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

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike Novak by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmnovak-at-msn-dot-com>

Your 555's are dying because they operate on the same supply as the output
source. To save any future 555's, make two seperate power supplies with a
common ground (one +9vdc, the other +24vdc). Results are much better also...
I don't particularly care for that specific circuit to drive a 555.  I much
prefer this setup:

*
-Pins bridged: 8 to 4; 6 to 2
-Ground pin 1
-Put a 10k potentiometer in series with a 1k resistor, and connect between
pins 6 and 3
-Insert a 10k resistor between pins 7 and 8
-Attach a .01µF ceramic or tantalum cap in between pins 1 and 2
-Connect pin 4 to positive supply (+9vdc well filtered or 9v battery)
-Pin 3 is output to a pnp transistor (2N3906) via a 300 ohm resistor to keep
both the 555 and transistor happy.
-Same positive supply goes to collector on the transistor, and the emitter
drives the base of the power transistor.
-Connect the emitter of the power transistor to the same ground as the first
supply
-Connect the primary of the coil (ignition, flyback, etc...) between the 24v
supply and the collector of the power transistor.
-Sometimes it may be desirable to use a 100 Ohm potentiometer between the
emitter of the 1st transistor, and the base of the power transistor.  this
provides good control of power output.
~Frequency (in my setup) is between 1kHz and 47kHz according to the
frequency counting function on my multimeter.
*

Note: You might have to use a slightly higher value for the capacitor when
using a particular ignition coil... Most coils i've seen have a low resonate
frequency (~400Hz?) and thus, a 555 timer may not be the best (simplest)
solution. Try a 5µF 330volt motor run cap in series with a lamp dimmer and
the coil... this works with 120vac (60Hz).

I always have heat troubles with my flyback setup. Right now I'm using a
huge heatsink with a cooling fan and tons of thermal paste along with a
thermal shut-off switch (VERY USEFUL). I would HIGHLY recommend using a
thermal shut-off switch (opens circuit at "unhealthy" temps) for any high
power transistor driver.  Also, I use a 2N6457 which is rated at 200 watts
where the 2N3055 is only rated at 115. The result is much less heat being
generated by the transistor itself.

Just thought I'd let everyone in on what's going on and hopefully no more
innocent 555s will be killed :-))

-Mike