Quote:
Originally Posted by thingstodo
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
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I just reviewed my test, using the slightly less capable DI-145.
I'll try to post some pics of the test rig and output.
The test was controlling a double DC motor powered differential-steer tracked vehicle. I wanted to find out how controllable it is and how much power it used. Each motor had a 12V lead acid battery that was connected directly on the positive side to a contactor and then the motor. The motors' negatives were connected together then through one lead back to the two batteries.
Control was on-off with the contactors. In the end it was pretty controllable with that simple set-up and I did a bunch of running up and down the driveway tests, along with hill climbs, etc.
To measure performance, I connected the 12V from the battery after each contactor directly to the DI-145. With this setup, I could later see which motor was on, voltage drop, etc.
For measuring current, I used a LEM current sensor. I ran the return line from the motors through the LEM. I have a bunch of little dc-dc converters that I use for random stuff. I just connected one of those so that 12V from the battery to supply 5V for the LEM. I just wired the LEM's output directly to the DI-145 and scaled the output according to the LEM's specs. I connected the DI-145 directly to my laptop on the seat next to me and went for a ride.
With this admittedly very hokey rig, I got some very useful and repeatable data. The DI-145 put out 4 data tracks that were sinc'd along with a time signature. I could see how extra current draw pulled the individual batteries down, how using only one motor (to turn) affected the overall power output, etc etc. The current measurement was so sensitive that I could tell by looking at the output where exactly I was in my driveway at a particular time. It would track the 425A startup spike along with the normal 50A +/- 10A (depending on conditions) running current.
I'll post some test rig pics tonight.
- E*clipse