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DMM measuring incorrectly
mclabop
Member
09-23-2017 04:45 PM - edited 09-23-2017 05:04 PM
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I've got a very simple setup in a lab assignment, but I fear that my DMM is damaged. The measured current readings are significantly off. The exercise has us measuring R1=330 Ω resistor and find current at different DC voltages. Repeat with R2=3.3 kΩ resistor. I am using the NI ELVIS II+ with 100 MS/s Oscilloscope, software is from the NI ELVISmx Instrument Launcher using the DMM and the Variable Power Supply.
The black alligator plug is connected to COM, and the red is connected to A. The resistor is on the terminal block with one end on line 48 (+ variable supply) and the other is connected to line 49 (ground). The alligator clips are then connected to each side of the resistor leads red on positive and black on the ground side.
The resistors are within 5% tolerance and measure as 325.19 Ω and 3.2415 kΩ respectively. The resistance and DC voltage meter function appear to function correctly. If I calculate current by taking measured voltage and measured resistance I end up very close to expected current. But if I just measure the current, it ends up with incorrect values, which I've listed below.
| Vs (Volts) | Calculated Current (mA) | Measured Current (mA) |
|---|---|---|
| 1 | 0.0030 | 0.0163 |
| 2 | 0.0061 | 0.0168 |
| 3 | 0.0092 | 0.0166 |
| 4 | 0.0123 | 0.0165 |
| 5 | 0.0153 | 0.0174 |
| 6 | 0.0184 | 0.0211 |
| 7 | 0.0215 | 0.0208 |
| 8 | 0.0246 | 0.0199 |
| 9 | 0.0277 | 0.0188 |
| 10 | 0.0307 | 0.0202 |
The measured current for the 3.3 k-ohm resistor is even stranger. At 1 volt it measures 0.835 mA (expected value of 0.303 mA), and then over the next few minutes, appears to slowly run down to fluctuate around 0.71-0.72 mA. I can disconnect and reconnect it as many times as I want, the "countdown" process starts over each time.
This is only the second assignment and I haven't done anything weird with it. So is it broken? Or is there a way to calibrate it? I should also mention, that I have also tried using the Null offset function and it yields similar results.
