12V PSU Remote Power-On Circuit With RC Delay, NPN (TIP31), LCC120 SSR Review

The discussion centers on designing a remote power-on circuit for a 12V power supply unit (PSU) that requires shorting two pins after a delay following mains power application. The original approach used an NPN transistor (TIP31) and an RC delay circuit, but feedback indicated a PNP transistor is more appropriate, with the emitter connected to +3V and the collector to the PSU "ON" pin. The capacitor should be connected correctly to avoid series placement with the transistor base. A simple RC delay causes a gradual voltage increase, potentially leading to unstable switching; thus, alternatives like a one-shot multivibrator or comparator with RC are suggested for cleaner transitions. The use of an LCC120 single-pole double-throw (SPDT) solid-state relay (SSR) was proposed to isolate control signals and enable remote activation. However, concerns about variable resistance and gradual voltage changes with the SSR output were raised, recommending testing or substituting with a mechanical relay. For continuous pin shorting to keep the PSU on, a mechanical relay with a capacitor across its coil is advised to provide the delay. The LCC120 input resistor value can be flexible, with 1kΩ typical, and lower values like 680Ω or 820Ω if needed. The emitter follower transistor configuration may cause voltage rise beyond 3V, risking PSU input chatter. A recommended mechanical relay circuit includes a 3V coil, electrolytic capacitor, and resistor to achieve a reliable delayed switch-on without unstable transitions. The discussion emphasizes correct transistor type, proper RC placement, relay choice, and circuit topology to achieve stable delayed remote power-on functionality for the PSU.
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