Richards, Justin P wrote:
>
> G'day,
> Any device I met (68HC11 and off the self LCD voltmeter) that does ADC seems
> to require an independant power supply from the voltage to be measured. I
> want to use it in a car and just hate chewing thru 9 v batts when there are
> 2 big 12volters up front.
>I thought this would be easy by doing the
> following (which I have unsuccessfully experimented with).
> a 555 IC running at almost any freq (100hz I tried). The output of 555
> driving the base of a transistor. Collector to +12v, Emitter to primary side
> of 1:1 transformer, then to ground. My limited theory tells me that just as
> long as it is AC looking a current will be induced in the secondary which I
> rectify and use a big cap. This was to be simply followed by a 7809. It does
> not need to be efficient, and a 20mA load is about all I need to drive
> Sounded simple enough and sound enough, but have had no real success having
> these problems
1. The voltage on secondary continues to ramp up which I
> monitoured with excitement until at 45v I remembered that volt ratings on
> caps should not be ignored.
2. Put a bit of a load on (1k) and then it did
> not want to go above 1.2 volts.
3. parts getting hot which I tried to  eliminate with resistors.
> What I want to know, is the theory sound and just a case of tuning freq, the
> transformer size, cap size, load tweaking etc to get a nice steady DC supply


Yes it should be an easy one. :o)
20mA is not much, you can just slap a 5.1v zener
on the output cap. This gives a convenient max (and
ALWAYS constant) current so you can just tune the
circuit frequency to the max efficiency to supply
your fixed current into a fixed 5v load voltage.

You can use a cheap 1:1 transformer from any
hobby shop, these are tiny (thumbnail size)
and my cataloge shows one at:

3kohm:3kohm 16mm 16mm 15mm 450mW 220ohm Rdc
May be too high resistance, but it has plenty
of power capacity in that sized package at
450mW. You only need about 150mW maybe? I think
it would work.

You can use a IN4148 for the output rectifier
diode, set the transformer up as flyback with
the rect diode, you may need to reverse the
tranformer output leads to get it in flyback
mode. Then stick a pot on your 555 timer and
crank up the speed until you get the required
5.1v on the zener diode at 25mA or so, using an
ammeter on the input and output. Since Vin and
Vout shoult be constant you can just measure
Iin and Iout and turn the pot until it's at
its most efficient.

I think your idea is pretty clever as a cheap
way to get isolated 9v from 12v. It will probably
run quite efficiently at a few kHz.
-Roman

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