On Tue, 27 Dec 2005 02:02:15 GMT, "James F. Mayer"
wrote:

I need to generate 6v DC and 90v DC from a 12v DC automotive electrical
system to power an RT-70A/GRC surplus military radio. I need about 250 mA
at +6 volts and about 75 mA at +90 volts. I was thinking about using the
guts from an old battery back up but it would be a bit of a kluge. Are
there any 90 volt regulators in the 78xx series? How do I get the voltage
up to where I can get something that I can get the 90 volts from. Getting
the 6 volts doesn't seem to be a problem. A 7806 off the battery should
work for that unless any of you can see a problem doing that. Maybe the
common common would be a problem. Right now I'm running it off of an HP6299A
and an HP6236B with commons jumpered. I'd like to be able to go portable
with it.



Hello James,
Speaking as a cheapskate ham/hobbyist, how
about salvaging parts from a computer power supply.
Find a dud power supply for free at computer
shops/dumpsters/rubbish tips/roadside etc.
Salvage the TL494 integrated circuit that can
be found in many computer power supplies.

You could build a 90 Volt power supply as
Ray Robinson has down here
http://www.shlrc.mq.edu.au/~robinson...verter_4W.html

Here is a link to the TL494 Data sheet
http://focus.ti.com/lit/ds/symlink/tl494.pdf

This link tells you how to use the TL494 IC
http://focus.ti.com/lit/an/slva001d/slva001d.pdf

Use another TL494 in a buck converter for your
12 Volts to 6Volts power supply module.
Look at the bottom of this page for more info.
http://focus.ti.com/docs/prod/folders/print/tl494.html
Your 7806 is probably a better idea, saves a lot
of messing about.
Or
you could use the guts of a mobile phone
car charger. The charger that fits into the
cigarette lighter in a motor car.
You can find them in pawn shops for a dollar
or two. Dud ones even cheaper. Usually
just a broken connection from rough handling.
http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF
Look at page 7 Figure 10 Step down converter.

In the few car phone chargers that I opened,
the IC was a MC34063 and the schematic was
very similar to that shown in figure 10
Just change R1 or R2 slightly so that you
get 6 Volts out.
You may have to check/change the value of
resistor Rsc to be closer to that shown in figure 10
You might even get away with not changing the
inductor. Try it and see if the original inductor
works well enough for you.

A ton of reading, bits and pieces for free
or dirt cheap.
What a great hobby, this is James, eh!

Regards,
John Crighton
Sydney