How to Test a Phone Charger With a Multimeter

How to Test a Phone Charger With a Multimeter

ByKen

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How to test a phone charger with a multimeter, you check only the low-voltage DC output side, with the meter set to DC volts and the charger plugged into nothing but the test setup you can safely build. The AC side that plugs into the wall is mains power and is outside this guide. The sources we opened describe DC voltage and continuity measurements in general, but they do not give a USB output range or a phone-charger specification. Read the rated output on the charger’s own label, and treat the meter as a check against that label, not as a rating tool.

Electrical safety first

A phone charger is a mains device. The plug, the prongs and the inside of the case carry mains voltage, and mains can kill. Do not open the case, do not probe the prongs, and do not test a charger that is plugged into a wall outlet. Fluke’s multimeter guide covers AC and DC voltage measurement in general terms, but it does not make a wall-side measurement safe for a home user. Panel, outlet and wiring work belongs to a licensed electrician.

The meter’s CAT rating must suit the circuit it touches. If your meter’s listing or manual does not state a CAT rating for the voltage involved, do not use it on mains. This guide keeps the meter on the low-voltage output side only, and the output of a phone charger is a small DC voltage.

Keep the charger out of wet areas and away from bare metal while you work. Do not leave a charger running unattended on a bench while a probe is in its output. Unplug the charger from the wall before you connect or disconnect anything on the output side.

Quick answer

  • Test only the low-voltage DC output, never the wall plug or the prongs.
  • Set the meter to DC volts, marked V with a straight line and dashes, and choose a range above the expected output if the meter is not auto-ranging.
  • Compare the reading with the rated output printed on the charger label. The sources we opened do not give a USB output figure to compare against.
  • Use continuity to check a cable’s conductor path with the cable disconnected from power, not a charger in use.

What you need

You need a multimeter with a DC voltage setting, a pair of leads with a rating that suits the output, and the charger’s label or manual, which gives the rated output in volts and amps. Fluke’s multimeter guide describes the common (COM) terminal and the voltage (V) terminal, and says the meter’s display shows the reading. Check your meter’s manual for the correct jack for DC volts, because the terminals differ between models.

A USB charger’s output usually ends in a USB connector. The sources we opened do not describe a method for reading a USB connector’s pins with a multimeter. If you do not have a proper USB test adapter, do not cut cables or push probes into a connector to make a reading. Use the charger’s label as the reference instead.

Step 1: Read the rating on the label

Find the output rating printed on the charger, usually shown as a voltage and current on the output side. Write it down. This is the figure your reading will be compared with. The sources we opened do not state a rating for any particular phone charger, so the label is the only reference you should use for the number.

Step 2: Set the meter to DC volts

Turn the rotary switch to the DC voltage setting, denoted by V-. Fluke’s multimeter guide says to set the rotary switch to the appropriate DC voltage setting, denoted by V-, and to read the DC voltage on the display. A charger’s output is DC, so an AC setting will not give a useful reading.

Step 3: Connect the leads to the output

With the charger unplugged from the wall, make the connection to the output side using a suitable adapter or the output leads your setup allows. Connect the black lead to the common terminal and the red lead to the voltage terminal. Keep your fingers behind the insulated part of each lead, away from the metal tips.

Step 4: Take the reading only where it is safe to do so

This guide does not describe powering a charger from a wall outlet for testing. The sources we opened do not give a safe bench setup for that, and a mains-connected charger is a mains device. If a reading must be taken on a running charger, leave that to a qualified technician. Fluke’s guidance on firm contact still applies to any connection you make: a loose connection can give a false reading. Read the display, and disconnect the charger before you change any lead.

Step 5: Check the cable with continuity

If you suspect a broken cable rather than a charger fault, use continuity mode with the cable unplugged from both the charger and the phone. Fluke’s continuity guide says to set the rotary switch to the continuity setting and that the meter beeps when there is continuity. Touch the probes together first to confirm the beeper. Then test one conductor at a time, from one end of the cable to the same conductor at the other end. A beep means the conductor is continuous, and no beep means it is broken or open.

Continuity testing is for a cable that is disconnected from power. Fluke’s continuity guide describes testing a component with the circuit de-energized and the component isolated. Never run a continuity check on a live cable.

Step 6: Compare and decide

A reading near the label’s rated output means the output side is working within the label. A reading well below the label, or one that drops sharply while the phone is charging, points to a fault, but the sources we opened do not give a numerical pass limit, so this guide does not state one. If the reading is off, replace the charger with one that has the same or a suitable rating, and do not try to repair the mains side.

How we chose

We chose sources that could be opened and read as text, and that came from a meter maker’s learn section. Fluke’s multimeter-use guide covers DC voltage and the meter’s terminals, and its continuity and resistance guides cover the cable check. We did not use retail listings or forums for technical statements. We did not select a USB-specific source, because none that we could read as text covered the output side of a phone charger. The reading date of 2026-10-27 applies to each source listed below.

What we could not verify

  • The sources we opened do not give a USB output voltage or current range, so this guide does not state one.
  • The sources do not describe a procedure for reading the pins of a USB connector with a multimeter.
  • The sources do not describe the internal design of a phone charger, its protection circuits or its ripple.
  • The sources do not give a pass or fail limit for phone charger output under load.

Mistakes to avoid

  • Probing the wall plug, the prongs or any mains-side part of the charger.
  • Opening the case of a charger to reach a board or a fuse.
  • Using an AC voltage setting to read a DC output.
  • Testing a charger connected to a phone, which can add a load that changes the reading.
  • Replacing a charger with one of a different rating without checking the label.

Frequently asked questions

Can I open a phone charger to test the board inside?

No. A phone charger contains mains-side parts. Do not open the case. Replace a faulty charger instead.

What voltage should a phone charger read?

The sources we opened do not give a voltage for any phone charger. Use the rated output printed on the charger label as the reference.

Which meter setting do I use?

Use DC volts, marked V with a straight line and dashes, for the charger output. Fluke’s guide says to set the rotary switch to the DC voltage setting, denoted by V-.

Can continuity testing check a phone cable?

Yes, with the cable unplugged from both ends. Fluke’s continuity guide says a beep means continuity, so a conductor that does not beep is broken or open.

Is a multimeter enough to decide a charger is safe?

No. A meter gives a reading at one moment on the output side. It does not check the mains side or the charger’s protection, and the sources we opened do not describe a safety test for a charger.

Bottom line

Read the rated output on the label, test only the low-voltage DC output with the meter set to DC volts, and check cables with continuity while they are unplugged. Never open the charger or probe the mains side, and leave any wall-side work to a licensed electrician.

For meter basics, see how to read multimeter symbols and dial positions. To confirm the meter works before you test, read how to check a multimeter is working correctly. For a 12 V check, see how to check a 12V battery with a multimeter.

Sources

  • Fluke, “How to Use a Multimeter: A Comprehensive Guide”: https://www.fluke.com/en-us/learn/blog/maintenance-monitoring/how-to-use-a-multimeter-guide (opened 2026-10-27)
  • Fluke, “A Guide to Continuity Testing with a Multimeter”: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-for-continuity (opened 2026-10-27)
  • Fluke, “How to Measure Resistance with a Digital Multimeter”: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-measure-resistance (opened 2026-10-27)

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