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.
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