Anything you want to know about dimmers


Dimmers have the capacity to improve the quality and function of our environment. They provide cost savings as well as convenience. With improvements through R & D and a lowering in the cost of manufacture, lighting controls are destined to become an invaluable part of many of our everyday lives.

The Triac or bi-directional Thyristor construction, is a device that can be used to pass or block current in either direction. It is therefore classed as an AC power control device. It is equivalent to two Thyristor in anti-parallel with a common gate electrode. As only one device is required there are cost and space savings.

Figure 2 shows the Triac symbol and a simplified cross section of the device.

The Triac has two main terminals. TE1/ TE2 (power in and load out) and a single gate connection. The main terminals are connected to both p and n regions since the current can be conducted in either direction. The gate is similarly connected, since a Triac can be triggered by both negative and positive pulses.

The ON state voltage/ current characteristics resembles a Thyristor. The Triac static characteristics show that the device acts as a bi-directional switch. The condition where terminal TE2 is positive with respect to terminal 1 is denoted by the term TE2+. If the Triac is not triggered the low level of leakage current increases as the voltage increases until the break over voltage V is reached and then the Triac turns ON. The Triac can be triggered below V by a pulse to the gate, provided that the current through the device exceeds the latching current I before the trigger pulse is removed. The Triac has a holding current value below which conductance cannot be maintained.

If terminal 2 is negative with respect to terminal TE2 the blocking and conducting conditions are similar to the TE2+ condition, but the polarity is reversed. The Triac can be triggered in either direction by both negative or positive pulses on the gate. The actual values of gate trigger current and holding current as well as latching current can be slightly different in the different operating quadrants of the Triac due to the internal structure of the device.

Cathode/ Anode voltage ratings
The voltage of the AC mains is usually regarded as a smooth sinewave. In practice there is a variety of transients, some occurring regularly and others only occasionally. Although some transients may be removed by filters, Triacs must still handle cathode/ anode voltages in excess of the normal mains voltage level.