[sdiy] Isolating a +-15v/+5v PSU?

[Roy J. Tellason]

On Wednesday 26 July 2006 09:37 pm, Jeff Farr wrote:
> I'm working on PSU schematic for my 808 clone today.  I've been
> sticking with the design of the CGS supply and adapting it to provide
> +5v as well, which seems as simple as adding a 7805 to the mix.  While
> researching I found this interesting little snippet about how to keep
> these two supplies seperate from each other at piclist.com:
>
> "Here is a simple idea that works really well to isolate noise
> generating sections of a circuit from sections that need clean power -
> it uses just 3 diodes and 1 extra capacitor. In the common leg of a 3
> terminal regulator you add a diode that raises the regulator's output
> voltage by the drop across the diode. You then fit 2 identical diodes
> to the output from the regulator to drop the voltage back down to
> where it would have been. Separate smoothing capacitors are then
> fitted to the 2 separated outputs."

Sounds good.  But if isolation was that important to me I'd just use separate 
regulators.

> Seems logical enough, but I still have a few questions.  First can the
> 7805 really handle +15DC at Vi?

As far as I can recall (though I'd have to look to see whose datasheet I'm 
remembering) you can go up to 35V on the input of those,  and up to 40V on 
the input of most of the rest of the 78xx parts.  What you need to watch 
though is your power dissipation,  which is gonna be (Vin-Vout)/Iout and they 
can get a mite warm at times.  My first attempt at a supply using a couple of 
7805s to drive a bunch of TTL and several digits worth of LED 7-segment 
displays cycled these -- they'd hit the thermal limit,  shut down,  come on 
again after they cooled off,  etc.  Not good for the longevity of the part.

> The national datasheet specifies a working range of 7-20v (AC?), but I seem
> to remember having to be careful with how much juice I applied to some
> Midibox projects (which this is powering), it was likely DC voltage tho, and
> I figure that is why.  So, if the 7805 can't handle high Vin's than the
> above technique won't work, right?

See above.  I'd look for some more datasheets and see what they say.

> In this case I have to connect the '05 before the other regulators and add
> smoothing caps to it, but where to connect?

Before them?  What were you gonna feed it?

> Will I get better isolation by giving the digial PSU it's own
> rectifier, or will I be fine using the same rectification for both?

Some stuff that I've seen uses a separate transformer and winding for 
low-voltage digital stuff and everything else.  I suspect that this is more a 
matter of giving it a lower input voltage to regulators and thereby running 
them a little cooler (and more efficiently) than anything else.  No point in 
giving the same thing to regulators that are supposed to give you +15V as 
you're wasting quite a bit of power there.

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