On Tue, 27 Dec 2005 02:02:15 +0000, 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.

Probably the easiest is to obtain one of the original power supplies.

Most installations using the RT-70 were in conjunction with an RT-66/RT-67
or RT-68 and an audio distribution amplifier. A separate vibrator power
supply was supplied those cases where the RT-70 was used 'stand alone'.
One approach to homebrewing a vehicular power supply is to mimic the
original vibrator supply. Provide the heater supply through dropping
resistors and emulate the vibrator supply by building an inverter using a
pair of switching transistors and a junk 60 Hz power transformer.
Regulation isn't necessary if you get the right transformer. The only
thing the least bit tricky with this approach is properly snubbing the
primary leakage inductance to prevent destroying the switching transistors.

Probably the most compact approach is to build a flyback supply with both
a six and 90 volt output. Regulate the 6 volt output and let the 90 volt
output track. The advantage of the flyback supply is that it can be
designed to operate over a wide range of input voltages while maintaining
a regulated output. In addition, the overall size will be much smaller due
to the high switching frequencies (300 kHz typical) used by modern flyback
controller chips.

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.