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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