Re: Ammeter Configurations

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

One useful trick on metering AC line current is to put a standard bridge
rectifier (with sufficient current capability) in series with the load. The
wiring is like

AC -> line
AC -> load
+ -> to DC meter +
-  -> to DC meter-

This solves the forward voltage drop problem.

Then, all you've got is the shunt/measurement problem.  If you want to use
cheap DPMs, they usually have a full scale of 200mV.  You want that to
correspond to 20A (or 2A...), so you need .2 V/20A or .001 ohms.  That's
about a foot of AWG 10 wire (call it 2mm diameter).  A bit of tweaking of
the DPM or the shunt length will get the calibration right.
Don't forget that you probably want to measure RMS amps, but the meter will
read some weird hybrid of peak and average (depending on filter time
constants and when it samples).

To filter out the AC fluctuations, you'll want to put a capacitor across the
DPM, too.  If you put a series resistor in front of the cap, it will change
the time constant.  Most DPMs have an input impedance of several Megohms, so
something like a 10k resistor and a  10 uF cap will give you a time constant
of 0.1 second. (the source is effectively zero ohm impedance driving it).




----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, October 04, 2004 7:16 PM
Subject: Re: Ammeter Configurations


 > Original poster: "Ozone Junkie" <OzoneJunkie-at-gmail-dot-com>
 >
 > G'Day Again,
 >
 > The problem is in getting a 0-250 full scale AC voltmeter.  I have
 > scoured all my usual parts sources here in Aus, and can't find any AC
 > meters for affordable prices.  The coil is only a 12/30 (360VA) but i
 > want to be able to read upto about 6A (1.44kVA).  From what I thought,
 > there is no point in trying to build a small current transformer for
 > this small amount.
 >
 > That is why I am trying to use the smaller, cheaper, easier to get DC
 > versions.  I was originally using 0.1r across a digital panel meter,
 > but it has died a rapid death.  I am trying to avoid using external
 > DMM's for a 'neatness' factor, but I am thinking that that might be
 > the only way.
 >
 > Tristan
 >
 >
 >
 > On Mon, 04 Oct 2004 19:06:38 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:
 >  > Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
 >  >
 >  > Tristin:
 >  >
 >  > This is how we do it.  Since the coil is powered by an AC transformer,
just
 >  > use a 0-250 full scale AC voltmeter that the dial faceplate is
re-calibrated
 >  > in 0-16,363 Volts.  We usually round this off to 16,500 VAC full scale.
 >  >
 >  > In normal operation the meter will read 14,400 Volts at 220 Volts
input.
 >  > This is the Erms potential and the cap will fire at 1.414 * Erms
(actually
 >  > this depends on your sparkgap setting for exact value).
 >  >
 >  > A cheap easy way to accomplish the task and no messy HV dividers
necessary.
 >  > Also, very accurate for TC work requirements.
 >  >
 >  > We also do this with NSTs, just using a different value.  The math is a
 >  > simple ratio proportion:
 >  >
 >  > X = (250 V. * 14.4 kV) / 220 VAC        X = 16.353 kV fullscale reading
for
 >  > a pole xmfr
 >  >
 >  > or X = (150 VAC * 12 kV) / 120 VAC    X = 15 kV fullscale for a 12 kV
NST
 >  >
 >  > Dr. Resonance
 >  >
 >  > Resonance Research Corporation
 >  > E11870 Shadylane Rd.
 >  > Baraboo   WI   53913
 >  >
 >  >
 > <SNIP >
 >
 >