11-10-2006 05:50 AM
11-13-2006 01:07 PM
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
11-15-2006 01:42 AM
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
11-15-2006 07:31 AM - edited 11-15-2006 07:31 AM
Message Edited by David Crawford on 11-15-2006 01:32 PM
10-31-2008 04:19 PM
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.
11-01-2008 04:40 AM
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