I'm doing something super simple and I can't get it to work. I want to use the CR6 to measure the input current on a controller we are using. I set up a 6 ohm resistor in series with the Power supply and Controller. I then connected a differential channel from the CR6 across that resistor to measure voltage and convert back to current. See rough drawing here on google drive.
The code is as simple as the drawing.
Public Measure, Current, Resist
'Main Program
BeginProg
Resist = 6
Scan (1,Sec,0,0)
VoltDiff (Measure,1,mV5000,U1,True ,0,60,1,0)
Current = Measure/Resist 'current will be in mA due to Voltdiff measuring in mV
NextScan
EndProg
I measure a voltage of 1.35V at the U1 and U2 terminals with a fluke meter but the public table only shows 0.002V (which I assume is in mV). I have tried switching channels and loggers. I started with a 0.02 ohm shunt resistor and sized up to the 6 ohm when that wasn't working. I must be missing something simple?
A couple of thoughts:
1) Are you sure the signal is a DC signal? Is the Fluke meter on AC or DC when getting the value you reported?
2) Related to 1, even if the current is DC, it may be fluctuating and the meter is doing a lot of averaging but the datalogger is taking a "snapshot" in time. If the measurement is made when the controller is in quescent power mode, you could get what you're seeing.
Thanks for the reply.
1. I am positive the signal is DC as it is being generated by a variable DC Power Supply. The fluke meter is measuring DC. Both the DC supply and the Fluke meter give the same output current reading.
2. If this was the case, could I take a burst measurement and average all values to get the same result?
Another idea is to try this on the low side instead of the high side of the circuit. The 0.02 shunt resistor should work perfect there. What is the high side voltage with respect to ground? The datalogger can handle up to 20Vdc without being damaged but its measurement range is still only ±5Vdc.
I attempted a burst measurement of 100 values (10 second scan). This still did not yield a single value that was anywhere near the measured voltage of the shunt. (logger measured at best 250mV when the meter measured 2.3 volts)
The high side voltage is 24V with respect to ground. This was the reason for using a differential measurement and not connecting the grounds of the CR6 and controller together. I'm doing my best to make the CR6 be a datalogging current meter for any DC setup.
Could you udpdate your drawing to show where the datalogger is grounded. To make the measurement on the high side becomes tricky. The CR6 would need to not just be floating, but be grounded at the + side of power, have additional power supply on top of that, and then a wireless connection to your PC or other device to prevent an additional ground point that would mess things up. It seems much easier to measure current on the ground wire and reference both systems to the same ground potential. I have this set up (current measured on the ground wiht a 0.01ohm resistor) on my variable power supply and it works great for me.
I've had troubles with current measurements when components and datalogger weren't sharing the same ground. As mentioned above, you need to show how the datalogger is powered and grounded, and if the two(?) separate power supplies are sharing the same ground.
As ZooKeeper mentions above, measure on the (-) side of the power supply, then you can ground everything together.
Also, if you measure on the (-) side you can use a smaller range on the VoltDiff command which will improve the resolution.