Um, err, what???
I thought the relay in most ATX PSUs was used to bypass the NTC Thermistor to increase efficiency by avoiding wasted heat in the Thermistor.
https://electronics.stackexchange.com/questions/361814/inrush-current-ntc-calculation
This article describes the function of the bypass relay. The only thing the relay switches is the Thermistor, nothing else. It does not stop half of the power supply from working when the relay contacts are open circuit, because the thermistor remains in series with the AC Line input and connects power to the switched mode PSU circuits.
https://www.tomshardware.com/reviews/power-supplies-101,4193-9.html
After the EMI/transient filter, a negative temperature co-efficient (NTC) thermistor is usually used to protect the other components from large inrush currents. A thermistor is simply a resistor that adjusts its resistance according to its operating temperature. The resistance of a "cold" thermistor is usually 6 to 12 ohms, and after the PSU is started up, the thermistor heats up and lowers its resistance to approximately 0.5 to 1 ohm. Typically, in high-efficiency PSUs, there is an electromagnetic relay that bypasses the thermistor after the PSU starts up to let it cool down and operate normally in a hot restart of the PSU. Also, by bypassing the thermistor, we raise the efficiency a bit, as no energy is wasted on it. A relay is used mostly in higher-end PSUs, while in lower-end units, the same task can be done with a diode.
Two examples of ATX PSU bypass relays and NTC thermistors.
Bypass relay (left) and (sleeved) thermistor (right).
Green thermistor (left), white bypass relay (right).
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