Phase Cut Dimmer

How does phase cut dimmer work?

Phase cut dimmer works by taking the line input power and modulating the signal to reduce the power to the load. By “chopping” the signal, the load experiences a lower voltage, resulting in a lower light output.

The two most common types of phase cut dimming (Phase cut dimmers) are forward phase dimming (Forward phase dimmer) and reverse phase dimming (Reverse phase dimmer), each with their own benefits and limitations.

Forward phase dimmer

The standard forward phase dimmer consists of a triac, a DIAC and an RC circuit. The resistance is controlled by a potentiometer, and the resulting RC time constant controls the delay before the thyristor is turned on. This delay is called the firing angle, and the part of time when the thyristor is turned on is called the conduction angle. The resulting voltage waveform is a phase cut sine wave.

This dimming type of work is very suitable for incandescent bulbs because incandescent bulbs are a pure resistive load. As the conduction angle decreases, the time average voltage on the filament resistance decreases to achieve natural and smooth dimming.

The most common technology used to produce forward phase dimming is Triac dimmer. In this type of dimming, a semiconductor acts as a switch, controlling both positive and negative half cycles. Due to the nature of how the triac works in forward phase, it is not interrupting current at the end of the cycle, but rather letting the signal go to zero. Triac dimmers are commonly used to control Incandescent, LED, CFL, Magnetic Low Voltage (MLV) or Fluorescent controls. MLV and some Fluorescent controls explicitly require forward phase controls due to their architecture. MLV loads convert the output from the dimmer to a low voltage output, so that smaller 12V or 24V lamps can be used. These “inductive” type loads require a forward phase control due to potential voltage spikes when current is interrupted in reverse phase. Electronic Low Voltage (ELV) loads generally require reverse phase controls and are not suitable for forward phase dimming.

Reverse phase dimmer

Reverse phase dimming modulates the input power by turning ON in the beginning of the cycle and turning OFF in the middle of the cycle. In order to increase the light output, the power would be turned OFF later; and to decrease the light output, the power would be turned OFF earlier.

The most common technology used to produce reverse phase controls are MOSFETS. In this type of dimming, two MOSFETS act as switches, one on each of the positive and negative half cycles. Due to the use of MOSFETS in the design, reverse phase dimmers generally require a neutral wire for operation. Reverse phase dimmers are commonly used to control LED, CFL, ELV and some other types of LED Drivers or Incandescent loads. Due to the architecture of ELV loads, reverse phase dimming is generally required and will result in proper performance.