Soft start issue


2005-03-20 5:39 am
Soft star issue

I built a soft start circuit based on Rod Elliot design this weekend. I have a 1kVA toroidal xfmr, 120 AC lines, and 56mF cap bank.

I used 3 33ohms/35W resistor in parallel to limit the current to 11A at switch on for about 2 sec.

After maybe 10 starts, I heard a pop coming fom the soft start circuit. Next time I started it a louder pop with a flash and some smoke came from one of the three resistors.

I'm not sure yet if the resistor is burnt but I don't really want to leave the circuit like that now. What's wrong?
You have got a failure due to too much transient power. I recommend a wire wound resistor of 50-100 ohms, 10 W at least, 0.5 sec time delay.

Your major problem is the inrush current from the transformer itself, therefore you won't need any long delay.

I have used 5 W, 100 ohms for 16 years now with 600 VA and 2 x 44000 uF. Delay time 0.5 sec.

I had the same probmem with 470 ohms (0.6 W metal film) i series with my 0.33 uF which got broken after 13 years. Now I have four resistors in series.

Check R6-R9 in the schematic
Hi there.....a 1KVA toroid I find needs at least a wirewound 25W perhaps 50W rating for soft start.....25-47 ohms for a start.

Becareful......Use a metal encapsulated version ( I know there' dearer) as if you used a standard wire ended enamelled version it could melt and start a fire if relay fails. The beauty of soft start is that the fuse rating can come more realistically in line with circuit needs.

alexl said:
This is interesting. You suggest to use a 10W/ 100ohms resistor to replace my 105W/11ohms. It looks the same to me except for the delay which is 4 time shorter. Isn't it?
Remember that the soft starter only is for saving the fuses in the wall, nothing else. This huge current is only for a couple of mains periods = x times 10 ms where this X is 5-10.

Note, if you have a weak rectifier bridge put in perspective to the smoothing caps, then you must increase the delay time and also the resistor wattage rating.