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Shaun

Wed Sept 19 2007, 02:48AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I'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?

Bennem

Wed Sept 19 2007, 05:51AM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

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.

Steve Conner

Wed Sept 19 2007, 12:28PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

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)

Terry Fritz

Wed Sept 19 2007, 12:28PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

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

Shaun

Wed Sept 19 2007, 10:20PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

wrote ...
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?

Shaun

Fri Sept 21 2007, 03:58AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

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?

Steve Conner

Fri Sept 21 2007, 02:57PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You are correct, the soft start is only to prevent blowing breakers and fuses.

The schematic you wanted is on the forum somewhere.

Marko

Sat Sept 22 2007, 09:40AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

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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