Simple Triac Triggering Circuits Explained

In this post I have explained the fundamental methods of triggering a triac, and also discuss the right way to connect the terminals of a triac.

Triacs are solid-state bidirectional thyristors that can switch across both the AC half cycles on a 120-volt or 240-volt Ac power system.

A triac could be activated (switched on and latched) with the AC line both synchronously or asynchronously.

However, if the triac's gate terminal current drops just below its lowest holding threshold, it will be switched off instantly at the completion of each AC half-cycle (180 electrical degrees).

WARNING: ALL THE CIRCUITS DISCUSSED IN THE FOLLOWING ARTICLE INVOLVE LETHAL 220 V MAINS AC. THEREFORE YOU MUST BE EXTREMELY CAREFUL WHILE USING AND TESTING THESE CIRCUITS, MAKING SURE THAT YOU APPLY ALL THE NECESSARY PRECAUTIONS DURING THE PROCEDURES. ALL THESE CIRCUITS ARE STRICTLY RECOMMENDED ONLY FOR THE EXPERTS.

Synchronous vs Asynchronous Switching

In an asynchronous switching, the triac is triggered ON randomly at any point of the phase cycle.

Due to this, asynchronous switching of triacs can produce substantial radio-frequency interference (RFI), especially at the first switching cycle.

In a synchronous triac switching, the switching periods consistently arrive on at the same moment for each AC half-cycle (typically right after the zero-crossing period) and therefore produce negligible RFI.

All the circuits I have I have explained in this article use asynchronous power switching. Figures 1–8 depict a number of asynchronous triac power-switching circuits for elementary ON/OFF AC line switching.

How to Connect a Triac

A triac has 3 terminals, which are MT1, MT2, and the Gate. The MT stands for main terminal.

Therefore, the main terminals MT1 and MT2 are used for switching heavy AC mains operated loads, through 220V or 120V AC mains supply.

This switching happens in response to a small DC voltage applied to the gate terminal of the triac.

New hobbyists often get confused and ask the question how the MT1 and MT2 terminals should be configured with the AC load and through a DC at the gate?

Remember, the correct method to connect the triac MT1 amd MT2 terminals is by ensuring that the AC load is always connected in series with the MT2 terminal, and the MT1 is connected directly with the other AC line of the mains supply.

Also, it is extremely important to note that the AC line associated with the MT1 terminal must be also linked with the negative or the ground line of the DC supply, which is being used for triggering the triac gate.

Failing to do this will not allow the triac to respond to the gate signals.

Triac Switching

A basic AC power switch using a triac is shown in Figure 1.

This triac circuit can be used to control the flow of AC power to lamps, heaters, motors, and a variety of other appliances and devices.

However, the triac for this circuit should have the appropriate power handling capacity to reliably switch AC power for the particular application.

All of the components in this article's schematics were chosen to switch only 120 volts, 50/60 Hz AC.

During the time the switch S1 is open, the triac is turned off and functions as an open switch.

However, when switch S1 is closed, it operates as a closed switch which is powered from the mains AC line via the load and R1 right at the start of each AC half-cycle.

When the triac is turned on, its main terminal voltage decreases to just a few hundred millivolts, thus R1 and S1 draw relatively negligible current.

Please remember that as soon as S1 is initially closed, the triac's turn ON threshold is not synchronized with the AC line, but it gets synchronized with the successive AC half-cycles.

The snubber network formed by resistor R1 and capacitor C1 reduces voltage spikes which develop whenever inductive loads are switched and when current and voltage are out of phase.

Most of the triac circuits discussed in this article incorporate snubber connections.

The triac works like a power switch that may be actuated by DC supply derived from AC supply, as shown in Figure 2 below.

Each positive line half-cycle charges capacitor C1 to +10 volts via resistor R1 and Zener diode D1.

When S1 is turned ON, the charge from C1 initiates the triac. Here the resistor R1 always gets exposed to approximately to the whole AC line voltage.

As a result, it demands a significant power rating (5 watts in ous case).

Due to the fact that all portions of this circuit are "active," it can create a fatal electrical-shock hazard.

Furthermore, since it lacks an isolator or complementing mechanism, this circuit is impossible to integrate with outer control circuitry.

Isolated Triac Control using Opto-Couplers

The next Figure 3 below demonstrates how to modify the circuit in Figure 2 to make it easier to connect with external control circuits.

