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Re: 555 timer driver for ignition coil

Although this circuit works after a fashion, there are a couple of potential
problems with it, either of which may  be what you are experiencing.
1. The 555 is being asked to source too much current, although some 555s
will cope with it, not all will. You could do a couple of things about this,
one is to raise the value of the base resistor from 100R to maybe 470R. An
alternative is to add another npn transistor buffer stage, this only needs
to be a general purpose type (e.g. 2N2222A or BC108).
2. Another problem is the back emf produced by the primary of the coil, this
can be several times the applied voltage. The 2N3055 will cope with this
ok,but the 555 has a max working voltage of 18V. I would suggest one
possible way round this. Connect the positive rail to the coil, and from
there take a 100R resistor to the 555 part of the circuit, then from the
junction of the 555 and the supply connect a 1A rectifier diode (e.g.
1N4001) to ground (cathode to ground).
----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, September 02, 2000 12:02 AM
Subject: 555 timer driver for ignition coil

> Original poster: "wsc1914" <wsc1914-at-hotmail-dot-com>
> I have finally suceeded in getting the driver circuit
>  http://www.geocities-dot-com/CapeCanaveral/Lab/5322/coildrv.htm ) for an
> ignition coil to work. However, when I disconnet it and connect it the 555
> timer chip sometimes burns out and fails to work any more. What should I
> I heard somewhere that I need a filter, but what on earth is that, and how
> do I connect it?
> Any help would be gratefully received at wsc1914-at-hotmail-dot-com !
> Many thanks!
> Oliver.