Olin wrote: This sounds like your current sense resistor is on the
return leg with one side ground.  If so, why do you need a differential
amplifier in the first place?

-----

I had originally planned to place the sense resistor between +5v and the
load. Now I am considering placing it between the load and ground. I
chose the first method out of habit. This is how I usually use an amp
meter when taking measurements.

I have never made a current sensor before so I'm looking for some best
practices to do this. Some of the posts here have lead me to think that
the ground side current sense might be the best. I have not had time yet
to build up a standard non-inverting amp so I'm not sure of the issues
with this method.

How do you guys build current sensors? Douglas Wood mentioned that there
are chips designed to do this. I might investigate one of these as a
solution but for now I'd like to learn how to solve this problem by hand
before going to a pre-built solution.

I'd still like to hear comments on the Hall Effect current sensors if
anyone here has used them.

Thanks again.
Chris


-----Original Message-----
From: Olin Lathrop [mailto:olin_piclist@embedinc.com] 
Sent: Sunday, May 08, 2005 1:53 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Problems using a differential amplifier to measure
current.

ChrisEGroup@rocklizard.org wrote:
> * I made an error on the schematic. The gain is 100x. I am using 
> 10K/100 Ohm resistors in each OpAmp voltage divider.

That's a lot for a single stage.  It will be difficult to get the
resistors matched well and get good common mode rejection.

> * The OpAmp inputs range from 0V TO 50 mV

This sounds like your current sense resistor is on the return leg with
one side ground.  If so, why do you need a differential amplifier in the
first place?


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