Power supply via USB is practically an industry standard for many devices nowadays. Originally only intended to supply power to small devices such as computer mouse or keyboard, the USB connection is now operated without a data line practically as a universal 5 V socket. Here, the intended limit ranges for voltage and current are usually operated far outside the specification (Table 1).

SpeciticationVoltage (V)Voltage permitted (V)Current (A)Power (W)
USB 1.0 / 1.1 (Low-Powered-Port)54,40 – 5,250,10,5
USB 2.0 (High-Powered-Port)54,75 – 4,250,52,5
USB 3.0 / 3.15 4,40 – 5,25 0,94,5
USB-BC 1.2 (USB Battery Charging)5 4,40 – 5,25 1,57,5
USB Typ C5 4,40 – 5,25 315
USB-PD (USB Power Delivery)5, 12, 205100
Table 1: Electrical specifications for USB ports

To get an overview of voltage and current when charging USB powered devices I built a simple fixed voltage lab power supply.


As a voltage source, I have chosen a simple power supply with an output power of 80 W corresponding to 5 V, 16 A. This provides enough power. In the first version, I decided against an adjustable current limiter for the time being and instead fused each charging port with a 5 A fuse. Should I determine during use that a freely adjustable current limit is necessary, I would still retrofit this.
Three USB ports and a few banana jacks are available as outputs. Each USB port and connected in parallel to the third USB port, the banana jacks have a separate voltage and current measurement. This allows a single port monitoring and a good assessment of the charging behavior or current hunger of individual USB powered devices. Voltage measurement is performed directly at the output sockets. The line loss, which is mostly caused by the much too thin USB cables with too small cross-section, can of course not be detected. This makes a supplementary measurement directly at the consumer necessary.

I used three digital combination voltmeters and amperemeters for the measurement. These are cheap, adjustable and sufficiently accurate for this application. The connection is unintuitive and confusing. Therefore, here is the correct connection diagram.

It difficult to find good USB mounting jacks. Most are very expensive or substandard for me. Instead, I decided to use bare sockets with a print connection without any mounting frame, which I then constructed myself.

If you want to charge your smartphone via the device, do not forget to connect pins 2 and 3 (D+ and D-). This gives modern smartphones the signal that the charger is fast charging.

3D printing of the housing

As a basis for the box I used the “The Ultimate box maker” .scad-customizer of the Thingiverse user “Heartman”. I have chosen the dimensions of the housing so that the power supply has enough space and on the front clearly the three displays, USB jacks and banana jacks have space. For the USB sockets I have, as already mentioned, constructed a separate mounting frame.


I am very satisfied with the result so far. Simple but extremely useful device for me. Completely without gimmicks and limited to the essentials. Three separate sockets and additional possibility to tap directly 16 A at a voltage of 5 V give me the greatest possible flexibility. The whole device is completely passively cooled, so I can run it at night without the annoying noise of the fans.