Measure Voltage of Phone Charger Using a Simple Basic Digital Multimeter

About: Systems Administrator and Software Programmer.


Recently I bought a cheap Phone charger running on 4 AAA. One AAA battery is rated at 1.5V. So 4 batteries makes the total voltage: 6 V (1.5 X 4). The smart phone must only be charged using 5V power supply. I thought there should be some electric circuit in the cheap charger that is bringing down 6 Volts to 5 Volts. I needed to test the cheap charger because I did not want to risk frying my phone. Then I had an idea to use my Digital Multimeter (DMM). However, my simple DMM is not equipped with USB test probe.


This instructable will show:

  • How to turn a USB cable into a USB test probe
  • How to adapt the DMM's test probe into a the USB test probe
  • How to test the modified DMM

This instructable does not apply to Phone chargers that do NOT have USB receptacle.

Target Users

Just about anyone with a smart phone.

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Step 1: Gather Tools and Materials

Gather the following tools:

  • Wire stripper
  • Screw driver

Gather the following parts:

  • 1 X USB cable with a male 'A' plug (See picture)
  • 1 X Digital Multimeter (DMM)
  • 2 X Wire Connector (See picture)

Step 2: Convert a USB Cable Into a USB Test Probe

Do NOT cut off the male 'A' plug

Cut the cable close to the other plug and discard it

Strip the cable insulator to expose 4 wires

Strip the insulator of black wire to expose metal conductor

Strip the insulator of red wire to expose metal conductor

Insert the metal conductor of black wire into wire connector

Tightened the connection by screwing.

Insert the metal conductor of red wire into a separate wire connector

Tightened the connection by screwing.

The picture shows how mine looks.

Step 3: Attach the USB Test Probe to the DMM Test Pin

Insert the black test pin of DMM into wire connector of black USB wire

Tightened the connector's screw.

Insert the red test pin of DMM into wire connector of red USB wire

Tightened the connector's screw.

Attached is a picture showing how it looks like

Step 4: Test DMM

Use a USB Charger with a output voltage that you know for certain. In my case, I use an official iPhone AC charger.

Set the DMM to measure voltage

Plug the USB test probe of the DMM into the USB socket of power supply.

Switch on AC charger.

Read the meter

In my case, the meter reads 5.07 Volts which is within the specified rating of the iPhone charger.

You now have a DMM that can measure voltage of Smart Phone Chargers.


The cheap portable charger that I mentioned in the beginning measured at 6.2 Volts. It is definitely not a 5 Volts charger typical of smart phone.

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    3 Discussions


    3 years ago

    i dont think you really have to do this as there are apps to measure your chragings and discharging currents. One is used is called "Ampere"

    1 reply

    Reply 3 years ago

    Current and voltage are two different things, he is trying to measure voltage using this method and not the current.

    Also, current and voltage are two different things. Higher currents will let your device to charge faster. Lower currents mean it will charge slower. Higher voltages will fry your device. Lower voltages will also break your device. Moral of the story: never plug your device into anything that isn't rated for the exact voltage it is asking for. Doing so will most likely fry it and there won't even be time to open Ampere and test it out before you are likely to have done irreversible damage. If you are sure the voltage is correct, however, and just want to measure charging current, then yes Ampere is a great little app for that.