[sdiy] 7915 regulators

[Seb Francis]

Thanks Scott for pointing this out .. I guess I didn't take too much notice of the current range specified in the datasheet since 78xx also specify the same current range, but have no problems getting the right voltage with no load.

Peter Blackett mailed me offlist and said that he too has had the experience that 79xx regulators need a load, whereas 78xx don't.  You learn something new every day!  I guess I'll be putting an LED + resistor across the negative rail.

Cheers,
Seb


Scott Gravenhorst wrote:

> According to the datasheet:
>
> (T=25 deg C) Load regulation:
>
>     Output Current         typ     max
> 5ma <= Iout <= 1.5A         15     100
> 250ma <= Iout <= 750ma      5      50
>
> So it seems from this that the mfr expects a load of at least 5 ma.
> To achieve this, your load needs to be 3000 ohms or less for a regulator
> operating at 15 volts output.  10K won't draw enough current to put
> the regulator in this range.
>
> Seb Francis <seb at is-uk.com> wrote:
> >Hi,
> >
> >Slightly OT .. does anyone have the experience that 7915
> >regulators are very poor under low loads? I've just built a
> >simple bench power supply (I wanted it to be kind of a worst-case
> >PSU, hence using cheap fixed voltage regulators). The 7815 & 7805
> >give near enough perfect voltage with only the load of a DVM. But
> >with -30V in, the 7915 gives out -22V with no load, and -17V with
> >a 10K load. My friendly local electonics shop was kind enough to
> >test their entire stock of 6 to try and find a good one, but they
> >were all the same, giving out <-20V with -30V in and no load.
> >Should I put some kind of load across the output? I've seen a few
> >78/9XX-based PSU circuits where a single indicator LED is across
> >the -ve rail output - I always thought it was just a arbitrary
> >choice to place the LED across this rail. BTW, the 7915 in
> >question (and the 78XXs) are all manufactured by ST, although
> >they all come from different locations (Morocco, China,
> >Malaysia!) Perhaps I should try and get some NS or ON versions of
>
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