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

danial stocks wrote:

>Normal SCR's will switch on only with positive gate current. The max 
>pulsed value of gate current permissible should be quite a good deal 
>more than cont. gate current rating - check the spec on the device you 

Very true.  Documentation from GE suggests that a x5 of the minimum gate 
trigger current is stringly encouraged, particular at low temps for with 
heavy loads with possibly large di/dt (cap discharge).

>off is higher than the gate current for turn on. TRIACS are AC switches 
>and not only will conduct in either direction but can also be triggered 
>by either polarity of gate current - as there are 4 possible operating 

Yup.  However, standard triacs aren't good for inductive loads.  You 
must use a pair of inverse parallel SCRs, or an alternistor-type triac.

I have abandoned the triac approach for the more traditional discrete 
phase control SCRs.

>may req around 100ma for trig , hockey pucks can require substantially 
>more than that. I think a higher gate current will lead to faster turn 
>on time, but di/dt rating should be observed - External circuit 
>inductances will have a large effect on this, with some very large scr 
>systems I have seen using di/dt limiting inductors on each device.

Again - very true.  The HF x-ray converter uses these techniques, 
including a 6-turn air core 3 uh limiting inductor.  It is essentialy a 
capacitive discharge into a transformer core, at about 6-10 KHz, 300-500 
amps peak.  This converter has a whole array of the "Press-Pak" 
hockeypuck SCRs! :)

Any clue what the max reverse gate to cathode voltage should be?  It 
isn't spec'd in any of my books - and I exceeded it bigtime last night - 
killed one of my scrs in the process... :(


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