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9 April 2009
Author: Giorgos Lazaridis
The Triac

What is a TRIAC

The name TRIAC comes from the words TRIode for Alternating Current. The TRIAC was discovered the time that SCR was discovered therefore those two elements have the same history. It is actually composed of two different SCRs connected in parallel and in opposite direction. The formal name of a TRIAC is bidirectional triode thyristor. The name itselfe indicates that a TRIAC can allow current to flow within it's leads in any direction. This makes a TRIAC more suitable than an SCR when AC voltage needs to be controlled.

As mentioned before, a TRIAC is actually two SCRs connected in parallel and opposite. The electronic symbol of a TRIAC is as follows:

And the equivalent circuit of a TRIAC using two SCRs is:

The PNP layers of a TRIAC are as follows:

The theory of operation for a TRIAC is based on the theory of operation of an SCR triode. The differences between those two elements are:

• The TRIAC has no forward or reverse polarity
• The TRIAC has no Anode or Cathode pin that needs to be connected reverse of forward in order to allow current to flow within the device. Once a pulse is applied to the gate of the TRIAC, it shall become conductive and shall allow current to flow from the bias no matter what polarity.

• The TRIAC can become conductive with both positive or negative pulses
• Someone would expect the TRIAC to become conductive with positive pulses. This is far from true. A TRIAC becomes conductive with bot positive or negative pulses. As mentioned in the previous point, it has no polarity and therefore the polarity off the pulse on the gate is irrelevant. It only needs to be greater than the gate threshold voltage to turn on the TRIAC.

• The TRIAC turns into non-conductive state only when the voltage difference between the bias is zero volts (0V)
• As it is already known from the theory of operation for the SCRs, an SCR goes into a non-conductive state when the the voltage difference between the anode and the cathode of the SCR is equal or smaller than zero volts. TRIACs goes off (non-conductive) only when the voltage between the two leads is zero volts

You can find a complete theory of operation for the SCRs in this page.

Applications

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. They can be used both into AC and DC circuits but the original design was to replace the use of two SCRs in AC circuits.

TRIACS may not turn on reliably with non-resistive loads, because due to the phase shift the holding current may not be achievable at trigger time. Therefore, a pulse train is sent to the gate until the load is turned on. Because of the fact that the gate current does not need to be maintained throughout the entire conduction angle, current can be limited significantly and this is a benefit when drive capabilities are narrow.

Relative pages
• The SCR (Silicon Control Rectifiers) theory
• Learn how dimmers work
• Basic transistor circuits
• 555 timer basic circuits
• The LED theory
• Peltier coolers (TEC) theory of operation
• The voltage divider theory
• Learn about the most popular PC Cooling methods
• International unit converter

 At 18 June 2012, 9:14:29 user vipin Duhoon wrote:   [reply @ vipin Duhoon]What is the selection criteria of a TRIAC????At 6 September 2011, 12:11:29 user satish wrote:   [reply @ satish]thnxx its so goodAt 3 May 2011, 14:25:12 user Kammenos wrote:   [reply @ Kammenos]@herctrap the current for the 720 watts lamps goes through the resistors? If yes, then it is normal to fry the resistors. There will be a max of 2 amperes flowing through them, which will require to dissipate some 500 watts of heat.At 2 May 2011, 7:13:59 user herctrap wrote:   [reply @ herctrap]in this video http://www.youtube.com/watch?v=hBvZnIQgESI there is a bt137 with 140ohm 7/4W resistor on pin3 (7 x 1Kohm 1/4W in paraller) now the other side of the reisistors goes to output of the thermostat relay (phase) the pin2 goes to directly to phase and pin1 goes to load ( 12*60W lamps )(the other side of the lamps is conected to the netural why the resistors ( not the traic ) lights up when the thermostat closes the relay? thanks a lotAt 22 April 2011, 5:35:02 user Kammenos wrote:   [reply @ Kammenos]TRIAC and SCR will act the same in DC, only that TRIAC can be connected either ways. When the gate is activated, then DC current will flow from within the device, and the only way from then on to stop the current, is to cur the DC voltage. That is why these parts are rarely used in DC current control.At 21 April 2011, 7:12:05 user Fung wrote:   [reply @ Fung]Without concerning the gate, the circuit is similar to a bi-polar LED, with one each other reversed. Is TRIAC suitable to use in direct current and have similar function as an SCR?

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