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?
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
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
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?
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.
richj
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.
richj
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.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?
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.
I use 4 really big encapsulated toroidal separation transformers for keeping the bugs inside the walls.
At switch-on they are each soft-started by nice Golden alloy 100 watts Dale resistors mounted on a piece of metal, call me over carefull.
At switch-on they are each soft-started by nice Golden alloy 100 watts Dale resistors mounted on a piece of metal, call me over carefull.
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