Re:Triac Power Control

To: tesla-at-pupman-dot-com
Subject: Re:Triac Power Control
From: "Bill the arcstarter" <arcstarter-at-hotmail-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
Date: Sat, 05 Sep 1998 11:29:42 -0600
Approved: terryf-at-verinet-dot-com

It was written:
>If I use only resistive ballast (heater elements) to limit my 5KW 
>pig, couldn't I also use a big Triac for power control?  I know 
>Triacs don't like highly reactive loads (like pole xfmrs!), but it 
>seems like the heater elements should smooth things out enough for a 
>reasonably rugged Thyristor to survive.  Of course, the resulting AC 
>would be messy, but Tesla coils don't require "clean" power, do they? 
> I'd use something like a 40A / 600V TO-3.  I realize such a control 
>won't give true variable AC voltage like a Variac, but it should 
>provide adjustable input power.  Has anybody tried this?

Greg, 
 About 6 months ago I started a thread along these lines.  Search the 
archives for the specifics, but here are my conclusions:

* Yes, it can be done.  My employer uses SCRs to phase control power 
flowing into an unballasted 60KW transformer.  You will have to use 
back-to-back SCRs as opposed to triacs due to a triac's poor reverse 
recovery rate.

* A proper snubber circuit (look in the data books) will allow the use 
of scrs with highly inductive loads w/o much problem.  The key is 
reducing the dv/dt across the device at shutoff...

* You will have to go to extremes to keep the HFRF out of the scr 
trigger circuits or all hell will break loose!

* Remember - the failure mode for most solid state devices involves 
going into a short circuit condition.  This means that full unrestricted 
power will be sent to your pig if your parts fuse.  This being the case 
-I suspect you would be wise to incorporate additional ballasting to 
limit the max draw under these conditions.

* D C Cox apparently tried this years ago, and he didn't like the 
failure mode aspect.  Neither do I, so I didn't ever build one.  

Thats all I have to say about that... :)
-Bill

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