[/color]

single phase ac voltage controller using triac diac combination

Abstract

The DIode AC switch, or Diac for short, is another solid state, three-layer, two-junction semiconductor device but unlike the transistor the Diac has no base connection making it a two terminal device, labelled A1 and A2. Diac’s have no control or amplification but act much like a bidirectional switching diode as they can conduct current from either polarity of a suitable AC voltage supply.In our tutorial about SCR’s and Triacs, we saw that in ON-OFF switching applications, these devices could be triggered by simple circuits producing steady state gate currents.When switch, S1 is open no gate current flows and the lamp is “OFF”. When switch S1 is closed, gate current IG flows and the SCR conducts on the positive half cycles only as it is operating in quadrant Ι.

We remember also that once gated “ON”, the SCR will only switch “OFF” again when its supply voltage falls to a values such that its Anode current, IA is less than the value of its holding current, IH.

If we wish to control the mean value of the lamp current, rather than just switch it “ON” or “OFF”, we could apply a short pulse of gate current at a pre-set trigger point to allow conduction of the SCR to occur over part of the half-cycle only. Then the mean value of the lamp current would be varied by changing the delay time, T between the start of the cycle and the trigger point. This method is known commonly as “phase control”.

But to achieve phase control, two things are needed. One is a variable phase shift circuit (usually an RC passive circuit), and two, some form of trigger circuit or device that can produce the required gate pulse when the delayed waveform reaches a certain level. One such solid state semiconductor device that is designed to produce these gate pulses is the Diac.

The diac is constructed like a transistor but has no base connection allowing it to be connected into a circuit in either polarity. Diacs are primarily used as trigger devices in phase-triggering and variable power control applications because a diac helps provide a sharper and more instant trigger pulse (as opposed to a steadily rising ramp voltage) which is used to turn “ON” the main switching device.