Determine power using DMM

[andrew_JKTA]

I am trying to determine the power consumption of a load by measuring the current through it and the voltage across it. I am thinking of using a DMM to do the measurements. Would a DAQ board be more appropriate?

However, I have only found two DMMs (PCI-4065 and PCI-4070) that can measure BOTH current and voltage but the current I am measuring seems to exceed that of the maximum current input range of the boards (-1-1A).

Is this the right kind of instrument I should be using and if so, could I configure the apparatus so that I can measure larger currents?

Thank you in advance,

Andrew

[JonBowers]

If you are going to be measuring current and voltage then a DMM would be far more suitable. Generally to measure high currents with DMMs you would use a current shunt. National Instruments provide a couple of these for use with our DMMs (see the link bellow). There are also a number of third party current shunts available that you can also use to extend your current input range.

http://sine.ni.com/nips/cds/view/p/lang/en/nid/12692

Regards

Jon B
Applications Engineer
NI UK & Ireland

[wildcat_600]

Andrew,

Be aware that a DMM can only take one measurement at a time.  The 4070 and 4065 can both sample very quickly, but since there is only one channel simultaneous sampling is not possible.  Therefore to measure power you would either need to switch between the two measurments with a DMM (not simultaneous, but may work if your power is fairly constant) or use 2 DMMs.  By using 2 DMMs you could send an external sample trigger to both DMMs so that they would acquire a voltage and current sample at the same time.  Then you could calculate the power based on the two.

Have a good day,

Brian P., DMM Product Support Engineer

[wildcat_600]

Andrew,

I also wanted to point out that the 4065 and 4071 can measure up to 3A without using any external shunt modules.  You can go up to 10A with the external shunts that Jon linked to above.  Below are a couple of other links that discuss power measurements with a DMM. 

You could also use a DAQ board to measure voltage and current (using a shunt), but your accuracy and resolution would be worse than if you were using a DMM.  You may also have to attenuate your signal in some way before feeding it into a DAQ board since they usually have smaller input voltage ranges.

• Measure Apparent Power with a Digital Multimeter
• Measure Real Power with a Digital Multimeter
• Measure Reactive Power with a Digital Multimeter

Brian P, DMM Product Support Engineer

[andrew_JKTA]

Thanks to both of you for the information.

I have decided to use PCI-4070 along with SCXI-1127 for my measurements

Thanks again