A dimmer is a device that is originally created to control the brightness of lamps. This is done by altering the total power delivered to the lamp and thus the brightness. The following schematic demonstrates a basic type of dimmer:

The resistor R is a protective resistor for the triac's gate. The potentiometer Rp along with the capacitor C, controls the time that the triac will be conductive, counting from the zero point of the input waveform.

The operation of the dimmer is based on the fact that, during a full cycle of an AC waveform, a thyristor will only allow a part of the waveform to be delivered to the load (lamp). Take a look at the following waveforms:

Both waveforms above comes from the same dimmer. The only difference is that the waveform on the left will bright the lamp higher than the waveform on the right. That is because, on the left waveform, the triac will be conductive earlier than the triac shown in the right waveform.

The time that the triac becomes conductive is symbolized with the Greek letter α (ALPHA) and is measured in angles from the zero point of the waveform. This zero point is the point that the voltage is 0 volts, and this happens 2 times every one full period of the wave form. When the α becomes smaller, then the dimmer becomes conductive sooner and the lamp is brighter. When the α becomes bigger, then the triac delays more to become conductive and thus the lamb is dimmer.

A full wavelength period is 180 degrees (2π). Due to the fact that during a full wave length the zero cross occurs twice, α can take values from 0° to 90 degrees (0 - π). When α = 0°, the full power is delivered to the load and when α = π, no power id delivered to the load.

A basic dimmer using a microcontroller

The zero cross detection circuit is the most critical part when designing a dimmer. This circuit will watch the input power waveform and detect when this waveform crosses the 0 point and becomes 0 volts.

Zero cross detection circuits are mainly used in cases when the dimmers needs to be controlled from a micro controller. In that case, the micro-controller needs to know the zero cross detection point of the waveform, so that it can calculate the angle offset to send the trigger pulse to the gate of the triac.

Here is an example calculation. Suppose that the AC power oscillates in a 50Hz cycle. This means that each cycle will take 1/50Hz = 200 mSec to be completed. During those 200mSec, the waveform will cross the zero point two times, one at the beginning and one in the middle of the cycle, that will be after 200/2 = 100mSec.

If we want the lamp to be half the way bright, then the microcontroller needs to send a pulse in the middle of each semi-cycle. Thus, a pulse must be sent after 50mSec after each time the waveform passes the zero point. For this to be done, the microcontroller will watch the zero cross detection circuit (ZCD) for a pulse. When the ZCD send this pulse, the micro controller will count 50mSec and then will trigger the gate of the triac.

The following circuit will perform a Zero Cross Detection circuit. This circuit is very stable and accurate, and has a controllable pulse width. Another great advantage is that because of the transformer, this circuit has a complete galvanic isolation with the mains supply so that it makes it completely safe and risk free of destroying the microcontroller due to power peaks.

A typical DC light dimmer / DC motor speed controller

The operating principle of a DC dimmer is completely different. When a triac becomes conductive, the only way to turn them back into a non-conductive state is to have 0 volts difference between it's pins. In the case of AC current, this happens twice every full period. But in the case of DC voltage, this would never happen and thus, when the dimmer become once conductive it will remain like that until the power is completely turned of from it's pins. This makes the triac inappropriate for DC power. Also, because DC power never crosses the zero point, the α parameter has no meaning to be used.

For those reasons, DC loads are controlled in a different way. The most popular and efficient way is the use of PWM switching signal to control the power delivered. A PWM signal between 1.5KHz and and 3KHz is applied to the base of a transistor. The transistor is used to drive the load. By altering the duty cycle of the PWM signal, we can change the brightness of the lamp. The higher the duty cycle, the brighter it lits and vice-versa.

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