In this circuit, a single relay limits the in-rush current and turns off the
startup circuitry for a switching power supply. Typically, switching power supplies
initially use a rectified ac line to power the control circuitry. Once the power
supply is running, an auxiliary supply winding provides power. Startup circuits
either use a wasteful high-value multiwatt resistor to charge a capacitor until
the controller turns on, or a more efficient but costly active circuit that
turns off once the controller is running.
High-power switching supplies often use resistors to limit the initial in-rush
current caused by charging the filter capacitors (see
figure, a). A single-pole, single-throw (SPST) relay shorts across the
resistor once the power supply is operating properly. Changing this relay to
a single-pole, double-throw (SPDT) relay creates a circuit that's simple and
inexpensive, yet wastes little power once the supply is running (see
figure, b).
When the supply is initially turned on, the common terminal of relay K1 is
connected to resistor R2 to charge up capacitor C1 and
supply power to the controller IC. R1 is left in-circuit to limit
the in-rush current. Once the power supply starts operating, K1 closes, shorting
across in-rush current-limiting resistor R1 and turning off the current
through startup resistor R2.