[sdiy] 2 multimeters showing different voltages

[Tim Parkhurst]

On 8/7/07, Scott <Scott at scottwick.com> wrote:
> I've got two multimeters, and each is showing a different voltage.  One
> shows my synth to be at a solid 15 volts, and the other shows it at
> 17.5.
>
> I tested my computer PSU to try to figure out which one is correct, and
> I still don't know!  One shows the PSU 12v output at 11.05, and the
> other shows it at 12.62.
>
> Both meters are less than 3 years old, and have little use.  One is a
> radio shack, the other is a Velleman.
> I'm starting to think that the one that shows 17.5v (and 12.6 on the
> computer PSU) is correct.  However, that would mean my synth PSU came
> calibrated incorrectly.  (which is possible.. its just a used power-one
> supply)
> Does anyone know how I can figure out which to trust?
>
> Also, how much of a difference would those 2 volts make?  If my synth
> PSU is putting out +/- 17.5v, would that explain some tuning issues I've
> been having?
>
> thanks!
>

Yipes! I would DEFINITELY find out which meter is telling the truth
and chuck the other one (or label it and use it only for continuity
testing). One way to test would be to bug a friend who works at a hi
tech firm or a repair shop, and ask him if you can bring your meters
in and check them against their (hopefully calibrated) fancy-schmancy
four or five digit bench meter (or even a 3 digit hand held unit, as
long as it has been recently calibrated). Otherwise, I'd get a 7805
and a few caps and make a quickie 5V reference. Stuff the 15 or 17.5V
from your PSU in there and let the 7805 regulate it down. The 7805
will get warm, since it's being fed a fairly high input voltage, but
it has a nice heat sink tab and should easily last long enough for you
to make a few quick measurements.


Tim (likes the word "yipes") Servo
-- 
"Imagination is more important than knowledge." - Albert Einstein