Quote:
Originally Posted by e*clipse
Hmm - yea - I was just reading the #'s on the DI-149 I have. I try to keep all the voltages down in the +/-5V range, just 'cause the microcontroller works that way. Most things like temp sensors, pressure sensors, etc etc work in the 5V range. The software makes calibration really easy, so reading the output of the current sensor and graphing it as +/- 300A vs 0>5V is easy to set up.
For something like battery voltage, a simple voltage divider will work to bring it down to an acceptable level for testing.
- E*clipse
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Scaling the voltage is pretty easy - the pack is 125V so 0 - 5V would require a divide by 25. Maybe a 26K in series with a 1K2. With a 10 bit A/D you have 1024 counts. So 0V = 0 counts and 1024 counts = about 125V. That resolution is pretty good at about 0.12V per count.
The current is a bit more of a challenge. The shunt resistor is 50 amps = 50 mv for a resistance of about 1 milli-ohm. The ad lists 10 bit A/D, so 1024 counts from 0 - 5V, or 4.88 mv per 'count'. With a 50 amp, 50 mv shunt there is 10 counts between 0 and 50 amps. The measurements are normally +/- 1 count for a lab, more like +/- 5 counts in my garage. I don't really expect more than 500 amps, so I'm using 100 counts out of the 1024 counts in the A/D for a short time. I would expect to be below 250 amps for the rest of the testing. IF I can get a DC controller working, I'd be under 125 amps for most of the testing.
I could add an amplifier, but that requires an op-amp and several more connections, plus many additional sources of noise for the signal being measured. In my limited experience with amplifiers, you don't get much additional accuracy on your measurements unless you make a PCB. Breadboarding is quick, but not perfect.
This may not be a good fit ... at least for the current measurement