Fluke 87v and current measurement mA

[cla]

Hi,

i recently came across a unexpected behavior with my Fluke 87v, which i not fully understand.

I was trying to measure a current. Because of my programmable DC power supply, i knew i have to expect around 120mA.
I measured with my Fluke, having the leads connected to COM and mA, since i was interested in an exact current reading. However my Fluke did show a lot lower reading than expected.

When i connected the lead to A on the meter, i got the reading i was expecting ~120mA.

This way i learned that the Fluke 87v does have an internal resistor of 1kohm on the mA plug.

This completely destroys my reading, and i have problems understanding it. My understanding was that for current measurement the internal resistor needs to be ideally zero.

So why does have the Fluke an internal 1kOhm resistor on the mA plug, which i expect to be especially for precise low current measurements, but only falsify my measurements?

Thanks

[Mr Eastwood]

Sounds like it could be burden voltage;  what was the lower reading?

[cla]

120mA current measured using the A plug on the meter.
down to 100mA current measured when switching to the mA plug on the meter.

Thats an error of ~16%

The lower current measured on the Fluke and also shown on the Powersupply...so it is clearly caused by the internal 1kohm resistance in the Fluke meter mA plug.

Powersupply gives me constant 12 Volt, the load i am currently trying to measure is a LED array of 48 white LEDs.

After you mentioning the term "burden" i found this on the internet:
http://www.fluke.com/Fluke/usen/community/fluke-news-plus/ArticleCategories/Electrical/BurdenVoltage.htm
and this one here delivers also some nice explainations
http://www.eevblog.com/projects/ucurrent/

However i am still having problems understanding it. According to the manual of the Fluke 87v the burden on the 6 A range is 0.03V/A and on the 400mA range is 1.8 mV/mA.
This would mean the error must be bigger when using the A plug/6V Range, and smaller using the mA Plug and 400mA Range...which would seem logical...however i am observing the opposite...bigger error on the mA plug.

Would be a great idea for Martin to explain low current measurements and their possible problems in one of his videos?

[Mr Eastwood]

However i am still having problems understanding it. According to the manual of the Fluke 87v the burden on the 6 A range is 0.03V/A and on the 400mA range is 1.8 mV/mA.
This would mean the error must be bigger when using the A plug/6V Range, and smaller using the mA Plug and 400mA Range...which would seem logical...however i am observing the opposite...bigger error on the mA plug.

Remember 0.03 V/A, means per Amp, not per mA.

[Mr Eastwood]

Could the 120 mA reading in the Amp range might be wrong?  if you read the specifications the 6A range has a resolution of 0.001 A,  and an accuracy of 0.2% and 4 counts;  so suppose the current was actually 120 mA, that reading would be potentially +- 28mA.   No wonder they added a mA range :-)

If you have a second meter you could measure the voltage on both sides of the device under test:-  the supply and the voltage dropped by the meter; then try the different ranges and see what you get.

[edit]
My guess is the 100mA current you measured in the mA setting is perhaps closer than you think?

[cla]

If you have a second meter you could measure the voltage on both sides of the device under test:-  the supply and the voltage dropped by the meter; then try the different ranges and see what you get.

Hmmm...

I measured both voltages with a second meter (Fluke 115).

Supply voltage stable at 12 Volt.

Using the "A" plug on the Fluke 87v, i am getting 24mV drop over the meter
Using the "mA" plug on the Fluke 87v, i am getting 220mV drop over the meter.

[cla]

For future references, mjlorton made a nice video explaining the problem:

Burden Voltage, How a Multimeter Takes Measurements and the uCurrent
https://www.youtube.com/watch?v=fRP98k3Rh1E

Thanks a lot.