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Before getting to the intricacies we need to understand the basis of mains power:

Mains power consists of an alternating current that flows in one direction and then another at a NZ frequency of 50 cycles per second (50 Hz). When viewed on a graph, each cycle is shown as a sine wave, resembling an ‘S’ on its side, with a horizontal line drawn straight through its centre.

Every time the sine wave (representing alternating current) decreases in amplitude it returns back to the horizontal line, where no voltage is present (known as the ‘zero crossing point’), and it is around that point that phase control dimmers strike.

A trailing-edge dimmer fades voltage by applying resistance as the sine wave decreases in amplitude and falls back to the zero crossing point (zero volts) whilst a leading-edge dimmer chops voltage abruptly at the zero crossing point and as the sine wave increases in amplitude.

Trailing-edge dimmers are more sophisticated than leading-edge dimmers, and usually use a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or IGBT (Insulated Gate Bipolar Transistor) switch rather than a TRIAC and coil. This benefits the user with smooth, silent dimming control, absent of any buzzing noise.

A trailing-edge dimmer has a lower minimum load (often 10W) than leading-edge dimmers, making it a better choice for dimming modestly sized low-powered lighting circuits.

Particularly beneficial for incandescent and halogen bulbs is the ‘soft start’ feature in trailing-edge dimmers, which prevents filament bulbs from dying or exploding of thermal shock when first switched on.

Leading-edge dimmer switches are cheaper and simpler than trailing-edge, and were used originally to dim incandescent and halogen bulbs or wirewound magnetic transformers. They use a ‘TRIAC’ (Triode for Alternating Current) switch to control power, and are sometimes called TRIAC dimmers.

Many existing leading-edge dimmer switches have a relatively high minimum load, which often rules out their use with modest LED or CFL lighting circuits. However, leading-edge dimmers are by far the most common dimming control in existence.

One example of an excellent leading-edge dimmer switch is the PS SC150-24, which is perfect for LED, incandescent and halogen dimming circuits with a 90-150W minimum/maximum load.