TRIAC Control Circuit Diagram | TRIAC Zero-Point Switching Circuit

TRIAC Control Circuit Diagram:

A TRIAC Control Circuit Diagram that allows approximately 180° of phase control is shown in Fig. 19-27(a). The waveforms in Fig. 19-27(b) illustrate the circuit operation. With the TRIAC (Q₁) off at the beginning of the supply voltage positive half-cycle, capacitor C₁ is charged positively via resistors R₁ and R₂, as shown. When V_C1 reaches the DIAC switching voltage plus the Q₁ gate triggering voltage, D₁ conducts producing gate current to trigger Q₁ on. C₁ discharges until the discharge current falls below the D₁ holding current level. The TRIAC Control Circuit Diagram switches off at the end of the supply positive half-cycle, and then the process is repeated during the supply negative half-cycle. The rate of charge of C₁ is set by variable resistor R₁, so that the Q₁ conduction angle is controlled by adjustment of R₁.

TRIAC Zero-Point Switching Circuit:

The TRIAC zero-point switching circuit in Fig. 19-28(a) produces a load waveform similar to that for the SCR zero-point circuit. The load power dissipation is controlled by switching the TRIAC on for several cycles of the supply voltage and off for several cycles, with switch-on occurring only at the negative-to-positive zero crossing point of the supply waveform, and switch-off taking place at the positive-to-negative zero point. Q₁ is a low-current SCR that controls the switching point of Q₂.

With switch S₁ closed, Q₁ is on, and the Q₁ forward voltage drop is below the level required for triggering Q₂, (V_G2 + V_D1 + V_D2), [see Fig. 19-28(b)]. Thus, no gate current flows to Q₂, and no conduction occurs. Q₁ switches off when S₁ is opened, so that I_G flows to Q₂ gate via C₁, R₂, D₁, and D₂ to trigger Q₂ into conduction, [Fig. 19-28(c)]. With Q₂ conducting, capacitor C₂ is charged via D₃ almost to the positive peak of the load voltage, [Fig. 19-28(d)]. The TRIAC switches off at the end of the positive half-cycle. Then, the charge on C₂ (applied to the gate via D₂) triggers Q₂ on again just after the zero-crossing point into the negative half-cycle. (It should be noted this is quadrant IV triggering.)

The initial Q₂ switch-on occurs only at the beginning of the positive half-cycle of the supply voltage. If S₁ is opened during the supply positive half-cycle, Q₁ continues to conduct until the end of the half-cycle, thus keeping Q₂ off. With Q₂ off, C₂ remains uncharged, and so it cannot trigger Q₂ on during the supply negative half-cycle. Q₂ triggering now occurs at the beginning of the next positive cycle.

If S₁ is opened during the supply negative half-cycle, Q₂ cannot be triggered into conduction, again because of the lack of charge on C₂. It is seen that Q₂ conduction can commence only at the beginning of the positive half-cycle of the supply voltage. Also, once triggered, Q₂ conduction continues until the end of the cycle.

To design the circuit in Fig. 19-28, the TRIAC Control Circuit Diagram is first selected to pass the required load current and to survive the peak supply voltage. Resistor R₂ is a low-resistance component calculated to limit the peak surge current to the Q₂ gate in the event that the peak supply voltage is applied to the circuit without Q₁ being on. Capacitor C₁ has to supply triggering current (I_G) to Q₂ at the zero crossing point of the supply waveform when Q₁ is off. Usually I_G2 is selected around three times the specified I_G(max) for Q₂, and C₁ is then calculated from the simple equation for capacitor charge; C = (I x t)/ΔV. In this case ΔV/ t can be replaced by the rate-of-change of the supply voltage at the zero crossing point, which is (2π f V_p). So, the C₁ equation is,

Resistor R₁ can now be determined by using the selected gate current for Q₂ (I_G2) as the peak anode current for Q₁; R₁=V_p/I_G2. The Q₁ gate resistor (R₃) is calculated from the Q₁ triggering current and the voltage of the de source; R₁ =(E – V_G1)/I_G1.

The Q₂ gate current is again used in the calculation of R₄ and C₂. To trigger Q₂ at the start of the supply negative half-cycle, I_G2must flow from C₂ into the Q₂ gate, so R₄ = V_p/I_G2. A suitable capacitance for C₂ is now calculated by again using the simple capacitance equation C₂ = (I_G2 x t)/ΔV. In this case, time t is selected much larger than the Q₂ turn-on time, and ΔV is approximately 0.1 V_P.

SCR Q₁ must pass the selected anode current (I_G2) and survive the peak supply voltage. The diodes must each survive the peak supply voltage and pass the Q₂ triggering current,

IC Zero Voltage Switch:

The functional block diagram for a typical Integrated circuit TRIAC driver, known as a zero voltage switch, is shown in Fig. 19-29. The device contains a voltage limiter and a dc power supply, so that it operates directly from the ac supply to the load to be controlled. There is also a zero crossing detector that provides an output pulse each time the supply waveform crosses the zero level. The zero crossing detector output is fed to an AND gate , and the AND gate output goes to the TRIAC drive stage that produces the current pulse to the TRIAC gate. An on off sensing amplifier is used to sense the voltage level from an externally-connected transducer; for example, a temperature sensing transducer would be used if the load is a heater. When the temperature drops to a predetermined level, the on-off sensing amplifier provides an input to the AND gate. The gate triggering pulse from the TRIAC drive stage occurs at the supply zero-crossing points only when the temperature is below the desired level.