How to Check a USB Charger With a Multimeter

How to Check a USB Charger With a Multimeter

ByKen

How to Check a USB Charger With a Multimeter

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The person typing how to check a usb charger with a multimeter wants to know whether a USB charger still puts out voltage, and whether a dead phone or tablet points to the charger rather than the device or cable. A multimeter can measure the charger’s output voltage at the USB connector, with the charger plugged into a wall outlet. It cannot show the charger’s full power rating, its safe current limit or its protection behaviour, because the sources we opened do not cover USB charger specifications. Read those figures from the label on the charger.

Quick answer

  • Check the charger’s label first. Note the rated output in volts and amps, so you have a reference for the reading.
  • Plug the charger into a wall outlet, set the meter to DC volts, and measure across the USB output pins with a suitable probe, not by forcing the probes into a port.
  • A reading close to the printed output voltage means the charger is producing voltage. A reading near zero, or far from the label, points to a charger fault.
  • A meter cannot confirm that a charger delivers its full rated current. Use a dedicated USB tester for that, and do not rely on a plain multimeter for current.
  • Do not open the charger or measure its mains side. A charger that is hot, buzzing, smoking or damaged should be replaced, not repaired at home.

Safety first

A USB charger plugs straight into mains. Its input side carries dangerous voltage even when no device is attached. Measure only the low-voltage output, with the charger plugged into an outlet and the probes kept on the USB connector’s contacts. Never touch the prongs, the input side or the inside of the housing while the charger is plugged in. Those parts carry mains voltage.

Use a multimeter whose CAT rating and voltage rating suit the work. The sources we opened describe the CAT system in general terms. A CAT rating on the meter tells you which circuit category it is built for. If you are unsure, read the rating on your meter and see the guide on what a CAT rating on a multimeter means. Stop if the charger shows scorch marks, a melted housing, a burnt smell or any exposed wiring.

What you need

  • A digital multimeter with a DC voltage range that covers the charger’s output voltage. Most USB chargers are rated at 5 V, but read the label on your charger, because some are rated higher.
  • Fine probe tips or a USB breakout cable. A breakout cable lets you reach the pins without pushing the meter probes into a port.
  • The charger’s label, which gives the rated output voltage and current.
  • A known-good USB cable, so that a bad cable does not lead you to blame the charger.

Step 1: Read the label

Find the output rating on the charger. It is usually printed on the side or the back, and lists an output voltage and one or more output currents. Write these down. The reading you take later will be compared with this figure. If the label gives several output ports, note which one you will test. Many chargers share one output rating across all ports, but not all do, so check the label for each port.

Step 2: Set up the meter

Turn the dial to DC volts, marked as V with a straight line over dashes. Set the range above the output voltage on the label. Most meters will auto-range, so check that the meter is on DC and not AC. Connect the black lead to COM and the red lead to the V jack. A USB charger output is DC, so an AC setting will give a meaningless reading.

Before you test the charger, check the meter on a known source. A battery of known voltage, or a working USB port on a laptop, can give a quick sanity check. If the meter does not read a sensible voltage, check the leads and the battery in the meter before you go on.

Step 3: Connect to the charger output

Plug the charger into a wall outlet. Do not connect a device yet. The simplest safe method is to use a USB breakout cable or adapter, so that you can reach the power pins with the probes. If you do not have one, touch the probe tips to the metal contacts of a USB connector only with great care, and keep your fingers well away from the mains side of the charger.

USB uses a small set of pins. The power pins carry the voltage the charger supplies, and the data pins carry the communication between device and charger. Measure the voltage between the power pin and the ground pin. Do not measure between two data pins, because that gives a meaningless result for power.

Note the reading. A reading close to the label’s output voltage shows the charger is producing its rated voltage at no load. Take the reading a second time after a few seconds, and check that it holds steady. A reading that drops or flickers suggests a fault in the charger.

Step 4: Read the result

  • Near the label’s voltage: the charger is producing voltage at the connector with nothing attached. That does not confirm it will deliver its rated current under load.
  • Near zero: the charger is not producing output. Check the outlet, the charger’s plug and any fuse or protection the charger has, then replace the charger if it still reads zero.
  • Far from the label, or unstable: the charger may be faulty. Replace it rather than trying to adjust it.

