Re: A Simple Lab Demo of Resistor Noise EMF.

[brian whatcott <inet@INTELLISYS.NET>]

At 14:17 3/19/01 -0500, Michael Edmiston wrote:
> John Denker is correct that the typical voltmeter reads zero if we set it to
> AC and feed it a DC level.  However, this is only true if the DC level is
> truly DC.  Hook a 1.5-volt battery to the voltmeter (set to AC) and it will
> give a "temporary" reading.  It initially responds to the change, and keeps
> responding if the change keeps occurring (e.g. connect, disconnect, connect,
> disconnect, etc.). /snip/

> Michael D. Edmiston, Ph.D.

Interesting observation. John mentioned he tried this out with
a "Fluke AC true-RMS measuring DVM". I would have thought that
the "AC true RMS value" of a DC 1.6 volt signal was 1.6 volts.
i.e.   sqrt(1.6^2) = 1.6
...or perhaps it's true RMS of each polarity, hence
       sqrt[(1.6^2 + 0^2)/2] = 1.131

How can the True RMS value of a DC signal be zero, I wonder?

I tried this check on two DVMs, first the Radio Shack 22-188,
  then an el cheapo $10 special, "M-830B".

RS on DC         1.592 volts (Rayovac alkaline cell)
M-803B on DC     1.603 volts

RS on AC         1.2 first sample, then decreasing,
                      either polarity on 2 V AC autorange.

M-803B on AC     2.8 volts constant for one polarity,
                 0.0 volts for the other, 200 V AC range.

I don't think anybody suggests either of these is
AC true RMS reading, for sure!



brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!