Thread: Gate triggered

The gate terminal of the triac is used to trigger the device into conduction. The higher the current into the gate, the smaller the forward break-over'voltage. Thus applying a sufficient pulse of current to the gate will switch the device into' on' state. Once triggered, the device continues to conduct until the current through it dr 'ps below a certain threshold value.

Two modes of gate triggering are employed in power control circuits: phase triggering and burst triggering. Phase triggering. In phase'triggering, AC power is applied to the load via a triac. The triac is triggered at any point during each half cycle through a variable-phase-delay network and a trigger pulse generator.shows the mode of phase triggering.

The triac unlatches at the end of each AC half cycle as the instantaneous supply voltage (and thus the load current) briefly falls to zero. If the triac is triggered after the start of each half cycle with zero phase, the load voltage equals the full supply voltage. If the triac is triggered with 90° phase delay, the load voltage equals half the supply voltage, so the load consumes one-fourth of the power. If the triac is fired with 180° delay, the load voltage will be zero and the load consumes oilly negligible power. The waveforms of AC and loads are shown in.

Phase triggering is highly efficient to vary the load power over a wide range as in lamp-brilliance control circuits. Since switching occurs at the AC frequency, lamp brightness can be varied without flicker. But one major drawback of phase triggering is that when power increases from zero to a high peak value especially with 90° delay, high-current surge generates radio frequency interference (RFI). So phase triggering is not suitable for high-current loads.



Burst triggering: More efficient power control is achieved through burst triggering in which power bursts of complete half cycles are fed to the load at regular line-frequency-related intervals. Thus if the burst is repeated at 8-cycle intervals, the load voltage is proportional to the full supply voltage if the burst is of 8-cycle duration, half voltage (equivalent to quarter power) at 4-cycle duration or 1!16th of the voltage (equivalent to 1!256th power) at one half-cycle duration. The burst firing method is shown in.

The advantage of burst firing is minimum RFI since the switching of power to the load is.Afery near the start of line half cycles, when the line voltage and load current are nearly zero. The zero-crossing detector circuit gated by a mark! space ratio pulse generator enables the gate of the triac at the start of the line half cycle.

The zero crossing detector-mark! space pulse generator circuit outputs only if it.is 'gated' on and the line voltage is beIow:q volts. The triac is hen triggered and switches power to the load. The mark! space generator requires 12V DC, which can be obtained from a DC power supply or rectified power from AC line. The burst fire method is efficient but enables power consumption of the load only in a number of half-cycle steps.