TRIAC is very commonly used in places where AC power has to be controlled for example, it is used in the speed regulators of ceiling fans, AC bulb dimmer circuits etc. If this gate voltage is above the gate threshold voltage then a current flows through the gate pin, which will be greater than the gate threshold current.
But since this is an AC power source the voltage will reach zero for every half cycle and thus the current will also reach zero momentarily. Hence latching is not possible in this circuit and the TRIAC will turn off as soon as the switch is opened and no commutation circuit is required here.
High caution is needed while working with AC power supplies the operating voltage is stepped down for safety purpose The standard AC power of V 50Hz In India is stepped down to 12V 50Hz using a transformer. A small bulb is connected as a load. The experimental set-up looks like this below when completed. The bulb will glow as long as the button is held pressed. When TRIACs are used as light dimmers or for Phase control application, the gate pulse that is supplied to the gate pin has to be controlled using a microcontroller.
In that case the gate pin will also be isolated using an opto-coupler. The circuit diagram for the same is shown below. This type of circuit is normally used for Lamp brightness control or motor speed control. That is when the MT1 terminal is subjected to sharp increase in voltage due to switching noise or transients or surges the TRIAC miss-interrupts it as a switching signal and turns ON automatically. Figure 15 shows the outline and lists the characteristics of a typical six-pin DIL version of such a device, in which the LED has a maximum current rating of 50mA, the triac has maximum ratings of V and mA RMS and a surge current rating of 1.
Optocoupled triacs are easy to use and provide excellent electrical isolation between input and output. The input is used like a normal LED, and the output like a low-power triac. Figure 16 shows the device used to activate an AC line-powered filament lamp, which must have an RMS rating below mA and a peak inrush current rating below 1. Figure 17 shows an optocoupled triac used to activate a slave triac, thereby driving a load of any desired power rating.
This circuit is suitable for use only with non-inductive loads such as lamps and heating elements. It can be modified for use with inductive loads such as electric motors by using the connections in Figure Here, the R2-C1-R3 network provides a degree of phase-shift to the triac gate-drive network, to ensure correct triac triggering action, and R4-C2 form a snubber network to suppress rate effects.
A synchronous 'zero-voltage' or 'integral cycle' power switch is one in which the triac invariably turns on just after the start of each power half-cycle i. In most power switching circuits shown so far in this article, the triac turns on at an arbitrary point in its initial switch-on half-cycle, thus producing a potentially high initial burst of RFI, but then gives a synchronous zero-voltage switching action on all subsequent half-cycles.
A truly synchronous zero-voltage circuit uses the switching system in Figure 19 , in which the triac can only be gated on near the start or 'zero-voltage' point of each half-cycle, and thus produces minimal RFI.
In the zero-voltage detector, Q4 or Q5 are driven on whenever the AC line voltage is more than a few volts set by RV1 above or below zero, thereby driving Q3 on via R5 and inhibiting Q2.
Thus, gate current can only be fed to the triac when SW1 is closed and the instantaneous AC line voltage is within a few volts of zero; this circuit thus generates minimal switching RFI.
Figure 21 shows the circuit modified so that the triac can only turn on when SW1 is open. Note in both cases that only a narrow pulse of gate current is fed to the triac, and the mean gate current is thus only 1mA or so.
SW1 can be replaced by an electronic switch or optocoupler, if desired, thus enabling the load to be activated by light or temperature levels or by time, etc. In practice, the simplest way of making a really efficient synchronous 'zero-voltage' triac-driving circuit is with the aid of a special-purpose IC that functions as an optocoupled low-power synchronous 'zero-voltage' triac that can easily be used as a slave for synchronously driving a normal high-power triac.
The next, and final, episode will give practical details of such circuits, together with other triac-related circuits and information. This special two-part article explains its basic operation and shows various applications it can be used in. Need to brush up on your electronics principles? These multi-part series may be just what you need!
Everything for Electronics. Forum Blogs Feedback Techforum Newsletter. All articles in this series: Basic triac principles and practical circuits. Part 1 Optocoupled synchronous power switching, plus other triac-related circuits and information. Part 2. Popular Stories Wirespondence! Turing Machines s Radio Applause Cards. Learning Electronics Need to brush up on your electronics principles?
A triac can be compared to a latching relay. It will instantly switch ON and close as soon as it's triggered, and will remain closed as long as the supply voltage remains above zero volts or the supply polarity is not changed. If the supply is an AC alternating current , the triac will open during the periods the AC cycle crosses the zero line, but will close and switch ON as soon its re-triggered. To switch-on a Triac, a gate trigger current must be applied on its gate pin G.
This causes a gate current to flow across Gate and terminal A1. The gate current could be positive or negative with respect to A1 terminal of the triac. The following diagram shows the simplified schematic of a Triac and also its internal silicon structure. When a triggering current is applied to triac gate, it is switched ON by means of its inbuilt diodes embedded back-to-back between G terminal and and A1 terminal.
These 2 diodes are installed at the P1-N1 and P1-N2 junctions of the triac. Triggering of a triac is implemented through four quadrants depending on the polarity of the gate current, as shown below:.
These triggering quadrants can be practically applied depending on the family and the class of the triac, as given below:. Q2 and Q3 are the recommended triggering quadrants for triacs, since it allows minimal consumption and reliable triggering.
While designing a triac control circuit, its gate triggering parameters become crucial. Non-isolated triggering of a triac can be done in two basic modes, the first method is shown below:.
Here, a positive voltage equal to the VDD is applied across the gate and A1 terminal of the triac. In this configuration we can see that the A1 is also connected to the Vss or the negative line of the gate supply source.
This is important otherwise the triac will never respond. This method is identical to the previous except the polarity.
Since the gate is triggered with a negative voltage, A1 terminal is now joined in common with the VDD line instead of Vss of the gate source voltage. Again, if this is not done, the triac will fail to respond.
The gate resistor sets the IGT or the gate current to the triac for the necessary triggering. Higher values will also work if your ambient temperature is rather constant. Alternatively, it can be also triggered from the existing AC supply itself. Here, the switch S1 has negligible stress on it since it switches the triac through a resistor causing minimal current to pass through the S1, thus saving it from any sort of wear and tear. Switching a Triac through a Reed Relay : For switching a triac by a moving object, a magnetic based triggering could be incorporated.
A reed switch and a magnet can be used for such applications , as shown below:. In this application the magnet is attached to the moving object.
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