I tested linear hall+ toroid,  it is easily to achieve >10mA till hundrends
A, without touching "trouble".
with EEPROM calibration, any non-linearity that you can sense can be
eliminated.





----- Original Message -----
From: "Chetan Bhargava" <cbhargava@gmail.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Monday, September 13, 2004 4:49 PM
Subject: Re: [PIC] Current measurement using PIC


> > 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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