Low power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances.
However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the AC power. Indeed, TRIACs can be very sensitive to high values of dv/dt between A1/MT1 and A2/MT2, so a phase shift between current and voltage (as in the case of an inductive load) leads to sudden voltage step that can make the device turn on in an unwanted manner.[2]
Unwanted turn-ons can be avoided by using a snubber circuit (usually of the RC or RCL type) between A1/MT1 and A2/MT2. Snubber circuits are also used to prevent premature triggering, caused for example by voltage spikes in the mains supply.
Because turn-ons are caused by internal capacitive currents flowing into the gate as a consequence of a high voltage dv/dt, a gate resistor or capacitor (or both in parallel) may be connected between the gate and A1/MT1 to provide a low-impedance path to A1/MT1 and further prevent false triggering. This, however, increases the required trigger current or adds latency due to capacitor charging. On the other hand, a resistor between the gate and A1/MT1 helps draw leakage currents out of the device, thus improving the performance of the TRIAC at high temperature, where the maximum allowed dv/dt is lower. Values of resistors less than 1kΩ and capacitors of 100nF are generally suitable for this purpose, although the fine-tuning should be done on the particular device model.[3]
For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load. However, due to the separate gates, proper triggering of the SCRs is more complex than triggering a TRIAC.
In addition to commutation, a TRIAC may also not turn on reliably with non-resistive loads if the phase shift of the current prevents achieving holding current at trigger time. To overcome that, pulse trains may be used to repeatedly try to trigger the TRIAC until it finally turns on. The advantage is that the gate current does not need to be maintained throughout the entire conduction angle, which can be beneficial when there is only limited drive capability available.
Application of Triac