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Re: Science Fair Project
From: ms291 [SMTP:ms291-at-mindspring-dot-com]
Sent: Wednesday, November 12, 1997 11:19 AM
To: Tesla List
Subject: Re: Science Fair Project
>
> I know that this is probably a bumb questions that shows my ignorance, but
> what is "12 volt square wave" . I get the idea it means just feed it
> bursts of electricity? I've read you can do this with a 555 timer- Is that
> true, I would think the voltage would be a little high for a 555 timer...
>
Matt,
seems to me the only dumb question is the unasked one...
The square wave gets its name from its appearance on an oscilloscope..
square. :) In this case you want a voltage that goes rapidly from 0 to
12
volts DC stays there for a period of time.. and then drops back down to
0
volts again.
You are on track with the 555 idea.. but to get sufficient energy, you
need to rapidly collapse the magnetic field in the ignition coil (just
like the points or HEI in a car). You also want to be able to provide
some reasonable amount of current in the primary of the coil.
I have a working circuit that uses a 7555 timer (that's the CMOS version
of a 555) a 14538 dual one-shot and a IRFZ40 HEXFET to drive the coil.
The
chips have a max voltage of 16-18 VDC so I can run the whole thing at 12
volts. The 7555 provides the square wave for the base frequency. The
one-shot triggers off the 7555 and allows me to adjust the pulse width
independent
of frequency. The output of the one-shot drives the gate of the FET.
The FET has a very low on-resistance, and allows me to put a nice pulse
of
current through the coil. I haven't tried it yet, but I don't see why
one
couldn't drive two FETs and two coils in parallel with the same circuit.
Once I get a decent cap constructed, I thought it might be fun to see if
I
could light off a small coil with it.. before I try the 15kv 60ma neon I
just got...
:)
good luck!
Jim