[jderou]

I have a servo power supply that is simply 120vac rectified with a large capacitor (not sure of the value at the moment). I cannot switch this on directly without tripping breakers, burning contacts, etc.
I have a couple different circuits I have drawn out to soft start this, I am looking for advice on which route to go.
The first method would use a timer, switching a resistor out of the circuit after a preset time. Simple enough.

alternate method:
This seems like more of a direct approach to the problem, or maybe its just neater. There is 0v across capacitor at start and by the time a voltage has developed, the inrush problem is gone. So why not put a relay with a 120vdc coil across the capacitor? This would have the same resistor circuit as the first method.
Good or bad idea?

[jderou]

Well I am using the second method, and it works just as planned. The only downfall here is that the relay coil stays in the circuit. Probably isn't an issue, it should just act like part of the bleed circuit.
There is a third method I have been thinking about, that would correct this issue, but I haven't worked out the details. The relay coil would be between across one leg of the rectifier-capacitor circuit rather than across the capacitor. This would mean the relay would be on immediately at startup, and off once the cap is charged. Not sure if it is worth pursuing this, other than for entertainment value.
Joe

[matth]

I used to use a little relay circuit that was published by Elektor electronics magazine years ago.
I used these on 1KVA toroid transformers (on 230v) for large monoblock amps.

Attachment 257438Attachment 257440Attachment 257442

[matth]

FYI, if a relay sticks or a timing cap fails then expect a loud bang as the shunt resistors let go!

[vargasmongo3435]

Hello guys

I have a question about the soft-star circuit for toroidal transformers, I have plans to use the circuit on a 600VA toroid but I'm using 120VAC from the outlet and not 220VA what would be the value of R4,5,6,7 ?

[handlewanker]

Hi.....I'm on this path too on another thread and the recommendation there is to use a NTC or thermistor in series with the live mains line to the transformer primary windings to control the inrush current at start up, switching out of circuit when the transformer is on line etc.

My choice, for simplicity, is to manually switch the NTC out of circuit with a make before break switch that can handle the current flow, otherwise the NTC, if left in circuit, will run hot (as it's designed to do) when the circuit current demand gets high.
Ian.