Soft Starting
Shaun, Wed Sept 19 2007, 02:48AMI'm working on a soft-start circuit for my DRSSTC so I can free myself from the last truly weighty component, the variac. I saw an IRFP260-based starter by Steve Ward, only I think it is only meant for SSTCs. I also thought about Steve Connor's solid state variac, but I would like something a little less complex so you can just turn it on and leave it alone (and so it can be mounted in the coil base with the other electronics).
I googled this first and found some schematics for motor drivers and big audio amps, some based on power resistors
, the rest on some combination of SCRs, Triacs, diodes, etc. 
(I have 8 TO-220 20A 600V SCRs to play with). Will any of these be suitable for a DRSSTC (~0.5-1 kW)?Or does anyone have a schematic that they have used and know to be good for this application?
Re: Soft Starting
Bennem, Wed Sept 19 2007, 05:51AM
Hi Shaun,
i have built Steve Connor's DRSSTC solid state variac circuit
to operate my DRSSTC's....it works fine. i highly reccomend the circuit.
I have built it as a seperate 'box' from the base of the coil
but you could easily incorporate it into the base as Steve does
and operate it from your intterupter box, however if you do it that way and you happen to
unplug your intterupter (ie disconnect the pot to the SS variac) it tends to
send the variac to full mains voltage....thats one reason i built it as a separate box,
plus i operate other coils from it too.
Bennem, Wed Sept 19 2007, 05:51AM
Hi Shaun,
i have built Steve Connor's DRSSTC solid state variac circuit
to operate my DRSSTC's....it works fine. i highly reccomend the circuit.
I have built it as a seperate 'box' from the base of the coil
but you could easily incorporate it into the base as Steve does
and operate it from your intterupter box, however if you do it that way and you happen to
unplug your intterupter (ie disconnect the pot to the SS variac) it tends to
send the variac to full mains voltage....thats one reason i built it as a separate box,
plus i operate other coils from it too.
Re: Soft Starting
Steve Conner, Wed Sept 19 2007, 12:28PM
Hi guys,
Yeah, I found out the "full power when unplugged" thing the hard way recently :( I need to change this circuit so it has a proper soft-start at power on, even with the voltage control knob up full or disconnected.
I do indeed have it mounted in the coil base and remotely controlled by a 0-15V DC voltage from the interrupter box.
Marko made a simpler SCR controller, but it can't be remote controlled like I described (the main reason why mine is complex)
Steve Conner, Wed Sept 19 2007, 12:28PM
Hi guys,
Yeah, I found out the "full power when unplugged" thing the hard way recently :( I need to change this circuit so it has a proper soft-start at power on, even with the voltage control knob up full or disconnected.
I do indeed have it mounted in the coil base and remotely controlled by a 0-15V DC voltage from the interrupter box.
Marko made a simpler SCR controller, but it can't be remote controlled like I described (the main reason why mine is complex)
Re: Soft Starting
Terry Fritz, Wed Sept 19 2007, 12:28PM
Hi,
I just used a resistor and a few switches:
In the stop position S1 is open. S2 either charges the main caps through the resistor or discharges the caps. So it also force discharges the buss too.
To run it, you put S2 in the charge position, wait a moment and then put S1 in the run position. To turn in off, put S1 in the stop position and then put S2 in the discharge position.
Cheers,
Terry
Terry Fritz, Wed Sept 19 2007, 12:28PM
Hi,
I just used a resistor and a few switches:
In the stop position S1 is open. S2 either charges the main caps through the resistor or discharges the caps. So it also force discharges the buss too.
To run it, you put S2 in the charge position, wait a moment and then put S1 in the run position. To turn in off, put S1 in the stop position and then put S2 in the discharge position.
Cheers,
Terry
Re: Soft Starting
Shaun, Wed Sept 19 2007, 10:20PM
Actually this is exactly what I was looking for. I don't need to be able to control it, as the interrupter will be enough control for me. I just want to be able to switch on the coil (and soft-start circuit) at the coil base and use the interrupter from there. Does anyone know the schematic for this controller?
Shaun, Wed Sept 19 2007, 10:20PM
wrote ...
Marko made a simpler SCR controller...
Marko made a simpler SCR controller...
Actually this is exactly what I was looking for. I don't need to be able to control it, as the interrupter will be enough control for me. I just want to be able to switch on the coil (and soft-start circuit) at the coil base and use the interrupter from there. Does anyone know the schematic for this controller?
Re: Soft Starting
Shaun, Fri Sept 21 2007, 03:58AM
Sorry for the double post, but I was just going over the soft-start thing in my head and I was wondering if you even need to soft-start (or variac) the coil anyway.
I'm using a half-bridge with 120VAC in, so the way I figure it, first each capacitor charges to about 170V thru the diodes, and then the two are alternatively switched thru the MMC/Primary by each IGBT. So if I am using the interrupter, aren't I abruptly applying full voltage to the IGBTs many times per second anyway?
Or is the soft-start not to protect the silicon but to keep you from blowing a breaker during the initial charging of the filter caps? If that is true, then I definitely won't need it because I have charged these caps by themselves off of rectified mains without popping any breakers.
Can anyone shed some light on this?
Shaun, Fri Sept 21 2007, 03:58AM
Sorry for the double post, but I was just going over the soft-start thing in my head and I was wondering if you even need to soft-start (or variac) the coil anyway.
I'm using a half-bridge with 120VAC in, so the way I figure it, first each capacitor charges to about 170V thru the diodes, and then the two are alternatively switched thru the MMC/Primary by each IGBT. So if I am using the interrupter, aren't I abruptly applying full voltage to the IGBTs many times per second anyway?
Or is the soft-start not to protect the silicon but to keep you from blowing a breaker during the initial charging of the filter caps? If that is true, then I definitely won't need it because I have charged these caps by themselves off of rectified mains without popping any breakers.
Can anyone shed some light on this?
Re: Soft Starting
Steve Conner, Fri Sept 21 2007, 02:57PM
You are correct, the soft start is only to prevent blowing breakers and fuses.
The schematic you wanted is on the forum somewhere.
Steve Conner, Fri Sept 21 2007, 02:57PM
You are correct, the soft start is only to prevent blowing breakers and fuses.
The schematic you wanted is on the forum somewhere.
Re: Soft Starting
Marko, Sat Sept 22 2007, 09:40AM
Simplest thing you can do is to use a PTC in series with your supply. It needs some time to cool so it isn't for fast power cycling but otherwise it should do the job. Use more than one in series/parallel depending on your wishes.
Apart from that, I would also recommend Steve's SSV for more serious approach. With it you could get rid of the variac for good.
Marko, Sat Sept 22 2007, 09:40AM
Simplest thing you can do is to use a PTC in series with your supply. It needs some time to cool so it isn't for fast power cycling but otherwise it should do the job. Use more than one in series/parallel depending on your wishes.
Apart from that, I would also recommend Steve's SSV for more serious approach. With it you could get rid of the variac for good.
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