DIY_EFI Digest V5 #518

[Tony Bryant]

>If
>you tried to switch HV DC directly to the coil, you'd have to use
industrial
>switching transistors rated for at least 600V, with a substantial HV diode
>behind it so the back EMF from the coil (at switch off) doesn't instantly
>destroy the transistor.  These devices are larger, more expensive,
disappate
>more power and require more drive than an SCR.  At least, this is my under-
>standing of the issue.
>


In a normal ignition system, you need a 600V rated transistor, and a HV
Zener
anyway!

600V transistors are trivial to find. TV horizontal flyback transistor are
often rated
to 1500V!

I'm using a 2N6678 which cost me all of 30c from a surplus place. For my
next
ignition I'll just use a 600V 6A mosfet, which from my local "radio shack"
equivilent costs US$4. Hardly a show stopper.

The HV zener is easy too, just use enough is a row, and feed the bottom to
the
gate/base, so the transistor absorbs the back BMF.




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