DC-DC converters

A follow up from my "Converting PCB from AC to DC" thread ... we have a
control board that runs off AC which is rectified and regulated to +12VDC
and -12VDC. I am converting this to run off a +12VDC regulated supply
instead of the AC and following the recommendations to use a DC-DC converter
to get +/-12VDC from a 12VDC input supply.

Question: DC-DC converters with come in 12VDC single output or in +/-12VDC
dual output. Is there such thing as a single channel -12VDC converter?
With this I could use the existing +12VDC input as my +12VDC 'leg' and use
the converter to get the -12VDC 'leg'.
that I need 100mA on the + leg and converters that supply this much juice
have a minimum rating of 12mA. At times my - leg only draws 7mA.

What to do?

 

Of course. You need an isolated 12 V to 12 V converter. Connect the

output + to ground, and the output - to the -12 V load.

Jon

 

I was hoping that's how it worked. I read the spec sheets etc but they
never actually come out and say this.
thx

 

The only specs necessary are that the output is isolated from the
input and the isolation is good for a lot more than 12 volts.

 

Afraid so. You need something like a pair of 13 volt zener diodes
across the supply, to derive a rough center voltage between the ends
of the 24 volt rails. I think that if you trace the current through
the series regulators you describe, it would be required, under some
circumstances for the current to go the wrong way through the pass
transistor.