mVA Problem: (Confirmed by DrGeoff)
I seem to have a definite measurement problem on the mVA range in DC. Can anyone confirm this on their meters?
If I have a test setup with a power supply and a precision 100ohm resistor and I am putting 5V across the resistor, then as long as the current flowing into the mAuA socket is positive, I get 5.0000V across the resistor, 50.000mA current and 250.00 mVA. All good. Just using absolute numbers here - not worrying about the +/- polarities.
If I swap the power supply polarity - nothing else changes, I get 5.000V, 0.0368mA and 55.215 mVA. The current is wrong and the mVA product makes no sense.
The configuration I used was the mAuA socket to the power supply, the multimeter common to one side the resistor load, the V/Ohms input to the other side of the load that also connects to the other power supply output. I am using this configuration so that the multimeter is measuring the actual voltage across the load. The problem happens whether or not the v/Ohms input is connected or not, so my meter just cannot measure negative currents over exactly 10mA in the mVA/VA mode.
The problem occurs exactly when the current reaches 10.000mA. It works perfectly at 9.999mA with both power supply polarities. At 10.000mA, it fails. This makes it look like a software bug.
The mAuA input works fine in A/mA switch setting. This is only a problem in the mVA/VA mode.
The same problem does not occur using the A input for current instead of the mAuA input - at least at up to 100mA. The problem might be there using the A input at higher currents, but I cannot test that right now.
I have not tested this in AC mode, but assuming the RMS converter is only supplying positive voltages to the measurement chip, I would guess that AC mVA mode probably is fine.
UPDATE:
The same problem happens with the A input when measuring VA. If the current into the A socket reaches -1.0000A it wraps around to zero. -0.9999A is fine. Same thing happens with the mAuA socket at -10mA. Both inputs work perfectly in the mVA/VA modes with positive input currents.
The problem seems to be connected to Autoranging with negative currents - if you use the range button, you can get the right numbers with negative currents. So it seems that the current measurement autoranging for mVA/VA measurements does not work properly for negative currents. It works properly for positive input currents. If the firmware can be fixed to get the measurement chip in the correct range for the applied negative DC current, this problem should be fixed.
It would seem that with negative currents, the mVA get stuck in the 5.4999mA range. It actually seem to keep measuring up to 9.9999mA - that is 80% overrange before rolling over to 0.00000 at 10mA. It then continues to measure correctly to 11mA (with the lost first digit) which is 100% overrange!. After that, I think the poor current input has had enough. At some point, the current input totally overloads and just produces rubbish numbers. The multimeter does not change to the 54.999mA range with mVA negative currents or to the 5.4999A or 10A ranges for VA negative currents at all if Autoranging is enabled.