Phase-Cut Dimming

A phase control (phase-cutting) dimmer adjusts the brightness of the connected load by varying the portion of the mains AC waveform that is passed through to the connected load.

In a 50 Hz system, a full mains cycle takes 20 milliseconds (ms), which consists of a 10 ms positive half-cycle and a
10 ms negative half cycle.

At the beginning of the cycle the mains voltage is zero ("zero crossing"), it then rises to reach its peak value
(~340 V peak in a 240 V rms system) after 5 ms, before reducing again to zero at 10 ms. The cycle then repeats with the opposite polarity.

The portion of this 10 ms half-cycle time for which the dimmer passes power to the load is called the Conduction Time, and might be expressed in milliseconds (ms) or microseconds (us) where 1 ms = 1000 us.

"Triac" and "MOSFET" are two main types of electronic switch used in dimmers to control the flow of power to the load.

Older Dimmer used Triac’s.

A triac can be turned ON at any point in the mains cycle, but once turned on, they cannot be turned OFF without reducing the load current to zero. Due to this limitation, triac dimmers can only perform "Leading Edge" (LE) phase control, where triac turns the power to the load on part way through the cycle and it then remains on until the current drops to zero as the voltage drops to zero.

To set a high brightness, the triac is turned on right at the start of the cycle, immediately after the zero crossing, and then allows power through the load for the whole cycle.

To reduce the brightness, the triac is turned on part way into the cycle, some time after the zero crossing. The longer the delay, the lower the conduction time, and the lower the brightness.

This means depending on where in the cycle the triac turns on, there can be a very large step change in the voltage applied to the load. At 50% conduction, the step change is equal to the peak of the mains (340 V in a 240 V rms system).

This works well for older style loads such as incandescent lamps and for inductive loads like iron-core low voltage lighting transformers or motors, which naturally limit the inrush current. It is not good for electronic loads such as electronic transformers or LED lighting, which have a capacitance at the input, as this capacitor looks like a short circuit to the triac, causing large inrush current spikes.

To overcome this older dimmer limitation:

Digital Dimmer uses MOSFETs, which can be turned both on AND off at any point in the cycle.

These designs allow "Trailing Edge" (TE) phase control, where the power to the load is turned on at zero crossing, and turned off after the desired conduction time.

Trailing phase control is ideal for modern electronic loads like LED lighting, since turning power to the load on at zero voltage means the inrush current is minimized.

However, it is not suitable for inductive loads, since an inductance resists rapid changes in current, resulting in generation of high voltage spikes when the load is switched off.

As a result, Trailing Edge dimmers have a protection mechanism whereby they will shut down if voltage spikes are detected, usually due to an incompatible inductive load being connected.

Since MOSFETs can turn on AND off, MOSFET dimmers can also be designed to operate in Leading Edge mode.

A dimmer which can operate in both modes is often referred to as a "universal" dimmer, which usually start in trailing edge mode (since this is the correct mode for most modern load types), and then switch automatically to leading edge mode if over voltage spikes are detected.

Automatic mode selection is not always reliable and often results in erratic or inconsistent behavior.