Measurement of current draw on a 28volt supply using M Series DAQ and SCXI.

[MarkL]

I need to measure the current draw on a 28V DC supply. Maximum consumption is in the order of 5A. I have a DAQ M series 6259 card and a SCXI 1000 chassis (1x 1540 LVDT module, 1 feed through module and a SCXI 1112 thermocouple module) with a spare slot available. Can I achieve this measurement using my existing kit or do I need to buy an additional module for the SCXI chassis? Ultimately the system will run under Labview RT so I guess use of USB type devices will not be possible.

Mark Lewis, HS-Claverham, Bristol

[DavidJCrawford]

Mark,

You could use a 0.1 Ohm (or 10 x 1 Ohm in parallel) series shunt and measure the voltage drop across the resistor. This will give a maximum of 0.1 Ohm * 5 Amp = 0.5V. Pick a resistor at the right wattage which needs to be >= 0.5W at 5 Amp through a 0.1 resistor.

David

[MarkL]

Hi David,

Thanks for your suggestion, I tried this but initially got some unexpected results i.e: current consumption of around 3amps even when the motor of the system under test wasn't running. This conflicted with a measurement taken with a DVM. I was confused by this especially as I had configured the input to work in differential mode. Someone in work then suggested that I ought to try placing the shunt in the 0V rather than the 28V line. This seems to work but I don't fully understand the reasons why (given the fact that I'm using differential mode). Which ever line I place the resistor in if I read the voltage drop with a DVM I see the same value but there is a difference in the reads when connected to the DAQ card.

If you can explain briefly why I see the above effect I'd be most grateful.

Thank you for your help

Cheers

Mark

[DavidJCrawford]

Hiya Mark

In a nut shell I don't know. However I was able to make a meaningful measurement using this this circuit.

The Z was an electronic load and I have a PCI-6013 DAQ. At 2 Amps I was seeing the correct mean with +/- 10 mA of noise using the 20K resistors. Without the resistors the voltage at A0 was 2.6V relative to AI GND using a multimeter which agreed with the differential measurement in MAX.

I found this link pretty http://zone.ni.com/devzone/cda/tut/p/id/3344 useful.

Hope this helps.

David

Message Edited by David Crawford on 11-15-2006 01:32 PM

[Scott Y]

Hello David,

I would like to know if you were using a 28 Volt supply in your experiment. I am running into a similar problem as the one described by the origional post. If I have a supply voltage under 11 VDC it works as expected. Over 11 VDC (I need to use about 16-18 VDC) the measurment no longer agrees with my handheld DMM.

If I understand the issue correctly your circuit will only work if the supply voltage is less than 11 VDC, due to the common mode voltage limitations of the 6013 and several other DAQ cards.

Scott.

[DavidJCrawford]

Hi Scott

Thats an old post you have dug up there. From memory, I used a standard 30V 2A benchpower supply set to 28V. I think it was one of the ones from Thurbly Thandar Instruments. I don't think the voltage sense leads were connected at the load but they should have been.

I am not an electronics engineer so I am not sure about the circuit not working below 11 Vdc due to the common mode voltage. I work with an electronics engineer so I will ask him and get back to you if no one else jumps in to help.

 Sorry that I couldn't be more help.

David