Triacs have some inherent drawbacks that reflect in their working: Careful designing of triac-based circuits reŽsults in a better working performance. The main.prawbac,ks bf triacs are rate effect, RF 'interference and backlash effect.
Rate effect-a snubber will cure it. Between the MTI terminal and gate of a triac, an internal capacitance exists. If tittle MTI terminal is supplied with a sharply increasing voltage, it causes enough gate voltage breakthrough to trigger the triac.

This condition is referred to as 'rate effect' -an unwanted effect caused mainly by the high transients in the AC line. Rate effect also occurs when the load is switched on due to high inrush voltage. It is severe particuŽlarly in driving inductive loads such as motors because the load current and voltage are out of phase.

An R-C snubber network minimises the rate effect and makes the switching clean. It is connected between MT1 and MT2 terminals of the triac as shown in. Radio frequency interference (RFI) - a filter will suppress. Unwanted RF generation is another major problem encountered in triac switching. Each time the triac is gated on its load, the load current switches rapidly from zero to a high value depending on the load resistance and supply voltage. This fast switching action (taking only a few microseconds) generates a pulse of RFI. It is the least when the triac is triggered close to 0° and 180° zero-crossing points but maximum in a 90° waveform. This is because switch-' on' current is minimum at 0° and 180° zero-crossing points. Switchat 90° producing very high RFI.

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The strength of RFl is proportional to the length of the wire that connects the load to the triac. RFl is annoying particularly in lamp dimmer circuits and can be
eliminated using a simŽple L-C RFl suppression network. Backlash effect-makes a lamp lazy. A serious control hysteresis or backlash develops in triac-controlled lamp-dimmer circuits when the gate current is controlled by a variable potentiometer. When the resistance of the potmeter increases to maximum, brightness of the lamp reduces to the miniri'lp.m. Thereafter, the lamp never turns on until the resistance of the potmeter is reduced to a few ohms, say, 50 to 70 ohms. This occurs due to discharging of the capacitor connected to the diac.

When the triac fires, the capacitor discharges via the diac and generates the backlash effect. This problem can be easily rectified by connecting a 47- to 100-ohm resistor in series with the diac or adding a capaciŽtor (C2) to the gate of the triac. This capacitor (C2) will slow down the backlash effect and the full turn effect can be obtained. The connection of the capacitor is shown in.