A meter reading is one check. It shows the charger’s output at no load, which is a different condition from a phone drawing current. A charger can read correctly at no load and still fail to charge a device.

Step 5: Rule out the cable and the device

If the charger reads correctly, the fault may be the cable or the device. Repeat the test with a known-good cable. A cable with a broken wire can stop charging while the charger still reads voltage. You can check a cable for continuity with the meter, as described in the continuity guide. Test the cable with the charger unplugged, and the cable disconnected from both ends.

Fluke’s continuity guide describes the same principle: with the circuit de-energized, connect the test leads across the part you are testing, and a beep means a complete path. Use that method for a cable, not for the charger’s mains side.

What a multimeter cannot tell you

A plain multimeter does not measure the current a charger can deliver under load, and it does not show whether the charger’s protection circuits work. It also does not measure how fast the charger will charge a device, because modern USB charging uses negotiation between the charger and the device. The sources we opened do not describe USB negotiation or current limits, so we do not state figures here. For current and charging behaviour, use a dedicated USB tester that is built to load the port, and read the instructions for that tester.

How we chose the method

The method uses a DC voltage reading at the output, and the continuity check described in the Fluke guides we opened on 2026-10-27. We chose it because it needs only a basic meter and a few minutes, and because it separates a charger that makes no output from one that does. We did not pick a meter or a charger model for this guide.

What we could not verify

  • The output voltage and current for any particular charger. Those values come from the label on that charger, and the sources we opened do not give ratings for specific models.
  • The USB power and current limits in the USB specifications. The sources we opened did not include a readable specification text, so we do not state those limits.
  • How a charger behaves under load. A no-load voltage reading does not show load behaviour.
  • Whether a given device will charge on a given charger. That depends on the device and the cable, and the sources we opened do not cover it.

Common mistakes to avoid

  • Measuring the mains side. The input prongs carry mains voltage. Keep your probes on the USB output only.
  • Forcing probes into a port. Pushing a probe into a USB socket can damage the port or short the pins. Use a breakout cable instead.
  • Reading the data pins for power. The voltage that matters is between the power pin and ground.
  • Trusting a no-load reading. A charger can show the right voltage and still fail to charge a device.
  • Repairing a damaged charger. A charger that is hot, cracked or smells of burning should be replaced.

Buying checklist for a replacement charger

  • Match the output voltage and at least the output current on the label of the device you are charging.
  • Check the charger carries a recognised safety mark on its label, and buy from a seller that lists the rating clearly.
  • Choose a cable rated for the charging current you need, and test it with the same method before you rely on it.
  • Keep the old charger out of use once it fails the check, so no one plugs it in by mistake.

FAQ

What voltage should a USB charger read?

Read the charger’s label first. It gives the output voltage the charger is designed to produce. Compare your meter reading with that figure, allowing for normal variation shown on the label.

Can a multimeter show if my charger is fast?

No. A plain multimeter does not measure charging speed or current under load. Use a dedicated USB tester for that, and check its instructions.

My charger reads zero volts. Is it dead?

A zero reading points to a charger that is not producing output. Check the wall outlet with another device, then the charger’s plug and any fuse it has. If the charger still reads zero, replace it.

Is it safe to test a charger in the wall?

Measuring the USB output while the charger is plugged in is the usual method, but the mains side is dangerous. Keep your probes on the USB output only, and never touch the prongs or the housing.

Why does my phone charge slowly on a charger that reads correctly?

The cable, the device or the charging negotiation can limit speed. Test with a known-good cable first. A plain multimeter does not show the negotiated current.

Can I test a laptop charger the same way?

Laptop chargers are a different type of power supply, with different connectors and ratings. The method is similar for the output reading, but read the laptop charger’s own label and the guide on testing a laptop charger before you test one.

Bottom line

Read the charger’s label, plug it in, and measure its DC output voltage at the USB pins with the probes kept off the mains side. A reading close to the label means it is producing voltage. A zero or unstable reading means replace it. A multimeter cannot confirm charging current, so use a dedicated USB tester for that.

Related guides

Sources

  • Fluke, “How to Test for Continuity,” opened 2026-10-27: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-for-continuity
  • Fluke, “How to Check a Fuse with a Multimeter,” opened 2026-10-27: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-check-fuse-with-multimeter

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