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I'm a bit confused about the range on my multimeter. I have connected the multimeter to a 5V USB wall charger rated at 1.3A.

When I'm measuring voltage, everything makes sense. It shows a 1 for 200m and 2000m, and 4.98V for 20, 200, and 500.

I'm confused with the current range. When it's connected to the 10 range, it reads 1.33A.

However, if I connect it to the 200m setting, it reads 13.3. How does this make sense? Shouldn't the display read "1" because the current is greater than the range (200mA). Furthermore, the 20m setting reads 1.33, 2000u reads 133 and 200u reads 13.3.

What do these numbers mean in terms of units (apart from when it's connected on the 10A range)?

Multimeter reading

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The USB wall charger is capable of providing up to 1.3A. Current draw is based on load, not on upon what the supply can provide. Obviously if a load requires more than a supply can provide, the load may not work correctly, or the power supply may fail (and cause damage if not protected).

A voltage meter places a high resistance across the test leads so that you can get an accurate voltage reading without drawing much current.

A current meter places a low resistance in series with the device under test so that you can get an accurate current measurement without adding a significant resistance. Your meter should have fuses for various current measurement ranges so that you don't damage the meter.

When you measure current provided by the wall charger, you should do so with something being charged. If you're just measuring current with your meter as the only connected item, you're effectively placing a short across the charger's terminals and it will therefore be unable to supply sufficient current.

When you connect your meter to the supply with any range that's under the 1.3A supply capability, you should theoretically blow the fuse in your meter for that range. I can't speak as to why the meter is showing unusual numbers for each range.

Connect a load to the charger, then insert the meter between the load and the charger to get a current measurement. Always start with the highest range and progress to lower ranges as applicable. Your meter measures 10A max in the 10 range, and the next lower range has a max of 200mA. Therefore, I would not measure current from your wall charger on any range other than 10 until you know for certain that the load is drawing less than 200mA - at which point you can safely switch to the 200mA range.

Note also that your meter indicates that the current measurement port is unfused and that it will tolerate a maximum value for only 10 seconds. Therefore, you might be getting unusual readings because the current measurement circuitry has been damaged. If you have only been very briefly measuring current with it, it might still be functional.

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  • \$\begingroup\$ If I wanted to test the maximum current the wall charger is capable of, wouldn't I have to be connecting the current meter directly to the terminals of the charger? \$\endgroup\$ – tgun926 Oct 27 '13 at 1:34
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    \$\begingroup\$ @tgun926, if you connect an ammeter across a power supply, you're conceptually measuring the so-called short-circuit current and that's fine. Practically, however, this is, in general, a really bad idea. For one thing, the short circuit current may blow the fuse, or worse, damage your probes, ammeter, and device under test (DUT). Unless you really know what you're doing, don't place an ammeter directly across the output of a power supply. You might get away with it once or thrice but eventually, you will experience a spectacular demonstration of why it's a really bad idea. \$\endgroup\$ – Alfred Centauri Oct 27 '13 at 2:01
  • \$\begingroup\$ @AlfredCentauri Would connecting a load based on the supply voltage and maximum rated current (in this case, a 6.5W 3.9ohm resistor) be sufficient to not cause fuses/circuitry to be damaged? Or should I not be attempting to measure something like this at all? \$\endgroup\$ – tgun926 Oct 27 '13 at 2:27
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    \$\begingroup\$ @tgun926 adding the load and measuring that is perfectly reasonable. The point is that with a short there's nothing to limit the current at all if the power supply doesn't do limiting itself. Many, many supplies don't limit current. The rating is simply a rating... there's no guarantee the power supply will behave if you go outside that rating. \$\endgroup\$ – darron Oct 27 '13 at 2:44

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