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DRSSTC Build again

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colin heath

Fri Jun 12 2009, 09:31AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

I guess I should have changed thread :) I was going down the resistor route for variac but then thought it would be a nice project to build the solid state version and ditch the variac for running this coil.

So what do you think of my cunning plan?

Ok so my plan now is to again to use whats available to me. I was going to
feed the synced PWM signal from UC2846 into a pair of UCC37321/2 chips on
the enable pin. Then feed a constant 4khz pulse onto the input pin so I will
get the 4khz pulses only when chip enables at correct section of phase. This
would feed a pulse transformer driving the gates of thyristor for isolation.
Sounds ok but reckon it would work? I am assuming small time delays with
chips etc isn't going to be major issue here as driving at 50hz.

Just trying to think of a way that's nice and simple although maybe not the
lowest part count or sleekest way of going about. I could ditch gate drive
chips and go with transistors but I think at my stage would like the
simplicity.

How does that sound please?

Steve Conner

Fri Jun 12 2009, 10:36AM

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

It sounds more cunning than a fox who was chair of Cunning Studies at Oxford University.

You might not need UCC chips, as SCRs are much easier to drive, they don't need anything like the abuse we dish out to MOSFETs and IGBTs.

My previous circuit just used two 555 timers:

I wound the pulse transformer myself for this one, it drove a Semikron 90A SCR brick. I ditched this circuit because of the difficulty of soft-starting and remote controlling it. But no reason why you couldn't copy the second 555 part.

colin heath

Fri Jun 12 2009, 10:46AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

LMAO! Everybody loves Black Adder :)

I am working on drawing now so will post up when complete for a sense check

colin heath

Fri Jun 12 2009, 03:17PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

OK to simplify things further I am thinking of slinging the uc2846 chip altogether as I cannot get any idoits guide to hooking the damn thing up to work! So I was thinking of using a hex inverter (again in the parts bin) and run through 2 stages to ensure correct signal. Not sure effective that would be though and tracking the phase? maybe some kind of balancing biasing would be needed to give a closer representation of the time window and zero crossing etc.

Once tracking the phase then feed into 555 timer which then delays the signal to give pwm. Not sure how effective this would be?

Failing this does anyone know of a good chip that will sync with mains input and carry out pwm that has decent literature for the layman. the UC2846 paper just tells whats in there with little application help.

Either that or i need to do more homework on getting the original chip working. I think best thing at this stage is to breadboard the lot and start scoping :)

Steve Conner

Fri Jun 12 2009, 03:22PM

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

Yes, the UC3526 that I used. :P

This page

shows where the "SYNC" signal comes from.

One problem I ran across is that the phase shifts the wrong way to fire SCRs. I had to invert the PWM output, and this made the chip's shutdown and soft-start functions useless, because they then started off at 100% and ramped softly down to 0. The rats' nest of wires between the UC3526 and TL494 are me using the TL494's error amps to make a new shutdown circuit.

(this is probably why you won't find any appnotes for phase angle control with these chips)

colin heath

Fri Jun 12 2009, 03:25PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

aha :) thanks steve. Ok perhaps I shouldn't work on reinventing the wheel at my stage of learning. I will have a decent look at your drawing tonight under the influence as it seems the only way for me to make sense of this stuff lol

101111

Fri Jun 12 2009, 03:58PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

Steve McConner wrote ...

The only problem with it used to be that you had to remember to turn the DC bus voltage knob down to zero before plugging it into the mains, or it would come on at full power and blow the fuse. I was happy with that for a while, but lately I've wanted to operate it with a footswitch that broke the mains for safety reasons, so I needed the coil to just start up at a preset power when the mains was connected. So I've modified it to include a soft-start that gets round this issue.

Hey, a little question: Do you have a updated schematic with the new modification? The modification sounds interesting because I am currently making making your DRSSTC drive including the SSV.

colin heath

Sat Jun 13 2009, 07:37PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

OK a little update. I have decided to do the whole thing with 2 555 timers and some ucc37321/2. I am using Steve Connors design as a basis althoug mine will differ slightly.

I hope I understand the design he has so far. I have my first 555 running as his does except I have used SN74HC14 chip to clean up sync pulses from dc line instead if schmitt trigger formed with transistors. again this is due to what I have to hand. The square pulses synced from AC line then get fed into trigger on 555. Then have wired the rest of timer as Steve has. Only other difference is the output pin 3 goes to enable on ucc chip. the other 555 will be astable with adjustable on and off time to get the hard firing pulses correct for transformer(as Steve did) this will feed input pin on ucc. So this should work and I have half of it on board so far (strip board) Only thing is as with steves design have to remember to turn pulse width right back down before power up. i am working on a ramp up control for soft start but that will come later.

Anyway not much more will be done before hols now so will be a few weeks time before back to it. may get chanced tomorrow if i am lucky

colin heath

Sun Jul 19 2009, 10:59AM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

OK I have my soft start built for the variac, well it's just a huge resistor 70ohm 500W and timer with contactor. It all works and variac doesn't pull breaker anymore.

I now have the FET in place to allow current limiting and have zeners across gates of bridge. So I am just waiting for a break in the weather as this things output is too big for inside garage safely.

Hopefully tonight may be the night for a full power run :)

Paul Benham

Sun Jul 19 2009, 04:25PM

Registered Member #570 Joined: Wed Mar 07 2007, 03:41PM
Location: Winchester Uk
Posts: 42

Hi Colin,

It's a tad windy today!

Good luck if you do fire it up.

Cheers,

Paul.

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