At 03:56 PM 5/8/2005 -0600, you wrote:
>
>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.

You can do this. You have to take care with latch-up gotchas on the
op-amp. The LM358 is prone to this. Also the input common-mode voltage range
and output range must include ground and the Vos should be much lower than
your signal (50mV?).


>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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