - /III - ).
Triacs have the greatest sensitivity to ignition in control types I + and III - . The other two types of ignition require a much higher gate current in some cases.
How does a triac work?
The principle or function of a triac can be illustrated very easily with the help of a dimmer.
If the complete sine wave of the mains voltage (U B ) is present at the illuminant (1), it lights up at full brightness.
To reduce the maximum brightness, a portion of the sine wave must be cut off.
And this is exactly the function of a triac (2). For this purpose, it is connected in series with the consumer (illuminant).
Without control, the triac is highly resistive. This means that the electronic switch is open and the voltage on the lamp is 0 volts. The lamp does not light up.
If a short activation pulse is present at the gate at time t 1, the triac switches through. It changes from high-impedance to low-impedance. The procedure is also referred to as ignition in the specialist language. The electronic switch is closed and the voltage at the lamp (U L ) suddenly jumps to the current value of the supply voltage. This supplies the lamp with power and starts to light up.
The ignited triac remains conductive, even if the control pulse at the gate is switched off again. Only when the alternating voltage crosses the 0 line at time t 2 and thus falls below the holding current of the triac, will it be blocked again. Experts say the triac is deleted. The lamp is no longer supplied with power.
The triac remains locked until time t 3 the next control pulse is applied at the gate and it is lit again. Since the triac is conductive in both directions, current will also flow through the lamp at the negative half-wave.
At time t 4, the holding current is undershot and the triac locks again until it is re-ignited at time t 5.
Since the ignition time t 1 is very early, the lamp receives a very large proportion of the mains AC voltage. Only a small part is cut off at the beginning of each half-shaft. This makes the lamp very bright.
If the ignition timing is later performed or moved further to the "right" position, the remaining part of the sine wave (U L ) is reduced on the lamp.
The lamp thus receives less energy and thus lights up darker.
At a mains frequency of 50 Hz, the lamp is switched on and off 100 times per second (50 times for the positive half-wave and 50 times for the negative half-wave).
The human eye can no longer perceive this fast switching sequence as individual switching processes. In addition, incandescent lamps also have a short "postheating effect" when switched off.
This results in a uniform change in brightness when dimming.
With an adjustable dimmer, only the time at which the firing pulse switches the triac is changed. To do this, the gate pulse must always be output at the correct position of the sine wave.
For this purpose, the control always requires a current reference to the current position of the sine wave.
In the diagram above, this connection was shown with a blue line.
Note:
In the example above, since the phase is locked at the beginning of the half-wave and then switched on, this control is a phase-angle dimmer. This type of dimmer is perfectly suitable for resistive loads such as light bulbs or high-voltage halogen lamps, but also for inductive loads such as conventional halogen transformers.
Capacitive loads such as electronic halogen transformers, on the other hand, require a phase angle dimmer. With this circuit concept, the voltage at the consumer increases synchronously with the mains voltage and is then switched off after a defined time. However, no triacs are used for this purpose, but switch-off thyristors or power MOSFETs or IGBTs are used as electronic switches.
What are the differences between triacs?
Even if the basic function of Triacs is always the same, the individual copies can differ considerably.
Design
One of the main distinguishing features is the design. Depending on how much current or power a triac has to cope with, the housing shape with integrated cooling surface and the design of the connections were optimized during the production. The higher the performance throughput, the greater the Triac will fail.
Technical specifications
But even with the same design, the differences can still be quite large. However, the differences then relate primarily to the technical data. In particular, the information on the max. voltage or the max. permissible current may differ considerably. In case of doubt, a look into the data sheet is essential.
What was to be taken into account when replacing a triac?
If a defective triac is to be replaced in a device, a replacement type with the same name must always be used. This is the only way to ensure that the replaced triac perfectly matches the existing electronics or circuit. Caution should be exercised when replacing the original part with a copy with approximately the same specifications. Because different semiconductor characteristics can cause not insignificant malfunctions.
Important:
As triacs are used in the mains circuit, you must be familiar with the applicable safety regulations when troubleshooting or replacing the circuit. There is an acute danger to life in the event of improper work or when troubleshooting under voltage. For this reason, you should contact a specialist if you are not familiar with the necessary procedures.