Re: SCR/Triac pig controller? (fwd)

---------- Forwarded message ----------
Date: Wed, 20 May 1998 23:18:58 +0300
From: Harri Suomalainen <haba-at-cc.hut.fi>
To: tesla-at-pupman-dot-com
Subject: Re: SCR/Triac pig controller? (fwd)

>>The basic problem is that once the spark gap fires the load is like
>>a short circuit. This will kill the controller.
>But the extra inductive and resistive ballasting (same as needed on 
>variac powered pigs) will reduce this short circuit current to a 
>tolerable level.

OK, it does. I did not know wheather you still intended to have them
(easier way) or not (the hard way).

Peak current is then tolerable. With suitably large inductance current
rise time (di/dt) is also limited. That blows up large SCRs too even
with small currents!

>I picked a particular phase-control SCR out of my Int'l Rectifier book - 
>the 80R1A40.  This stud-mounted component will carry an average rms 
>current of 125 amps, with a Itsm of 1700 amps.  If my understanding of 
>this rating is correct - this component will survive a momentary (one 
>half cycle?) pulse of 1700 amps.

Databook values vary. Times can be different but usually it is for
*nonrepetitive* pulse of one half cycle.

>>A proper approach would be to detect the overload and then 
>>commute the tryristor/GTO/whatever off before current has got too 
>How the heck do you commute an SCR off before the end of the half 
>cycle???  Please give me a hint!  Is this technique used in the 

Basically you include a charged capasitor in parallel with the scr. When
another scr is triggered current will come all from the cap for a short
moment and the heavier scr will turn off. Designing those is not fun
and component count gets large.

>>Further complications would be stuff like imbalance in the 
>>transformer core which might saturate it if this is not handled with. 
>>Some other problems exist too.
>Any way this can be handled w/o having an extra sense wire on the 
>transformer core?? :(  My pig didn't come with one! :)

This is the difficult task. How much imbalanse is allowed depends on
the transformer and from the driver. Timing differences etc will
cause some imbalance. If you're lucky plain mains-syncronized
firing of SCRs will do. If not you deep into problems.

Syncronisation point might then need some minor adjusting according
to the current going to the transformer. Try things out and see how
it will perform. Do apply multiple pulses to fire SCR's to make sure
they're fired even slightly delayed if they fail to fire for some reason
at the first pulse.  Do consider the current rise times to make sure
current has rised to high enough level to keep scr on after firing

Then there is only one problem to be solved: snubbing. Having pretty
heavy snubbers are a must. Simple task to do probably.

Like I said: I'm sure it can be done without huge effort. It would be
smaller and nicer than variac. Still I think reliability and cost issues
make variacs simpler. Blowing up even a few handfull of heavy
scr's during development might cost quite a lot!

>Thanks for any suggestions.  Perhaps we should continue this discussion 
>off the list?  I'd be interested in your comments along these lines...

I prefer list. Many other people might have good comments on subject
too. Others might be interested in this one also.

Harri Suomalainen     mailto:haba-at-cc.hut.fi

We have phone numbers, why'd we need IP-numbers? - a person in a bus