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Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
I am working on a table top DRSSTC and I need a soft start circuit that I can use to limmet the inrush before startup. anybody have any good links or idias. thanks
Registered Member #2161
Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247
Yeah, I have had good experience with the good ole relay+resistor set-up.
Here is a circuit i found through google just now:
The capacitor combinations determine the amount of delay. (first one limits current, the amount of time it takes for the relay to switch depends on how much capacitance you have on the capacitors after the bridge). The time delay must be set to quite a short interval, otherwise you will burn the resistors. I suggest testing different configs, with no actual load being soft-started.
Unlike the circuit, I am using 8x 10ohm resistors, for 5ohm. They seem to be holding up. My AC is 220v@50Hz. The rest I cannot remember of the top of my head.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Here is the circuit I use. You calculate the series capacitor so that it supplies the right amount of current to the relay coil, the electrolytic capacitor sets the time constant (delay) after which the relay is closed. Approximately T=RC where R is relay coil resistance. Use something like 0.2-0.5s (depending on your filter caps value).
(with the series resistor value as shown, this is really just a peak inrush current limiter which prevents the switch contacts from welding/rectifiers from damaging, if you wanted e.g. a soft start for a power amp, you would use a bigger resistor.)
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
thanks guys for the help, I was pluging around on my proto board and came up with this. a time controled relay like the desplayed circuits are what I went with first until I turned it on and the breaker triped out before the time constant of my charge resister and caps was compleat. So I went with a circuit that switches a solid state relay to close the resistor when I get to a certan charge voltage. I think I am going to add a opto isolator in series with the LED to disable the DRSSTC till charge comlpeat. I found that even after 3-4 time constants that there is still quite a surge so I added enough zeners to reach close to charge compleat before the SSRL turns on. I spose it would be pausible to add a transistor and trimmer to adjust the turn on. chears
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Solid-state relays (AKA triacs and thyristors) are not as good for inrush limiting purposes as a good ol' relay and power resistor. The reason for this is something called dv/dt induced turn-on.
Basically a thyristor or triac is apt to turn on spuriously if the rate-of-rise of the applied voltage is very high. So if you turn on you appliance right at the peak of a mains cycle, (just where the inrush into those empty caps is going to be largest!) the SCR can turn on due to the high dv/dt when the mains supply is suddenly applied.
You can protect against dv/dt induced turn-on, but using a mechanical relay is easier and has less losses in the on-state than a silicon switch.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Richie, also maost all SSRL turn on at 0 deg and a few will turn on at 90 deg for inductive loads. As long as my cap are at about 4 time contants the inrush should be minimised. the circuits above are good so I will use them later on another circuit. one of the other problems I have is a lack of space and I found a small ssrl that fit the bill. thanks all
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looking for a swoft start circuit
