A.C. Soft Start -or- ‘Long Life and Prosper’ to any A.C. load


Sometimes, in my home, the general power switch turns off at the exact moment I turn the Pc on.

The reason is the DeskTop Pc power supply (600W) draining a big peak current at power-up.
Monitor, computer, printers and all Pc peripherals are not directly connected to the mains, I use a switch to turn the loads on and off. This is useful, when not in use, to zero power load and to save electronics against sudden power bursts.

I tried to connect a UPS but nothing changed; the UPS on-off relay becomes damaged after say 200 (a couple of years) switch-on operations.

Thats why 5 years ago I decided to build a soft start and see what’s going on … happily working since then.

A soft start avoids to directly apply the full mains voltage to the the protected power target; in modern SMPS (switching regulators) the mains directly loads electrolitic capacitors (via a bridge rectifier) and until capacitors are fully charged a hig current is drained. Well there is always a standard protection that avoids excessive current: a resistors and/or NTCs.

The higher is the power, the higher is the current peak at power-on. In case of auto-ranging SMPS (Switch Mode Power Supplies) 100-240 V this is particularly true specially if ac mains is 230V 50 Hz (as in my country).

I believe this soft start will lead to a longer life (and prosper 🙂) of the power supply.


The 160 W halogen lamp is series-connected to the load. A simple n.o. relay is parallel connected to the output.

At power on transient, the output is seen as a short circuit, the bulb lights-up; when the output voltage (i.e. the load) reaches the relay toggle voltage (near 170 V in my case), the bulb is shorted, thus giving full power to the output.

The device has 2 protections: a standard fuse and a thermal circuit breaker: should the load or the power present an unpredictable behaviour, the circuit opens up avoiding fire hazards. Thermal breaker is thermally coupled with the bulb. It switches off at 60°C (140 °F) and back on again at 50°C (120 °F).

A really simple pure-electric solution, but with a side effect: the relay itself, when on, drains 1…2 W.

Components are designed for 230 V 600 W SMPS load but can be easlily changed for other voltages and powers.

The box for this device better not to be metallic as the full circuit is exposed to mains and cannot be plastic too couse the bulb may easily reach high temperatures (should not but one can never know); so I went for a pcb case: optimal electrical and thermal insulation. Four surfaces of the parallelepiped are soldered together leaving the pcb (soldered) inside; top and bottom lids are simply duct taped.

Here schematic, pcb & pcb-case.

IMPORTANT SAFETY NOTES: should you decide to make your own protector, please remember:

(1) the circuit is directly connected to AC mains, so do not touch any part of it when disassembled and input-powered.

(2) do test the protection by creating a short circuit to the output: the bulb must stay on until the thermal protection does its job by toggling off-on-off-on indefinitely. Themperature must remain safe (around approximately 60 °C)

Soft Start animation

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