A repost

[Bruce Bowling]

A couple of people said they missed this, so here it is
again:

A simple fuel injector cleaning circuit. The injector is operated at
a very high rate, the hope is that it helps "bang" off of the varnish/crud
off of the pintle. One can back-flush the injector while it is running,
thus helping the cleaning process. 

> 
> 
> 
>   +12       +12                   +12
>    |         |                     |
>    |         |                     |
>    |     ----------             Injector
>   30K    |  4,8   |                |
>    |     |        |            b   |c
>    |-----|7 555  3|----330ohm--- TIP120
>  130K    |        |               NPN
>    |     |6,2 1  5|                |e
>    |     ----------                |
>    |      |   |  |                gnd
>    --------   |  |
>    |          | 0.01uf
>  470pf        |  |
>    |         gnd gnd
>   gnd
> 
> 
> The 555 timer is running at 10 Khz, at about 55% duty cycle. The drive transistor
> is a TIP120 Darlington (NPN). Due to the duty cycle, the drive current should
> be fine for any injector 2 ohms and up.
> 
> All parts are stocked at Radio Shack, and the whole circuit will cost under $10.00, 
> including perf board. This is as DIY as it gets.
> 

BTW, Tom, you are right - the rating of 5 volts for the TIP120 is
the emitter-base reverse voltage, so the above circuit is O.K. I
did not put a zener or some other back-biased diode across the transistor's
collector/emitter (used to keep blowing up the transistor from inductive
spikes) simply because I surmise that since the injector is not actually
on for very long, there is not much of a kickback. I have run the
circuit without problems (and the transistor costs $1.39), and if
I end up blowing up a transistor, then I will put in the zener.
Some may want to put it in beforehand - it cannot hurt anything.
Use like a 30 volt unit, at least 1 watt. Connect either across the injector,
or the transistor (collector/emitter) - its your choice.

- Bruce
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                 Bruce A. Bowling
                 Staff Scientist
   Thomas Jefferson National Accelerator Facility
    12000 Jefferson Ave - Newport News, VA 23606
                 (804) 249-7240
bowling at cebaf.gov  http://devserve.cebaf.gov/~bowling
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