Whipper Snipper Project :)

[Tom Parker]

Wes Branchflower <hemi_265 at hotmail.com> wrote:

>I'm relatively inexperinced with electronics. I'm now wanting to use a
>transistor to switch the current to the coil, instead of the slow relay.

>I have a 2N3055 PNP transistor, which is rated at being able to switch
>15AMPS.

>Is this all I need to switch the coil? Is it that simple? Are there special
>automotive grade transistors for this purpose?

The coil is not a simple "spark making" black box. It is, literally, a coil of
wire and has significant inductance. You can't simply switch it on and off,
and I am a little surprised you didn't destroy your relay with it, you
probably damaged it.

The problem is that when you turn off the current in a coil, the coil will
create a voltage that tries to keep the the current flowing (Lenses law). This
will likely destroy your transistor. A design I have here uses a MJ10012
(darlington, I think) transistor and a 450V zener diode as protection. This
protection diode is very important, as is the voltage rating on your
transistor.

Go to motorolla's website and look up the MC3334 ignition controller IC. This
ic is designed to manage a coil and do exactly what you want to do, it will
accept a logic level input and do the dwell and transistor drive for you.

There are some schematics on diy-efi that use the MC3334 which should give you
an idea what is needed:

http://www.diy-efi.org/diy_efi/projects/ddis/ddis.htm

--
Tom Parker - tom at carrott.org
           - http://www.carrott.org

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