G'day,
had a look at Romans smps hoping it would solve my problem which is ..
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). [Additionally I
would to like power a payphone from a 12v solar radio site. The payphone
power must be independant of the telephone LINE power all of which is
powered from the same bank of 12v batts. The payphones were designed to be
powered from 21v AC which is done with a 240v to 21v transformer behind the
fluro and thus magnetically decoupled.]
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
? Or am I barking up the wrong tree and a more complex cct required.
Justin
PS Is there a nice, easily available drawing package for doing quick simple
schematic diagrams so I could have drawn a cct for above. I have used orcad
but dont know how to paste into a mail msg and are a little overcomplex,
paint is not really suitable, drawing tools in word is ok but still a little
cumbersome.


Justin Richards (T388)
IS&W Meekatharra (Northern Remote)
'  0428 921 153
6 08 9980 1255
* Justin.Richards@team.telstra.com

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