Controlling SCR firing board: PWM or potentiometers

Jun 26, 2006 7 Replies
I

I have an SCR firing board controlled with 0-5V potentiometer input and separately a microcontroller. This microcontroller can output



0-5v.

I have a dilemma. I can either:


1) Let the pots control SCR firing board output, and let the microcontroller simply turn it on and off


2) Let the PWM outputs (filtered to DC) control the SCR firing board.



Which is best? In the long run, 2 is more versatile and could allow me to do fun stuff like TIG pulsing, "Lift Arc" starts without high frequency, etc, with just some BASIC programming.



On the other hand, it is a little more dangerous if I make a mistake in code (which sometimes happens ;).



A common sense approach would be to wire the board so that I have two sets of outputs, PWM and pots, and connect SCR firing board to pots first. When I get some tig welds done, etc, I could experiment further with PWM control of the board.



What am I afraid of? That I make a mistake and programming and somehow the welder would go very crazy.



Is what I am saying, making any sense? Any words of wisdom?



i
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T

Have someone else, preferably someone you don't really like, test new code for you?

Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
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I

That's a good idea, but no, no one else tried it.

Anyway, I got the PWM output to work. (meaning that there is output, not that the SCR controller likes it).

My programming is basically done and so is wiring of this control board. I will do some final testing tonight.

i
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L

I'm not sure just how this may apply to your situation.

I do all the fancy control stuff with micros, and all of the safety stuff with simple electronics or mechanics. In other words, design so you can survive a microcontroller failure.

Luhan

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I

Yeah. I really like your approach.

What safety stuff do you mean that you do with simple electronics or mechanics?

I definitely want to first use regular pots for voltage and current control, until I get my first welds.

i
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L

I've used microswitches to directly cut motor power at the travel limits of servos.

Fuses or circuit breakers for maximum current cutouts. In some instances, incandescant lamps in series with power feeds: their resistance swings up under high current loads.

Ground Fault Interruptors feeding power to my test bench outlets.

I don't know your entire situation - just keep any safety stuff as foolproof as possible.

Luhan

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I

Thanks Luhan. I will definitely try to follow those approaches where possible.

I also have a lot of code safeties, assertions, watchdog (thanks to Rich), making sure that I do not hang the code, one loop for everything, etc.

i
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R

Do some PWM until you're confident that your code is OK, then divert the pot outputs to inputs of the uC and have it decide who's in charge - the pots or itself.

Of course, this exacerbates the potential problems of non-robust code. ;-)

Good Luck! Rich

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