Aha! I think I found some of the units you were referring to. Isolated DC-DC converters. Murata seems to be a big maker of these. Perhaps something along the lines of the MEE1S1509DC would do the trick, I think. Input voltage (Vin+ to Vin-) min 13.5, max 16.5. Output voltage 9VDC, current 111mA. And small too!

Interestingly, I have a similar kind of thing at the front of my regulator. I take the battery input, which can range from over 10 down to, well, almost dead. And it stabilizes it to 9V. It's a Pololu S10V3F9. If I were to use the Murata when the external voltage is available, then I could place the relay between the DC-DC converters and the rest of the regulator. Dreamy!

See anything wrong with this plan?

And if you have better / alternate products to the one I found, I'd very much appreciate it if you could please point me to them !

Thanks !

 

How big are those ready-made DC-DC converters? And they'll take a bipolar input? Can you point me at one please?

Not that big:

http://www.digikey.com/product-detail/en/RS3-1212S/945-1564-5-ND

You connect it between -7 and +7 so it gets 14V in. You get 12V back which is isolated, so you can refer it to your ground.

You can select a part with any output voltage and it can replace your existing regulator as well (unless you have a reason to keep battery tied to your ground).

 

See anything wrong with this plan?

Our posts crossed.

I would connect battery (in series with a diode) parallel to -7V/+7V source (in series with a diode too). The two diodes would make sure they don't feed each other. -7V/+7V would always win over the battery when present because it has higher voltage. Then I would connect these two to the input of the converter.

This way you can remove your old regulator, which should save you some space too.

 

Well, my regulator actually feeds a 3V to 9V input into an LT1054, which converts unipolar to bipolar, with +/-18V outputs. Those go to a LT1085 and a LM337, which step those to +/-15V respectively. Then caps, and a TLE2426 rail splitter for virtual ground. There's also an L7805 for 5V power to another unit, with a different function. So to sum up, no, I'd just as soon keep the regulator. But I'm happy to bypass the Pololu step-up-step-down, or parallel it with the new DC-DC converter.

The only problem with paralleling it, is that then the battery would still be feeding the Pololu, because the relay would be "downstream" from it. So I prefer your idea of paralleling the power sources into the new DC-DC converter, and then from there into the rest of my regulator.

I'll have to get an order to Mouser together. Relay, DC-DC converter, rectifier diodes. And, naturally, a few other odds and ends too.

Parting thoughts ??

 

You can look at the module which produces the isolated voltage you need (-15 to +15) directly. This way you can get rid of all this stuff except 7805.

IMHO: Two diodes - converter module - 7805 - that's all you need.

 

Found this 4.5-18V to +-15 DC-DC converter:

http://www.digikey.com/product-detail/en/IP1215S/1470-1512-5-ND