Hmmm...  can't believe I forgot about those, especially since I have a few of 
the 75A versions here (ACS750LCA-075), but haven't tried them yet.  Very 
nifty devices.  The thing about these is that they're ~$6 each, plus the 
op-amp is required, which I would think would be best kept near the sensor.  
Then I'd need regulated 5V there as well.  I'm guessing about $9 per station, 
without counting the PIC, etc.

However, I'm now re-thinking that I could eliminate the op-amp and use a 50% 
voltage divider for Vref-.  And Vref+ would be fed with Vdd directly.  This 
would let my full-scale output (for a 100A sensor) be (2.0/2.5) * 1024 = 819 
steps, which is pretty good for my requirements.  Also, whereas storing the 
0A output level in EEPROM would be easy (I already do this in a 
pressure-sensing application), I'd prefer to avoid this if possible.

Still not the few bucks per station I was hoping for, but the simplicity is 
very appealing.

BTW, I remember thinking (ages ago when I picked these up), that the holes for 
the large terminals would be a pain to drill since slots add a significant 
amount to the PCB cost.  Not sure if they'll let me put a few holes right 
next to each other, especially since it needs to have the pad around it.  How 
do you handle this?

Cheers,
-Neil.



On Sunday 17 July 2005 01:26 pm, Charles Linquist scribbled:
> Really low-ohm resistors are difficult to do, because even the solder
> connections to the part change the resistance. Individual calibration of
> each sensor becomes mandatory if you want any decent accuracy.
>
> Have you thought of using the Allegro hall-effect sensors (like the
> ACS750SCA-100) instead? I use them all the time with great results.
> I use one op-amp to move the normal Vcc/2 offset to near gnd, and
> amplify the output by about 1.8.  I then take that output and feed it
> into my PICs A/D.  I have a "calibrate" button that takes the A/D
> reading at zero current and stores that value in EEPROM. All subsequent
> readings subtract out that value.
> No pots, no adjustments and 2% accuracy to 100A.
>
> Charles Linquist
>
> PicDude wrote:
> > I need to measure DC current up to 100A (at 12V) at multiple points and
> > planning to use the ZXCT1010 current monitor and a sense resistor for
> > this. A drop of 100mV or even a bit more is acceptable; few percent
> > error/accuracy preferred.
> >
> > (a) Calculated ideal is a 0.001-ohm, 10W, 1% sense resistor, or .0005-ohm
> > 5W, but can't find either at the usual sources (Digikey, Mouser, etc).
> >
> > (b) I can use 4 sense resistors in parallel -- 0.005-ohm, 3W resistors,
> > but these add up to ~$6.  Replicating this a few more times adds up fast.
> >
> > (c) Shunts are and option, but are really expensive ($19 ea. at All
> > Electronics).
> >
> > (d) The other option is to use a couple inches of say 10-ga wire, but I'm
> > not sure how consistent this wire/resistance is.  Google hasn't found me
> > any info on this accuracy.  I do have the option of calibrating these
> > individually, but without any specific calibration equipment, I'd like to
> > avoid doing this. I'm also not sure if I should be running 100A through a
> > piece if 10-ga wire.
> >
> > (e) Finally, I thought of using a PCB trace for this, which is an elegant
> > solution, but some high-level calcs of 1oz or 2oz copper yield some large
> > trace sizes -- ~2 inches wide and 5+" long and really high temps.
> >
> > Any ideas on this?  Or perhaps a source for the ideal sense resistor in
> > (a)?
> >
> > Cheers,
> > -Neil.


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