I’ve wanted one of these for years, but I’d not seen any in the electronics stores. Eventually, I found some cheap($10) LCD panel meters, so I decided to make my own.
The first version was built inside a small box and ran off a 9V battery. It was a complete failure, as you can’t run two panel meters in voltage and current modes off a single battery without smoking the current meter. I blew two before I realized it wasn’t my wiring at fault and there must be a current path inside the LCD driver chip.
The second version uses a larger box which can accommodate two 9V batteries. It has a 3A bimetallic fuse and 0.1 ohm 5 Watt resistor between the positive input and output. With the current panel meter on the 200mV scale, the LCD shows up to 1.999A across the resistor. The voltage panel meter measures across the terminals of an output socket and goes up to 19.99V. I don’t consider one less digit on the voltage meter to be important for the accuracy I want.
The inputs and outputs have 2.1mm, 2.5mm and USB sockets in parallel. The input side has a square USB type B socket and the output has the rectangular type A. The USB data pins are linked by a short length of USB2 cable, with the sheath soldered to the socket shrouds. A number of 2.5mm to xyz cables have been made, so that different devices can be plugged into the output side regardless of what size their socket actually is.