Soft start

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Soft start is an electronics term describing any circuit which is reduces excess current flow during initial power up.

[edit] Need for soft start

Many electronic devices will consume a large initial current when first turned on (i.e. Inrush current) that can cause voltage fluctuations and affect the performance of other circuits connected to a common power supply. The source for this problem is often large capacitors with very low Input impedance. To counteract this issue, components can be added in series to throttle back the current initially as the device comes online.

Soft starters are also used to start some types of lamps. A cold tungsten filament has a much lower resistance by a factor of 8-10 than a hot filament, and allows a large inrush of current. This inrush coupled with uneven filament wear causes local temperature overshoot in hotspots during startup, further evaporating the thinner filament sections. While soft start has little effect on GLS lamp life, it can make a sizeable difference to a halogen lamp's life.

High initial current can cause damage to other components such as semiconductors if they are not rated for the initial high current of loads such as filament lamps, motors or capacitors.

Soft starts are sometimes used on larger equipment as well, such as electric motors in various applications. The current drawn by an electric motor during a start can be 2 to 10 times the normal operating current, and this can exceed the supply's ratings if not controlled.

Finally soft start is widely used on hand held tools to prevent the tool being jerked out of position when switched on. Jumping of electric drills was a widespread issue in the 1970s when soft start was usually not fitted.

[edit] Soft starting methods

There are various ways to implement soft starting.

The most popular for appliances is triac control which ramps up duty cycle over several cycles.

A method used with some motors is to start with windings in series, switching them to parallel when partial speed has been reached. This is commonly known as Y-Δ start.

Increasing supply impedance has a semi soft start effect on motors, with initial current still being above run current, but by a much reduced amount. This method is not widely used.

Current limiting and voltage ramp-up in electronic supplies are common methods of soft starting low voltage loads.