USB Charger Doctor – Voltage and Current Display Use the USB Charger Doctor to measure the voltage and current output for any USB port and project. This blue plastic dongle plugs between a USB device, and passes the data lines through. There’s a 0.05 ohm resistor in line with the power pin that is used to measure current draw. The Doctor’s digital display alternately displays the USB Voltage and the current draw. You can use this device from 3.5-7VDC input voltage (although most USB devices tend to hover around 4.75 to 5.25) and up to 3A of current. It flips between voltage and current readouts every 3 seconds or so. It’s a very handy tool for testing USB devices, checking load levels and debugging battery chargers and boost converters.