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The USB Charger Doctor is a handy tool that measures the working voltage and current output for any USB port and project. Plug the Doctor between a USB port and your USB device, and it will check the current draw with a 0.05 ohm resistor in line with the power pin. Data lines pass through as normal. 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 roughly every 3 seconds.