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

>Original Poster: "Bill the arcstarter" <arcstarter-at-hotmail-dot-com> 

>The question is how to size the gate resistor.  Too large a resistor 
>won't allow a quick turn-on of the part, 

..or won't allow turn-on of the part at all!

>but too small a resistor would 
>allow a possibly large gate current. 

Large currents are preferred. You should check the data sheets for mininum
current needed to trigger the device. With large SCR's this may be around
100mA or so!

Current required depends also on polarity. Typically negative currents needed
are smaller than positive current. Negative current is therefore preferred.

Then there is the current rise time: scr will shut down when current falls
certain level. This means also that during turn-on pulse current must rise
above this value or scr will turn off as soon as the trigger pulse ends.

>This problem appears to be 
>aggravated by the fact that the voltage across the resistor depends on 
>the firing angle.

One possibilitity would be using something like a cap charged by the rest
of the system as a "floating supply" for trigger current. There would be no
problems then.

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

Snubbing is a must. Without suitable snubber inductive spike at turn-off
can kill the scr. If it does not kill the scr there can also be very high
which is coupled to the gate through small parasitic cap. This can cause
spurious turn-ons.

These subjects are covered nicely on some SGS app notes available at
their web-site. (Sorry, no url)

>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...

That simplifies things. Why would you not use one?