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

Tesla List wrote:
> 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
> has
> >been:
> >
> >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:
> http://www.geocities-dot-com/CapeCanaveral/Hangar/6160/temp/triac.html
> 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)

You probably want to check your RC snubber, too... Those snubbers have
to be designed empirically (you build the circuit, fire it up, and
adjust the R and C until the triggering works like you want it to). The
primary reason for the snubber is to limit dv/dt when the device turns

Motorola Ap Note AN1048, "RC snubber networks for Thyristor Power
Control and Transient Suppression" has some info on this, and is really
a must read... It is available from their web site (although it may be a
fax back)...

Most designs use back to back SCR's rather than triacs. That way, the
device has a whole half cycle to recover, and, SCR's don't have the
dv/dt retriggering problem that triacs have.

> 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...

pulse transformers are much, much preferred... good isolation from the
timing to the load circuitry, and you can give it that nice spike with a
back porch waveform that the SCR manuals all recommend.

The transformers are cheap too, about a buck from Mouser, etc.

Jim Lux                               Jet Propulsion Laboratory
ofc: 818/354-2075     114-B16         Mail Stop 161-213
lab: 818/354-2954     161-110         4800 Oak Grove Drive
fax: 818/393-6875                     Pasadena CA 91109