Re: My findings on 555 Ign. Coil spark maker thingys

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Original poster: "anthony butler by way of Terry Fritz <twftesla-at-qwest-dot-net>" <anthony-at-glasspusher43.fsnet.co.uk>

Bob,

The standard 555 timers can only around 200mA of current. Try to do any more
than this it WILL fry. (I've fried a few >g<).

You can limit the output current flow with a resistor of around 470R quite
well. particularly if you are using an emitter follower stage with a 3055 or
somesuch. put the resistor between pin 3 of the chip and the base of the
following transistor.

If you drive an inductive load directly, be aware that the chip will change
state in about 100 nanosecs. A substantial back emf will be generated across
the inductive load which can also fry the chip. You can protect against this
by reverse biasing a diode across the load. This will shunt the back emf.


Apologies if I'm teaching you to suck eggs.

Regards,

Anthony Butler
G7JWH
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