Re: SCR based coils (was High-Power Car Ignition Coils)

Eddie Burwell <eburwell-at-HIWAAY-dot-net> wrote:

>I've been working on an SCR based coil for a while now and hopefully 
soon I
>should be posting a webpage about it. My number one resource for design 
>SCR Manual
>Sixth Edition
>General Electric
>I belive it's available through amazon-dot-com.
>I also have found the Powerex SCR databooks to be of use.
>Check out www.pwrx-dot-com

Unfortunately the GE SCR manual appears to be out of print. :(

I'm trying to do phase control of an inductive load (welder).  See the 
following circuit:

The question is how to size the gate resistor.  Too large a resistor 
won't allow a quick turn-on of the part, but too small a resistor would 
allow a possibly large gate current.  This problem appears to be 
aggravated by the fact that the voltage across the resistor depends on 
the firing angle.

If anyone has any comments on that - I'd love to hear them.  My present 
design is less than reliable. :(  I'm not sure if I've been starving the 
gate or not.

(The circuit doesn't show the 100 ohm 0.1 uf snubber which is attached 
across the triac terminals)

Now I know why most industrial designs use a small pulse transformer 
between the cathode and the gate - rather than shunting current from the 
anode into the gate...


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