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

two ideas - 1, use a current source rather than a simple resistor (just
takes another transistor to do that),
2, use a capacitor to dump constant energy into the gate when the time is
----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, January 10, 1999 12:47 PM
Subject: Re: SCR based coils (was High-Power Car Ignition Coils)

>Original Poster: "Bill the arcstarter" <arcstarter-at-hotmail-dot-com>
>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...
>Get Your Private, Free Email at http://www.hotmail-dot-com