PicDude wrote:
> 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.

I use a $1 quad National Semi Rail-Rail I/O Quad op amp to handle 4 
sensors.  They don't really have to be close to the Allegro parts, since 
I use an R/C filter ahead of each one to filter out any garbage.

> 
> 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.  

I'm sure this will work.  I couldn't do this in my application since I 
had to measure ground-referenced DC voltages with the other channels.

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.

Don't know why this would be something to avoid.  Works great. You could 
even make it automatic by checking for a flag in EEPROM. If the flag 
wasn't set, calibrate, then set the flag.  This way, first-time power-up
would calibrate the unit automatically.

> 
> 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?

We had a couple of hundred boards made, and the incremental cost of the 
obround holes was insignificant.

Charles Linquist



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