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

Hi Bill, All,
    A friend of mine from long ago, who designed switching inverters,
told me +ACI-to hit the gate of the SCR as hard as you can without cooking
it+ACI-.  He explained that the more area of the die that was turned on
the less the possibility for hot spotting and a dead die.  I have
found by trial that a 50 Ohm resistor between the gate and ground and
a resistor and capacitor between the gate drive pulse source and the
gate gives good service.  The resistor and capacitor values are around
0.1 to 0.01 uF and less than 10 ohms.  The waveform is an overdamped
double exponential, i. e., less than 1 us rise time and 10 to 50 us
fall time.  10 volt 1 watt back to back zeners are a nice touch across
the gate also as long as their junction capacitance doesn't degrade
the pulse risetime.

-----Original Message-----
From: Tesla List +ADw-tesla+AEA-pupman-dot-com+AD4-
To: tesla+AEA-pupman-dot-com +ADw-tesla+AEA-pupman-dot-com+AD4-
Date: Sunday, January 10, 1999 9:38 PM
Subject: Re: SCR based coils (was High-Power Car Ignition Coils)

Original Poster: +ACI-Bill the arcstarter+ACI-

Eddie Burwell +ADw-eburwell+AEA-HIWAAY-dot-net+AD4- wrote:

+AD4-I've been working on an SCR based coil for a while now and hopefully
soon I
+AD4-should be posting a webpage about it. My number one resource for
+AD4-SCR Manual
+AD4-Sixth Edition
+AD4-General Electric
+AD4-I belive it's available through amazon-dot-com.
+AD4-I also have found the Powerex SCR databooks to be of use.
+AD4-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
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
design is less than reliable. :(  I'm not sure if I've been starving
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
anode into the gate...


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