amperagemultimeterspark
I wanted to measure the current of a generic lithium ion battery charger. I set I plug my red lead in the A 10A max fused and black into COM. The charger is rated at 600 mA and my other lead plug is fused to 250 mA so obviously that wouldn't work.
So I put my leads on either side of the battery and there's a small spark. Did I break something?
Situation: AC meter range reads double DC value when AC range is used for DC:
I have a meter of the same brand and very similar model (UT33B as opposed toU33C) and my meter behaves exactly as reported. Details at end.
At one time it was extremely common for AC meter ranges on analog meters to read about double the correct value when DC was applied.
What you are seeing is almost certainly somebody using the old style circuit.
Battery voltages vary with state of charge and during charging and discharging. Nominal values may be quite different from actual value. eg
12V Lead acid - about 10V very flat to 13.7 V under normal charge and over 14V in some situations.
3.6V Lithium Ion - under 3V fully discharged and about 4.2V fully charged (per cell)
1.2V NimH - About 1.45V on charge when fully charged, 1.35 V just after charging., 1.3V fully charged after a while, 1.0 - 1.3 during discharge, under 1V sometimes when fully discharged.
NiCd - similar to NimH
AA Alkaline - 1.60 to 1.65V very new. 0.9 - 1.5V when discharging.
AA carbon zinc or heavy duty - 1.5V new. 0.9 - 1.5 discharging.
Update - reported results independently confirmed:
I recognised the meter from the picture :-).
I bought a number of those a while ago to allow multiple simultaneous sanity check measurements on some equipment. I just now checked the AC/DC behaviour with DC in and, as expected, results are as reported. As noted above, this is consistent with what I have seen in meters in the distant past.
Applying a few semi random voltages to 200 VDC and 200 VAC ranges I get
2.9 5.9
6.4 13.5
11.9 25.6
38.1 83.4
On 500 VDC / 500 VAC ranges the last voltage gave
38 83
Slightly more than 2:1 and increasing slightly as Vin rises.
I'll try to get around to tracing he circuit sometime soon.
(3:30am now and morning appointment so ...)
Not to point out the obvious, but reading the manual seems like a good idea:
I know you said you were measuring current correctly, measuring voltage versus current on most multimeters involves moving one of the test leads to a different jack on the meter. Usually current measurements are all done from a jack dedicated to current measurement. However steps 2 through 4 point out that measuring current at different ranges makes use of different ports. In this case, measurements from 0 to 400mA both use the same jack as for voltage measurements (V/mA/μA/CAP/Ω/Hz/Temp). I find this a bit unusual, but it could be somewhat convenient for low-current measurements.
As to why the meter shows the same digit (6) at different ranges, most likely because the meter was trying to read current from the jack which wasn't connected to anything during measurements.
Best Answer
You placed an ammeter in parallel to a voltage supply, which created a short circuit through the meter and most likely blew the 10A fuse in the ammeter socket.
What you want to do is put the meter between the charger and the battery, and then connect the other end of the battery to the charger. Diagram will help:
simulate this circuit – Schematic created using CircuitLab
Edit: If everything still works fine, it's likely you tripped some kind of over-current protection in the charger before the fuse in the meter blew.