Products > Test Equipment
Brymen, difference in reading between mA and uA scale
MarioBros69:
Hi,
These days I have used a Brymen BM869s, an impressively robust multimeter
But I have found the following, I do not know if it is my ignorance or a problem with my unit
I put it on the mA scale and measure the current through a 2k resistor connected to a DC 10v source, this should give 5mA, the measurement on the mA scale is very accurate 4.999 mA
The accuracy stated in the specifications for the mA scale is
50,000mA 0.15%+20d 3.3mV/mA
Now I turn the selector and change to the uA scale (micro amps) and the measurement gives a value of 4670.4 uA
We have a difference of 230 uA between the two scales for the same circuit. If I didn't know the current of the circuit I'm measuring, I wouldn't know which of the two is correct.
The precision stated in the specifications for the uA scale is
5000.0 0.1%+20d 0.15mV /uA
I do a little math and the result is that for the uA scale and for this measurement the error can be plus or minus 6 uA but not 230
I am also aware that the internal resistance (Shunt) according to the specifications is 3.3 ohms for the mA scale and 150 for the uA scale.
My question is, should the two scales show the same measurement value or is it correct that they appear differently because the internal resistance is different on each scale?
Could someone with a fluke or a Uni-t perform this simple test?
Performa01:
--- Quote from: MarioBros69 on August 13, 2023, 08:15:55 am ---...
I put it on the mA scale and measure the current through a 2k resistor connected to a DC 10v source, this should give 5mA, the measurement on the mA scale is very accurate 4.999 mA
...
Now I turn the selector and change to the yA scale (micro amps) and the measurement gives a value of 4670.4 yA
...
The precision stated in the specifications for the yA scale is
5000.0 0.1%+20d 0.15mV /yA
...
I am also aware that the internal resistance (Shunt) according to the specifications is 3.3 ohms for the mA scale and 150 for the yA scale.
My question is, should the two scales show the same measurement value or is it correct that they appear differently because the internal resistance is different on each scale?
...
--- End quote ---
Of course the measurements will differ, thats why Daves µCurrent exists - to lower the burden voltage for current measurements.
The instrument measures current. You are not using a constant current source but a voltage source together with a resistor instead, both with a tolerance most probably exceeding the one of the multimeter.
Ohms law says: I = U/R:
10 V / (2000 + 3.3) ohms = 4.99176359 mA
10 V / (2000 + 150) ohms = 4.65116279 mA
Seems like your measurement results are reasonably plausible.
Fungus:
--- Quote from: MarioBros69 on August 13, 2023, 08:15:55 am ---My question is, should the two scales show the same measurement value
--- End quote ---
No.
--- Quote from: MarioBros69 on August 13, 2023, 08:15:55 am ---is it correct that they appear differently because the internal resistance is different on each scale?
--- End quote ---
Yes.
That's the entire point of having two different switch positions instead of just autoranging. Each position has an internal resistance that's optimized for the range of currents that will be read there and so have the minimum influence on the circuit being measured.
J-R:
Side note, double-check your specific DMM yourself and don't assume the manual is perfect. Resistances on my BM869s:
A - 0.0276 Ohms
mA - 1.758 Ohms
uA - 101.543 Ohms
And my BM789 is 102 Ohms for uA, not 200 Ohms as stated in the manual.
BILLPOD:
I thought a yA was a Yactoampere :scared:
Navigation
[0] Message Index
[#] Next page
Go to full version