Thyristor Application Circuits

Suppose a student builds the following TRIAC circuit and finds that it does not work:

When the pushbutton switch is actuated, nothing happens. What is wrong with this circuit? Hint: the problem in this circuit is very subtle, and may be very difficult to discern.

A student builds this simple TRIAC power control circuit to dim a light bulb:

The only problem with it is the lack of full control over the light bulb’s brightness. At one extreme of the potentiometer’s range, the light bulb is at full brightness. As the potentiometer is moved toward the direction of dimming, though, the light bulb approaches a medium level of intensity, then suddenly de-energizes completely. In other words, this circuit is incapable of providing fine control of power from “off” to “full” light. The range of control seems to be from full brightness to half-brightness, and nothing below that.

Connecting an oscilloscope across the light bulb terminals (using both channels of the oscilloscope to measure voltage drop in the “differential” mode), the waveform looks like this at full power:

When the potentiometer is adjusted to the position giving minimum light bulb brightness (just before the light bulb completely turns off), the waveform looks like this:

Explain why this circuit cannot provide continuous adjustment of light bulb brightness below this level.

Predict how the operation of this AC lamp dimmer circuit will be affected as a result of the following faults. Consider each fault independently (i.e. one at a time, no multiple faults):

Potentiometer R_pot fails open:

Capacitor C₁ fails shorted:

Capacitor C₁ fails open:

DIAC fails open:

TRIAC fails shorted:

For each of these conditions, explain why the resulting effects will occur.