[sdiy] Ohms-Per-Volt

[jhaible]

Hi Glen,

If the text is rather old, 1000 Ohm / Volt might mean "at least
1000 Ohms / Volt". (I remember a time when recommendations
to buy a decent voltmeter actually said something like this.
We're spoilt with the high impedance of modern instruments
nowadays.

(Of course you can always connect a simple resistor in parallel to
your DVM if you need a lower impedance ...)

But I see a different problem here:
Frequency response.
Many DVMs can do 50Hz accurately in their AC mode, but
get much worse on higher frequencies.

I would use a scope and "watch" the amplitude.

JH.




> I was reading a calibration procedure in an old service manual, and the
> goal of one test in particular is to measure the AC voltage present at the
> instrument's output terminals, while different musical notes are played on
> the instrument's keyboard. The manual says to connect a voltmeter to the
> instrument's output terminals. (So far, so good.)
>
> Then it says to be sure and use the 1000 Ohms-per-Volt setting of your
> multi-meter to make the measurements. This is almost certainly intended to
> make sure your particular voltmeter meter "loads" the circuit to the same
> degree theirs did.
>
> The problem is, my volt meter is a modern DMM with a very high impedance.
I
> would like to emulate the impedance of their older d'arsonval style meter,
> but I really don't know how to calculate their meter's impedance, since
> they only gave the Ohms-per-Volt spec of their meter, but didn't tell me
> the full-scale voltage of their meter. If I knew the impedance of their
> meter, couldn't I just add a shunt resistor across my test leads to
emulate
> their older meter with lower impedance? I think this would work if only I
> knew the original impedance.
>
> Anyone have any suggestions?
>
> For reference, the signals to be measured should fall between 0.2 VAC and
> 16 VAC. That should help narrow things down ever so slightly. Of course,
> there is also the problem of meter impedance varying with frequency. I
have
> no way to estimate how their meter and mine compare in that regard, but I
> thought that it would be good to at least try and emulate the "DC" (or
> should that be 60 Hz?) loading of their meter.
>
>
> thanks for any suggestions,
> Glen Berry
>