[sdiy] Power Supplies

[Tom Wiltshire]

What's the evidence that drawing more current from a linear regulator causes its output voltage to fall? I thought that was exactly the problem regulators were supposed to solve?

If the 7812/7912 regulators are doing their job, you ought to be able to run different currents on the two lines and still get the same output voltages, or within the specs (100mV for one particular reg: http://docs-europe.electrocomponents.com/webdocs/1385/0900766b813857f0.pdf  )

I'd be very surprised if 100mV difference between the rails makes any odds to anything. If you're running signals hot enough that 100mV of offset makes a difference to headroom, you're going to be running into op-amp clipping distortion of 10s of percent anyway.

HTH,
Tom

 
On 10 Oct 2015, at 08:26, Rob Spencer <Rob at gmsn.co.uk> wrote:

> Hi All,
> 
> I’ve been thinking a lot about power supplies recently and was after some advice…
> 
> The first PSU I built was the classic 7812/7912 with a nice toroidal transformer which sat at the back of the nice big case I was using, however I’m now trying to slim things down. I’m also keen to remove mains power from the box completely.
> 
> I’ve been having a look using buck converters in an inverting configuration so I could use a 15v DC input, however I would prefer to stick to linear, as switching PSUs don’t quite feel right for me in analogue audio applications. I’m now toying with the idea of doing the heavy lifting in a PSU enclosure and then bringing the Dual 12v in through a multipole connector.
> 
> During my research I stumbled across this post: http://www.electronicspoint.com/threads/what-is-a-dual-tracking-regulator-for.3354/ which talks about a "dual tracking regulator" adjusting both voltages so they remain the same potential difference with respect to ground. If one voltage drops the other will be adjusted by the same amount, which got me thinking….
> 
> If I have the classic 7812/7912 PSU, then run a 7805 of the +12v, load it up with an assortment of digital and analogue modules from a variety of designers, I would expect the loading on each rail to be different. As I approach the 1A mark, the voltages on each rail will be quite different with respect to gnd. The affect this will have on critical audio op amps will be supply voltages not sitting evenly around zero volts, which, I would have thought, would produce a signal either with a DC offset or some other kind of distortion.
> 
> It would be great to incorporate a “dual tracking regulator” to ensure the voltages remains the same level to ground, but they all appear to be rated in the 100s of mA. Is it possible to use a power transistor to draw a larger current from rectified, but unregulated supply, limited to the regulated voltage or are there other tried and tested solutions to the overall problem?
> 
> Many thanks in advance.
> 
> Rob
> www.gmsn.co.uk
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