A USB Meter/Tester is a measuring device, That is connected in series between a USB power source that provides power (As a power bank or a wall adapter) and a USB power sink that consumes power (As a mobile phone).

These meters can do various functions, The two most basic functions that every USB Meter should have are Voltage and current measurement of the USB power line.

They can also have other features as:

Power consumption calculation
Capacity calculation
Saving data sets of measured data
Triggering and testing fast charging protocols from the fast charging capable power sources

In this instructable, I am testing 3 identical cheap power meters that i bought from AliExpress and my target is to see how they perform over their working range and see if i can depend on them in some electronics projects.

I am going to perform 3 tests on them to see their voltage and current measurement accuracy and their self-consumption current, Then i will disassemble one of them to figure out the schematic of the board inside.

Supplies

For these tests the following devices are needed:

USB Receptacle breakout board (To act as a power source output port)
UT61E Multimeter (You can use any accurate enough multimeter for these tests)
Handskit SW3010D 30V-10A Variable power supply (To test the meters at different voltages)
DC Constant current load device (For applying a constant current load on the meter)

Step 1: Make the USB Power Breakout Board

I made this breakout board to to connect it to the variable power supply from its input wires side and connect it to the USB meter from its output connector side

A shunt jumper and pin headers are also added to allow current measurements on the board

Step 2: Voltage Measurement Accuracy Test

This test is intended to determine how accurate these meter in measuring their USB input voltage

We are going to test their accuracy along their operating voltage range (3.5V TO 7V) but i am going to stop at 6.5V to keep a safety margin below the meter maximum operating voltage.

Test steps are:

Increase the input voltage in steps of 0.25V (starting from 3.5V to 6.5V)
Record the voltage reading with the used multimeter (UT61E in this case)
Record the voltage reading displayed by each of the three meters

After finishing this test i was delighted by how accurate these meters are, They are off by maximum of 5 counts which is something that i can depend in most of my electronics projects

Step 3: Current Measurement Accuracy Test

This test is intended to determine how accurate these meter in measuring the current passing through them

I am going to increase the applied current using a constant DC load device in steps of 0.25 Amperes Until I reach 3.5 Amperes.

Test steps are:

Increase the load current in steps of 0.25A
Record the voltage reading with the used multimeter (UT61E in this case)
Record the current reading displayed by each of the three meters

As you can see from the measurements table and graph, these meter are not as accurate in current measurement as they are in voltage measurement, They are also not accurate enough in the <100 mA range so you can't depend on them with microcontrollers and development boards.

Step 4: Self-consumption Current Test

In this test, I am measuring the DC current consumed by the meter itself.

Test steps are:

Connect the multimeter in series with USB meter to measure the consumed current and disconnect any load from the meter
Increase the input voltage in steps of 0.25V (starting from 3.5V to 9V) *(You can see why i reached 9V instead of 6.5V in step 6)
Record the multimeter DC current readings vs applied voltage

You can see that the self-consumption current is approximately increasing linearly with the applied voltage.

At 5V, the self-consumption current is 15.2 mA

For voltages above 4.5V you can calculate the theoretical current consumed by the meter according to this equation:

I(self-consumption in mA) = 1000* ( ((Vusb - 3.325)/120) + (Vusb /30000) )

Step 5: Minimum Operating Voltage Test

When decreased the input voltage to the meter it continued to display until it reached 2.4 Volts.

But between 3.5 and 2.4 Volts it just displays 3.5V despite what the actual applied voltage is.

Step 6: Teardown and Circuit Analysis

As you can see from the obtained schematic, The meter has:

A 50 milliOhm shunt resistor, wired for low side measurement and will cause a 50 mV/A voltage drop ( Take care that due to the low-side measurement configuration this will cause output side ground voltage rise by 50mV/A over the input side ground pin )
A TL431 Voltage reference IC working as shunt regulator to supply 3.325V theoretically for the meter IC VCC pin, This 3.325 value is calculated using the TL431 Vout formula (Vout = Vref*(1+R6/R5)) using the values of R5 and R6 on the board which are (1000 Ohms and 330 Ohms respectively)
A voltage divider with a ratio of 1/3 which divides the input voltage by 3 to keep it under the 3.3V Supply for the IC
120 Ohm current limiting resistor (R4) Which limits the current consumption of the TL431 under the maximum value of 100mA, This resistor also determines the current consumption of the device

As the meter is using the TL431 IC for its power supply and due to the accuracy of this voltage reference, The meter can measure the USB bus voltage accurately as we have seen previously. In addition to, using precision resistors.

When I found that the VCC of the IC is around 3.3V and the voltage divider for the USB voltage has a ratio of 1/3, I figured out that the maximum operating voltage of the meter can be more than advertised, It might be more than 9V!!

Where did this number come from ?

Well, by assuming that the input voltage on the voltage measurement pin can safely reach the VCC value so by multiplying the VCC value by the divider ratio we get 9.975 V

So I tested the meter by gradually increasing the input voltage to 9V and guess what ?

It didn't blow up and it also measured the applied voltage accurately!!

Also at 9V the regulation current passing through the 120 Ohm resistor is 48 milliAmperes which is still less than the maximum limit of the TL431 which is 100mA.

Step 7: Conclusion

After measuring the meter performance in voltage and current measurements, I can now know when to use this meter and when not to.

Note that when voltage readings will differ from the input value when load is applied due to voltage loss in the VCC PCB trace, The meter is measuring the output voltage not the input voltage because the voltage divider resistors are placed near the output port not the input port so there will be unmeasured voltage drop between the input port and the voltage divider resistors.

You can use this meter for:

Voltages from 3.5V TO 9V MAX. (See steps 2,5 & 6)
Current values up to 3.25A (But you will need to neglect having up to 200mA errors in higher ranges) (See step 3)
When you don't need more that 10mV or 10mA of resolution

I do not recommend using this meter for:

Current measurements that need high accuracy (See step 3)
Low current applications <200 mA (See step 3)
Applications working with USB Voltages above 9V (See steps 2 & 6)
Any fast charging protocol power source or sink device (Higher voltages used in fast charging can destroy the meter and higher current may cause extra heating in the PCB tracks and shunt resistor)