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.
Advantages of Triac as Static Switches
- Triacs can be effectively replaced for mechanical switches or relays for controlling loads in AC circuits.
- Triacs can be configured to switch relatively heavier loads through minimal current triggering.
- When triacs conduct (close) they do not produce debounce effect, as in mechanical switches.
- When triacs switch OFF (at AC zero crossing), it does this without producing any transients, due to back EMFs etc.
- Triacs also eliminate fusing of contacts or arcing issues, and other forms of of wear and tear which are commonly seen in mechanical based electrical switches.
- Triacs feature a flexible triggering, which allows them to be switched at any given point of the input AC cycle, through a low voltage positive signal across gate and common ground.
- This triggering voltage could be from any DC source such as a battery or a rectified signal from the AC supply itself. In any case, the triac will go through switch OFF periods whenever each half cycle AC waveform moves through the zero crossing (current) line, as depicted below:
How to Switch-ON a Triac
A triac consists of three terminals: Gate, A1, A2, as shown below:
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. A1 terminal may be wired in common to the negative VSS line or positive VDD line of the gate control supply.
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.
Triac Triggering Quadrants
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.
Q4 triggering quadrant is not advised as it calls for a higher gate current.
Important Triggering Parameters for Triacs
We know that a triac can be used to switch high power AC load across its A1/A2 terminals through a relatively small DC trigger supply at its Gate terminal.
While designing a triac control circuit, its gate triggering parameters become crucial. The triggering parameters are: triac gate triggering current IGT, gate triggering voltage VGT, and gate latching current IL.
- The minimum gate current required to turn on a triac is called gate triggering current IGT. This needs to be applied across the gate and the A1 terminal of the Triac which is common to the gate trigger supply.
- The gate current should be higher than the value rated for the lowest specified operating temperature. This ensures optimal triggering of the triac under all circumstances. Ideally the IGT value should 2 times higher than the rated value in the datasheet.
- The trigger voltage applied across the gate and the A1 terminal of a triac is referred to as VGT. It is applied through a resistor which will discuss shortly.
- The gate current that effectively latches a triac is the latching current and is given as LT. The latching can happen when the load current has reached the LT value, only after this the latching enables even while the gate current is removed.
- The above parameters are specified at an ambient temperature of 25 °C, and may stat showing variations as this temperature varies.
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.
The second method is by applying a negative voltage to the triac gate as shown below:
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.
Calculating the Gate Resistor
The gate resistor sets the IGT or the gate current to the triac for the necessary triggering. This current increases as the temperature drops below the specified 25 °C junction temperature.
For example if the specified IGT is 10 mA at 25 °C, this may increase up to 15 mA at 0 °C.
To ensure that the resistor is able to supply sufficient IGT even at 0 °C, it must be calculated for the maximum available VDD from the source.
A recommended value is around 160 to 180 ohms 1/4 watt for a 5V gate VGT. Higher values will also work if your ambient temperature is rather constant.
Triggering through External DC or Existing AC: As shown in the following figure, a triac can be switched either through an external DC source such as battery or solar panel, or an AC/DC adapter. 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. Whenever the moving system gets past the reed relay, it triggers the triac into conduction through its attached magnet.
Reed relay can be also used when an electrical isolation is required between the triggering source and the triac, as shown below.
Here, the copper coil of suitable dimension is wound around the reed relay, and the coil terminals are connected to a DC potential via a switch. Each time the switch is pressed causes an isolated triggering for the triac.
Due to the fact that reed switch relays are designed to withstand millions of ON/OFF operations, this switching system becomes extremely efficient and reliable in the long run.
Another example of isolated triggering of triac can be seen below, here an external AC source is used for switching a triac through an isolation transformer.
Yet another form of isolated triggering of triacs is shown below using a Photo-cell couplers. In this method an LED and a photo-cell or photo diode are integrally mounted inside a single package. These opto couplers are readily available in the market.
An unusual switching of the triac in the form of off/half-power/ full-power circuit is shown in the diagram below. To implement 50% less power the diode is switched in series with the triac gate. This method forces the Triac to switch ON only for the alternate positive AC input half-cycles.
The circuit can be effectively applied for controlling heater loads, or other resistive loads having thermal inertia. This might not work for lighting control, since the half positive AC cycles frequency will result in an annoying flicker on the lights; likewise, this triggering is not advised for inductive loads such as motors or transformers.
Set Reset Latching Triac Circuit
The following concept shows how a triac can be used for making a set reset latch using a couple push buttons.
Pressing the set button latches the triac and the load ON, while pressing the reset button beaks the latch.
Triac Delay Timer Circuits
A triac can be set up as a delay timer circuit for switching a load ON or OFF after a set predetermined delay.
The first example below shows a triac based delay OFF timer circuit. Initially when powered, the triac will switch ON.
In the meantime the 100uF begins charging, and once the threshold is reached the UJT 2N2646 fires, switching ON the SCR C106.
The SCR shorts the gate to ground switching OFF the triac. The delay is decided by the 1M setting and the series capacitor value.
The next circuit represents a delay ON triac timer circuit. When powered the triac does not respond immediately. The diac remains switched OFF while the 100uF capacitor charges to its firing threshold.
Once this happens the diac fires and triggers the triac ON. The delay time depends on the values of 1M and the 100uF.
The next circuit is another version of a triac based timer. When switched ON, the UJT is switched via the 100uF capacitor. The UJT keeps the SCR switch OFF, depriving the triac from the gate current, and thus the triac also remains switched OFF.
After sometime depending on the adjustment of the 1M preset, the capacitor is fully charged switching OFF the UJT. The SCR now switches ON, triggering the triac ON, and also the load.
Triac is one of the most versatile components of the electronic family. Triacs can be used for implementing a variety of useful circuit concepts. In the above post we learned about a few simple triac circuit applications, however there are countless ways a triac can be configured and applied for making a desired circuit.
In this website I have already posted many triac based circuits which you can refer to for further learning, here's the link to it: