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How to get good isolated / idiot proof 9VDC supply from +/-15VDC rails?
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K3mHtH:
So I have a modular synthesizer with a +/-15VDC power supply and I'd like to power something (guitar effects pedal) that requires 9VDC.

I was initially planning to just use a 7809 off the +15 rail to get my 9V power supply, but realized that several of my pedals actually ground the positive rail to get a -9V internal supply. My thoughts was to run the 7809 through a 1:1 transformer, but I've never specced out a transformer before for isolating a DC supply and have no idea where to start.

I'd be looking at supplying about 250mA max.

I'd also like this supply to be idiot proof.. so if I short something out, or draw more than 250mA - I don't want to damage anything, or take down my +/-15VDC supply.

Maybe there's even IC's designed to deliver a 9VDC isolated supply, etc... I'm totally OK with the supply being switchmode too.


Buriedcode:
You can get off-the-shelf DC-DC converter modules, often these are unregulated, and *can* be quite noisy, have fairly poor regulation, but isolated.  It can be common practice to follow the output of these with a linear regulator, even a cheap 7809 (as you mentioned) will do, as this can clean up the line and provide short-circuit protection.  An example would be 12V in 12V out DC/DC module, with a 7809. 

The 250mA output current means 9*0.25 = <2.5W, but I would go for at least 4W for headroom.  Aside from digital/multieffects or boutique reverb pedals, most draw very little current.  I suppose if you can get them cheap enough you could use one DC/DC module per pedal output, in which case you could go with the lower 2W.

If you're ordering from Farnell or Mouser, I would buy a few from different manufacturers (if they're fairly cheap like $4) just to test them and see how noisy they are - the noise on the output can be sorted with a regulator and/or filtering, but EMI, radiated noise, would require shielding so is harder to sort. 

There are indeed IC's designed purely to provide isolated low voltage supplies, often just simple push-pull oscillators, but the transformers they are designed for can be difficult to source.  Also I can't see to find any part numbers.  As this isn't for high volume, I would go with modules.
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