USB meter: Charge Doctor



Official specifications:

• 21mm x 53mm x 15mm / 0.8" x 2.1" x 0.6"
• USB Plug: 13mm / 0.5"
• Weight: 16.2g
• USB 1.x and 2.x - all data speeds. Does not have USB v3 pins so USB v3 connections will automatically downgrade to v2
• Working Range: 0~3A, 3.5~7VDC
• Resolution: 10mV / 10mA
• Operating Temperature: 0-60ºC

This meter exist in a couple of different versions with minor differences (SMD vs. leaded parts).


How does it look






One of the digits is used to show U or A, depending on what is shown on the display.






The difference between the version is the leaded components, they have been replaced with smd components.



Not very nice looking, the circuit board has not been cleaned after soldering. The measuring resistor can be seen here, it is marked R050, i.e. 0.05 ohm (50mOhm).



Measurements

• Voltage is within 0.03V when unloaded (Tested from 3.5 to 6 volt).
• Current is within 0.06A.
• When loaded the voltage is somewhere between input and output voltage.
• The meter uses 22.3mA current
• Works down to 3.27 volt
• Internal resistance is about 0.14ohm including connection resistance.
• USB data works fine.

M1: 57,5°C, HS1: 66,9°C
Notice that the hot spot is in the connector, it has a rather high connection resistance and is getting hot.



Conclusion

The internal resistance is acceptable and the same with the precision.
With two digit resolution for both voltage and current it is a good usb meter.

It looks like there is a new generation of these on the market where the resistor is changed from R050 to R100, i.e. the resistance is 0.05 ohm higher.



Notes

How do I make the test