[sdiy] Power Supplies

[Richie Burnett]

Remember that regulation using negative feedback only guarantees the correct 
voltage at the point where the negative feedback is being taken from, (often 
at the output terminal block or posts of the power supply.)  A synth module 
at the opposite end of an enclosure from the power supply is going to see 
less voltage because of the voltage drop in the cable or PCB tracking, due 
to this module's current draw and all the other modules connected to that 
length of cable.  Current draw *will* cause a local voltage drop.  It's the 
same behaviour that makes the lights dim in your house when someone takes a 
shower.

On expensive power supplies with external sensing terminals, you can run 
another two wires to sense the voltage at the far end of a long power cable. 
This would keep the regulation very tight at this point, but the voltage at 
the power supply end would then go higher than spec when there's a heavy 
load near the end of the cable.  Maybe a "ring main" type of wiring 
arrangement would be best!?!?  I don't know, this might even be common 
practice in modular synths for all I know!

-Richie,


-----Original Message----- 
From: Mattias Rickardsson
Sent: Saturday, October 10, 2015 6:13 PM
To: mskala at ansuz.sooke.bc.ca
Cc: synth-diy diy
Subject: Re: [sdiy] Power Supplies

On 10 October 2015 at 12:38,  <mskala at ansuz.sooke.bc.ca> wrote:
> On Sat, 10 Oct 2015, Tom Wiltshire wrote:
>> 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 output voltage doesn't decrease when the current increases, then
> the real part of the regulator's output impedance is zero or negative,
> which is very likely to create stability problems.  You want to to have a
> small, positive, and almost purely resistive output impedance.

Is this the reason why synth power supplies seem to be a little bit off?
Almost all +/- 15 V rails I've measured so far have been something
like +14.9 V and -15.1 V.

/mr
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