Common Issues/Questions:
- Why does my PSU "click" when I shut down and after I power up?
- When a power supply is initially turned on, there is a high rush of current entering the circuit in order to charge the bulk capacitors. This rush of power could potentially cause damage if not kept into check. We call this rush of current "in rush current" and the act of limiting the amount of current delivered is called "inrush current limiting".
Typically, the act of limiting this inrush uses an NTC (negative temperature coefficient) thermistor. The thermistor's high resistance slows the inrush current. As the current goes up the thermistor's temperature increases and resistance decreases and eventually, once the bulk cap is fully charged, only the amount of current required by the load is flowing through the circuit. The problem with this is that the NTC is never "removed" from the circuit and always adds some resistance to the circuit.
Lately, the strive to make electronics more and more efficient has become paramount. Removing something that adds resistance to a circuit while literally doing nothing is a good place to start.
This is where your "click" noise comes into play. By using a relay, the NTC can be bypassed once the bulk cap is charged and the NTC's function is no longer required.
Picture of an actual relay used in a PSU
- When a power supply is initially turned on, there is a high rush of current entering the circuit in order to charge the bulk capacitors. This rush of power could potentially cause damage if not kept into check. We call this rush of current "in rush current" and the act of limiting the amount of current delivered is called "inrush current limiting".