Novice Question: Multimeter Amp Readings
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Novice Question: Multimeter Amp Readings

by PippaPeppa on Fri Aug 26, 2011 6:00 pm

Hello and thank you for having this site! I have a novice question please: why does the listed output amperage of many power adapters specify DC "A" when it really is AC "micro-A" instead? And why is the decimal position often off by two places?

For example, I'm using a Klein Tools MM200 multimeter to test the output of a small USB hub power adapter with the following printed specs on the adapter:

Input: 100-120V ~ 50-60Hz
Output: 5V = / 2.6A

When I test the DC voltage, it's correct (5.27 DCV), but when I test the DC amps, it seems totally wrong (.006A). However, when I switch to testing amps in AC mode, the number matches better (.025) but is off by magnitudes. Why do I need to be in AC mode for a DC output adapter? Why is the decimal two places incorrect?

I have another laptop adapter whose amperage output is only correct if I switch to AC mode and move the decimal over by one (the device's listed output is DC 1.5A, but measured at 0.031A in DC mode, but 0.12A in AC mode).

Finally, when I test the output of a second laptop charger, the voltage is correct and the amps read correctly in DC mode, but while the charger lists the output amps at 3.3A, the measured output is actually 0.033A. The decimal is wrong by two places again.

I know that if I move the multimeter selector from A to mA to uA, the decimal will be "fixed", but the reading doesn't match what's printed on the adapter/charger, namely, just "A" and not "mA" or "uA".

Thank you for any insight!
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Joined: Fri Aug 26, 2011 5:21 pm

Re: Novice Question: Multimeter Amp Readings

by brimus on Sat Aug 27, 2011 1:24 am

OK first thing a,ps cannot be read with a meter like volts can you have to have a load attached and will read the amperage the load is using. so if you have a circuit that has a load of 100 mA connected to a 500 mA power supply and take a reading it will show 100 mA not 500 Ma because you are reading the load being used not the capability of the power supply.

Also reading a DC circuit with an AC amp probe will not give you correct readings. If you are using a clamp on type amp probe you must have an AC / DC probe not just an AC probe in order to read DC amps. If you are reading by connecting the meter inline with the circuit then you can read AC or DC.

Check out this page for a brief explanation on reading DC amperage.

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Re: Novice Question: Multimeter Amp Readings

by PippaPeppa on Sat Aug 27, 2011 10:52 am

Thank you Brian. I see now that amp readings cannot be done directly, I have to create a circuit with my multimeter in-line with the current flow (like my Kill-A-Watt meter does).

I also found out that you shouldn't put the red wire into the 10Amp left-hand plug on the multimeter and then stick the two probes into a banned electrical socket. I think this blew the 10A fuse in the multimeter, though, I have to admit, I don't know why this happened. After all, if I can put the red wire into the right-hand plug on the multimeter (which says "400mA max", which is much much smaller than 10A) with no blown fuse while testing a banned electrical socket, then why would the larger capacity option blow the fuse??? Also, I've seen at least two tutorials that say, when in doubt, test the amperage with the 10A plug first, then, if it's below 400mA, use the lower capacity plug for more accuracy... (????)

I wonder if the FAQ should have a list of "Top 10 Things NOT To Do As A Newbie".

Thanks Again!
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Re: Novice Question: Multimeter Amp Readings

by scott_42 on Sat Aug 27, 2011 1:46 pm

PippaPeppa wrote:I wonder if the FAQ should have a list of "Top 10 Things NOT To Do As A Newbie".

#1 on that list will be something like "Don't put anything in an AC banned socket until you understand the concepts of electricity and the possibility of death/fire/disaster should you make a mistake."

Hence why you don't stick a multimeter's leads in the Amps position into a socket. That will basically just close the connection between positive and negative and blow any fuses. If there were no fuses, you would have fried the wiring and possibly caused a nice electrical fire.

Please don't play with electricity (AC or DC) until you understand what it is and how to use a multimeter safely.

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