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High Current Measurements





---------- Forwarded message ----------
Date: Wed, 15 Oct 1997 16:23:21 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Cc: FutureT-at-aol-dot-com
Subject: High Current Measurements   



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Input power measurements   
> Date: Tuesday, October 14, 1997 11:31 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Tue, 14 Oct 1997 21:08:40 -0400 (EDT)
> From: FutureT-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Re: Input power measurements  
> 
> > My mistake :(    I was looking at the voltage.  I'll redo the test and
> > check the current.  Sorry for the confusion.
>  
> > John Freau
>   >>
> Thanks Jim for pointing that out.  I'm replying to my post here: 
> Now that my brain has started working again (at least for the 
> moment)  :)    I went ahead and observed the current waveform
> on the scope.  There is some distortion, but not too extreme,
> but probably enough to make the meter read wrong.  I also 
> compared the phase of voltage and current, and there's an
> inductive power factor of about .9 or so.  I compared the readings
> of wattmeter, and voltmeter and ammeter, (for VA), and also
> calculated a power factor of about .87, so there's pretty good 
> agreement.  Now if I just had a hot-wire ammeter or something
> similar.  I do have a small thermocouple heater unit, Weston
> model 9997 i believe, that says "range uA 100".  I'm not sure
> how or if this can be used.
> 

This is a thermocouple that at the rated feed-through current will cause the
thremocouple to produce a current of its own that can be sent through a 100
microamp meter (full scale). Let's say the unit you have is rated at 10 amps
and you wanted to convert it to read 100 amps. You would have to fashion a
parallel current shunt that diverted 90 amps (10+90=100 total amps for 100
microamps full scale).

I normally eyeball the existing shunt, or take a few quick measurements. Then I
build a shunt that I KNOW will take MORE current than I want. I firmly bolt the
new shunt and old shunt together (bolting is the only way to describe it!!).
Then I borrow a "real" calibrated current meter able to measure 100 amps and I
jerry-rig some kind of a load to make about 100 amps flow. I apply the load for
short periods of time and quickly compare the 100 microamp meter reading and
the known current flow. Both high current meters must be in series. I then use
a file to remove a bit of my homemade shunt and test again. This is repeated
until I get identical readings, or slightly higher. Now the modified
thermocouple meter is calibrated. Pain in the you-know-what, but it works. 
Unit should be periodically recalibrated. It is always better to have it
reading a little too high rather than too low, as you can trim out a too-high
reading by adding a couple of hundred ohm potentiometer across the 100
microammeter, and adjust for proper reading. For proper operation the entire
shunt assembly should have ambient air flow. Sticking it in a small enclosed
box will create errors.

One really nice feature of this sort of current meter is that it measures true
RMS current from DC to many Mhz, and is self-averaging.

> The distortion is causing the very peak of the sine waves to be
> pinched somewhat.  Also, the positive and negative half cycles
> are not symmetrical, the positive one is wider.
> 

John, I think you may simply be seeing the current limiting kicking in here.

> John Freau
> 
> 

Hope this helps.
Fr. Tom McGahee