> read the data sheet and see what the accuracy is in that current range
> compared to what you need.  it's probably fine, particularly since you
> don't seem concerned about the difference between average and rms
> current in an ac circuit, and they can be very different.

Between 0-25A the output of the sensor would be 2.5v - 3.5v

So if a 10bit adc measures 0-5v the smallest step would be .005v and
there would be 200 steps in 1v.

So the least count of the sensor would be .125A

Do I understand it correctly?

 
> yes, i if you can tolerate a 5V drop and want to dissipate 25W in your
> sense resistor, a lower value is much smarter, with an amplifier if
> necessary.  for one thing, the temperature change at different current
> levels in the resistor will contribute nonlinearity.  you'll also want
> to use at least a 50W power resistor for reliability and to allow some
> safety margin, and in either case you'll have to mount them on a
> heatsink.  on the other hand, a .01 ohm resistor at 20 amps will only
> dissipate 4W, or at 5 amps 1/4W.  of course if you are dealing with ac
> you'll have to rectify the voltage drop which can be done with a bridge
> rectifier with the ac legs in series with the load and the resistor
> across the dc legs (again it has to be rated for your maximum current
> and may need a heatsink), or you can sample it in phase with the power
> line or fast enough to rectify it in software.  this is another problem
> solved by the rms to dc converter.

Lowering the value of sense resistor would be ideal. But which opamp
should I use. I haven't worked with opamps much so I would appreciate
if you can suggest some TI opamp as they would be easier to sample.

 
> actually it depends on what type of load you'll have, if it's purely
> resistive, then average is the same as rms, if it's reactive like a
> motor or worse a fluorescent lamp or a switching power supply the
> difference can be large and the average may be misleading.  i think the
> rms to dc converters are about $3 which isn't that bad, and of course
> maxim and some others may be willing to send you a sample.

Looks like www.maxim-ic.com is down. :-(
 
> knowing more about the application would greatly help in suggesting
> appropriate methods.

I want to integrate a small logger in a power supply or with a 12v
battery that will measure the load current and send it to an LCD with
Ah display.


Thanks a lot,

Chetan
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