Surge Limiter

1. This limiter is used where there is a significant switch on surge compared to the normal current consumption. The values shown were used in a 420V 200mA DC power supply to limit the inrush current while the smoothing capacitors are initially charging and typically takes about one second to activate the relay. Values may may need to be selected depending on the actual currents involved in the constructors equipment. The particular relay used mounts on a PC board and has contacts rated for 8A at 240V AC.

The 47ohm series resistor limits the inrush current to a reasonable value and during that period the voltage drop across the resistor means that there is insufficient voltage to energise the AC relay. As the surge current reduces in value the AC voltage across the primary of the mains transformer increases until it is sufficient to energise the relay and short out the series resistor.

The series resistor should be a metal cased type that can be bolted down to the chassis and should be capable of absorbing the peak current, the resulting voltage drop and the voltage between the centre connection and case. If in doubt err on the large size in terms of dissipation. You may need to use a 50W or 100W depending on the application.

The resistor value and dissipation  may be changed for use with higher or lower consumption power applications and on a 115V supply. A separate limiter should be used for each mains transformer requiring surge limiting in a power supply.

This mechanism is not suitable for use where there is a high constant current into the mains transformer primary winding compared to the peak value although it should be fine for use with filament transformers by proper selection of the resistor value.


2. If this simple current limit is not suitable for a particular application than a timer mechanism may be used instead which will require a small separate mains transformer to supply a low voltage regulated supply for a 2 - 3 second timer and driver transistor to activate an equivalent relay to that shown above but with low DC voltage coil, say 12V. Current requirements will typically be 100mA maximum