[sdiy] 7905 output voltage, FAQ probably

[Florian Teply]

Am Sonntag 08 Februar 2009 01:02:59 schrieb Seb Francis:
> Jason Proctor wrote:
> > thanks for the response.
> >
> > i currently have a little circuit breadboarded up that has an opamp
> > sucking on it, and the output voltage is down to -5.7v, so it's
> > heading in the right direction. i fully expect the one that's soldered
> > in to start behaving itself when i hook it up to various things.
> >
> > any idea why this affects only the -ve regulator? the 7805 i wired up
> > with nothing sucking on its output gave a reliable +5.
>
> This is normal in my experience - 79XX regulators need quite a lot of
> load to get within the spec'ed voltage.  78XX on the other hand seem ok
> with no load.  However the datasheets state minimum of 5mA current draw
> for either type, so could be that even some 78XX need load.
>
Well, i'd extend that to both types of regulators, 79xxs and 78xxs.
Already had virtually all kinds of those behaving weird (all means: positive 
and negative regulators, name brands and noname) as in putting out too much 
or too little voltage without load as well as oscillating due to under- or 
oversized caps. I've found them to be more or less picky about the capacitors 
ESR, so it's usually a good idea to look closely on the datasheet.
And, a resistor and LED makes also for a cheap power on/off indicator, so i 
regularly put one of those into regulator circuits. Has served me well so 
far.

Just my 2 cents,
Florian