How does triac function

When mT 1 is positive and mT 2 is negative then left SCR will be in forward bias and as we trigger the gate terminal it will start conducting. In the negative cycle other SCR will conduct. It is same as SCR, since it works in also negative voltages, so the negative characteristics are also same as positive SCR characteristics.

Figure given above shows the construction of triac. Semiconductor layers together forms two SCR equivalent construction. In this mode current flows through P 2 , N 1 and N 4. Figure given below is the circuit diagram of triac switching circuit and its graph. Since voltage of AC line goes negative as well as positive. When we close the switch, gate terminal connects to either positive voltage or negative voltage.

Based on gate trigger voltage and mT 1 , mT 2 polarity it conducts block or conduct according to two SCR analogy. There are many AC dimmer circuits for AC dimming. But, we will see and understand the simple circuit using resistor , capacitor , diac and triac.

This is the mostly used application of triac and diac in home for controlling the speed of ceiling fans. This is also called triac dimmer. For dim AC power we use phase angle. By controlling the phase angle we can control the power of some electrical appliances.

For control the power of an AC electrical appliance we control the phase angle of AC supply. For this we use a low frequency power switch. We trigger the switch at different angles of AC signal in both cycles. For switching purpose we use a triac. Figure shown above is graphical representation of phase angle control. Now we will see that how we can design a circuit for phase angle control.

This device controls the AC precisely and frequently used in controlling the speed of the motor, AC control devices, AC circuits, light dimmers, pressure control systems, etc. It finds applications in switching, phase control, chopper designs, brilliance control in lamps, speed control in fans, motors , etc. The power control system is designed to control the distribution level of AC or DC.

Such power control systems can be used to switch power to appliances manually or when temperature or light levels go beyond a preset level.

It is equivalent to two SCRs connected in inverse parallel with the gates connected together. As a result, it functions as a Bidirectional switch to pass the current in both directions once the gate is triggered. The Gate can be triggered either by a positive voltage or negative voltage.

It includes four layers as well as six doping regions. The designing of its gate terminal can be done with an Ohmic contact using two regions namely the P region and N region so that this device can activate through both the polarities.

Even though it is a bidirectional device, where the current and voltage of this can be specified with MT1 like the reference to diminishing confusion. Thus, the current flow between these two terminals can be decided through the layers in the device. Likewise, the MT2 terminal is made negative when a TRIAC is connected in reverse bias with respect to the MT1 terminal through gate open, then this device works in the mode of reverse blocking mode.

The negative gate signal can be used in case of a minor activating capacity. When supply is ON at positive cycle then the current flows through lamp, resistors , and DIAC provided triggering pulses are provided at pin 1 of optocoupler resulting in pin 4 and 6 starts conducting gate and reaches the supply and then only lamp glows for that half cycle directly through the M2 and M1 terminal of the TRIAC.

In the negative half cycle, the same thing repeats. Thus the lamp glows in both the cycles in a controlled manner depending upon the triggering pulses at the optoisolator as seen on the graph below. If this is given to a motor instead of a lamp the power is controlled resulting in speed control.

Like a transistor, a triac is made up of different layers of semiconductor material. This includes N-type material, which has many free electrons, and P-type material, which has many "holes" where free electrons can go. For an explanation of these materials, check out How Semiconductors Work. And, for a demonstration of how these materials work in a simple transistor , see How Amplifiers Work.

You can see that the triac has two terminals, which are wired into two ends of the circuit. There is always a voltage difference between the two terminals, but it changes with the fluctuation of the alternating current. That is, when current moves one way, the top terminal is positively charged while the bottom terminal is negatively charged, and when the current moves the other way the top terminal is negatively charged while the bottom terminal is positively charged.

The gate is also wired into the circuit, by way of a variable resistor. This variable resistor works the same basic way as the variable resistor in the old dimmer switch design, but it doesn't waste nearly as much energy generating heat.