I was looking at some AC panel voltmeters in my friendly neighborhood surplus shop the other day. There seem to be two kinds, "rectifier type" and "iron vane". They make rectifier type meters that go as low as 1V FS - for instance, see
How do these "rectifier type" meters work? Specifically, how do they cope with the rectifier's forward voltage drop, when the fullscale voltage is so low?
FS -
so
Germanium diodes would help. Perhaps the forward voltage drop behaves nonlinearly at low voltage? If so, then the meter movement could have a compensating nonlinearity. Or, a truncated meter scale.
I had undergraduate physics lab before they threw out the Bakelite meters, and there was a "lab tip" -- meters were traditionally calibrated for maximum accuracy at 70% of fullscale reading. Experimenters were cautioned to construct experiments so that readings were obtained near the point of maximum accuracy for the meter in use. The loss of accuracy at the low end might seem intolerable by modern standards.
There were (also?) a number of (old "Bakelite") Weston meters that were guaranteed to indicate within 1/2% of full scale that had a mirror behind the needle, and were at least as good as 1% of reading(or better) all te way down to zero. In most cases, these were far better than modern VOMs, and comparable to 3-digit DMMs. Tossing out such reliable and dependable instruments was stupid.
neighborhood
"rectifier type"
as 1V
they cope
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It's not at all difficult to make an accurate AC voltmeter down to 1 volt f.s., or even 1 millivolt f.s. if you have an external source of power to run the meter. If the meter must work using only the power that can be drawn from the metered source, 1 volt f.s. is not going to work very well.
If you never have to read a voltage lower than 1 volt AC, it's possible to draw power from the source and create a DC operating voltage sufficient to run the meter. I certainly wouldn't do it, if I could avoid it.
Norm Strong
Be advised that the full scale sensitivity of most analog meters is in the range of 50 to 250mV, so that a 1 volt scale is trivial. I have a Triplett 5 microamp movement, which has 16K meter resistance; calculate the full scale voltage sensitivity: 80mV. The common 1mA movements will be higher. Furthermore, i spoke of reading Weston meters way down from full scale and still getting 1% (or better) accuracy of *reading* (NOT of full scale, which would be worse).
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