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TRIAC
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A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. The formal name for a TRIAC is bidirectional triode thyristor. This results in a bidirectional electronic switch which can conduct current in either direction when it is triggered (turned on) and thus doesn't have any polarity.
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A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two silicon-controlled rectifiers (SCRs/thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. The formal name for a TRIAC is bidirectional triode thyristor. This results in a bidirectional electronic switch which can conduct current in either direction when it is triggered (turned on) and thus doesn't have any polarity. It can be triggered by either a positive or a negative voltage being applied to its gate electrode (with respect to A1, otherwise known as MT1). Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, the holding current, such as at the end of a half-cycle of alternating current (AC) mains power. This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents. In addition, applying a trigger pulse at a controllable point in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (phase control).
The thyr- part of the name comes from the Greek qura, door.
Application
Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances.
However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the AC power.
A snubber circuit (usually of the RC type) is often used between A1 and A2 to assist this turn-off. Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply. Also, a gate resistor or capacitor (or both in parallel) may be connected between gate and A1 to further prevent false triggering. That, however, increases the required trigger current and / or adds latency (capacitor charging).
For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load. However, due to the separate gates, proper triggering of the SCRs is more complex than triggering a TRIAC.
In addition to commutation, a TRIAC may also not turn on reliably with non-resistive loads, because due to the phase shift the holding current may not be achievable at trigger time. To overcome that, pulse trains may be used to repeatedly try to trigger the TRIAC until it finally turns on. The advantage is that the gate current does not need to be maintained throughout the entire conduction angle, which can be beneficial when there is only limited drive capability available.
Example data
| Variable name | Parameter | Typical value | Unit |
|---|
| | Gate threshold Voltage | 1,5 | V | | | Gate threshold Current | 10 - 35 | mA | | | Gate controlled turn-on time | 2 | µs | | | Repetitive peak off-state Voltages | 600-800 | V | | | RMS on-state current Non-repetitive peak | 4-16 | A |
Alternistor
Alternistor is a trade name for a proprietary class of TRIAC with an improved turn-off (commutation) characteristic made by Teccor and other companies.
These devices are made specifically for improved commutation when controlling a highly-inductive load, such as a motor, an application which causes problems for "normal" triacs due to high voltage/current angles. Most triacs' commutation with inductive loads can be improved by use of a "snubber network", but alternistors are made specifically for this purpose and they dispose of the snubber requirement altogether.
This improvement is achieved at the expense of the ability to trigger the device in the 4th quadrant (negative voltage and positive gate current). However, this is usually no problem, because this trigger mode is seldom used since even normal TRIACs are least sensitive there.
The majority of other semiconductor manufacturers, such as ST Microelectronics and Philips have their own versions of improved commutation triac, but they are not marketing them under the proprietary "alternistor" moniker (ST uses the trademark "SNUBBERLESS").
See also
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