Bipolar junction transistor Q1 is used in instead of switch S1 and is operated by the output stage of an optocoupler (or optoisolator) IC1.

An infrared light-emitting diode (IRED) is optically linked to a phototransistor in this system. Any of the commercially available transistor-output optoisolators can be implemented in these application.

The opto couplers like TIL111, TIL 112, 4N27, and 4N28 are among the several. Using resistor R1, a 5 volt or larger DC source could be used to power the optocoupler.

Only after switch S1 connects the input circuit supply to a 5 volt or larger power source, the triac is switched on.

Typical isolation values (Viso) for optocouplers are 5000 volts AC, with some having ratings as large as 7500 volts AC.

This implies that the DC input circuit is completely isolated from the triac output side circuit powered by the AC line.

By substituting S1 with an appropriate electronic detector, this fundamental triac switching circuit may be modified to provide any desired type of automated "remote" triac switching.

Figure 4 below shows a modification of the circuit seen in Figure 3.

Using capacitor C1 and the series resistor R1, along with the back-to-back Zener diodes D5 and D6, the triac is AC actuated on each line half-cycle in this design.

The amount of the triac gate current is determined by C1's AC line impedance, while the power dissipation of the capacitor C1's is almost around zero.

The series connection of Zener diodes D5, D6, and R3, that is loaded by transistor Q1, is coupled across the bridge rectifier built using diodes D1, D2, D3, and D4.

The bridge is essentially open while transistor Q1 is off, and triac TR1 switches on following the onset of each AC half-cycle.

As soon as  transistor Q1 is turned on, an almost a short circuit like condition is developed across D5, D6, and R3, which shuts off the Triac gate current, eventually turning off the triac TR1.

The optocoupler from the isolated external input stage drives transistor Q1, thus the triac is typically on, but it switches off as soon as the switch S1 is closed.

Using DC for Triggering a Triac

Figures 5 and 6 as given below, illustrate how to use a DC power source from a transformer power supply and a transistorized switch to activate a triac AC power switch.

When S1 is closed, both the transistor and the triac are both turned on, and as soon as S1 is open, both the devices are turned off.

In Figure 5, switch S1 can be substituted by a sensor device that can detect and respond to physical changes.

Transistor Q1 can be a BC557 transistor, not shown in the diagram.

For example, If the ambient temperature decreases below a predetermined level, a thermistor, can be incorporated to activate the triac circuit.

Similarly, photoconductive cell can be installed to detect light levels, a pressure sensor may detect changes in air or liquid pressure, and a flow metre can react to variations in liquid or air flow rate.

Please be cautioned that the  Fig. 5 circuit, is "live" and poses a lethal shock threat.

Figure 6 below demonstrates how to adapt the Fig. 5 circuit to use an optocoupler for its control.

This circuit could be actuated through a completely independent and isolated external circuit due to the presence of the optocoupler.

Triggering through Unijunction Transistor

Figures 7 and 8 as shown below, depict many different methods for triggering a triac through a completely isolated external circuit.

A unijunction transistor (UJT) placed in a pulse-generating relaxation oscillator provides the triggering operation in both of these circuits.

The oscillator circuit, which contains UJT Q2, provides the triggering pulses in these two circuits.

It works with a frequency of many kHz and feeds its output pulses to triac's gate via the pulse transformer T1, that ensures the intended isolation.

During the ON periods of the oscillator, the triac is turned on immediately at the start of each AC half-cycle due to the UJT device's relatively high working frequency.

With a resistor R3 is connected between the emitter and the base B2 of the UJT Q2, and a capacitor C1 hooked up between the emitter and the base B1, the UJT Q2 now works like a relaxation oscillator.

In this configuration UJT is able to switch rapidly to charge/discharge the capacitor at high speeds, as soon as the capacitor voltage reaches a certain threshold.

The time consumed by the capacitor to discharge could be evaluated, using the sawtooth's frequency calculations which is around 1/ time.

Since Q1 is in series with the UJT's primary timing resistor R3 in the Fig. 7 circuit, the UJT and the triac only switch on when S1 is closed.

On the other hand, in Figure 8 above since Q1 is in parallel with the UJT's primary timing capacitor C1 in the Fig. 8 circuit, the UJT and triac only switch on when S1 is open.

S1 could be substituted by a sensor or transducer in each of these circuits to provide an automated power-switching operation as mentioned previously.

The Q1 in the above figure should be an NPN transistor, such as a BC547.