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From: dbell@cup.portal.com (David J Bell)
Newsgroups: sci.electronics
Subject: Re: Geting my A/D converter to read current
Message-ID: <32659@cup.portal.com>
Date: 10 Aug 90 19:14:54 GMT
References: <5064@uafhp.uark.edu>
Distribution: na
Organization: The Portal System (TM)
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Joel Kolstad kolstad@cae.wisc.edu asks:

>Hi...  I'm currently working on a project where I need an A/D converter to
>read the (average) current going through a load.  The power supply for 

>filiament.  There is a shunt (if you can call it that) resistor that goes
>after the load and to ground.  It is approximaterly .025 ohms.  By looking
>at this on a scope, I can watch the power going through the filiament and
>the load resistor.  Since V=IR, I can measure the voltage, I know the
>resistance, and therefore calculate I at any point.  If I look at the
>angle that the TRIAC fires at, I can integrate all this (v*t, in effect) and
>get the average current going through the filiament.

>HOWEVER... I need a computer to be able to read this.  So, I need some sort
>of integrator circuit that's LINEAR with respect to current -- I need to 


OK, first off, do you really need to measure (at high speed) the instantaneous
load current, then integrate it, or do you *really* want a measure of the
effective ("average", actually R.M.S.) current?

If you want instantaneous current, just read it. Your A/D will need to
read both input polarities, or you could amplify and offset the voltage
across then shunt, giving a value from 0 to 10V, for example, with
0 volts corresponding to max negative, and 5V corresponding to zero current.
Then integrate in software...

If all you need is RMS current, you will need to rectify the signal from
the shunt, to produce a series of positive-only half cycle pulses. Then
a capacitive filter will give you the effective current, filtered over
several power line cycles.  Read that result with the A/D.....


Dave       dbell@cup.portal.com