This is a tool for measuring current used by a USB device. It contains:

  • an upstream micro-USB port that you attach power to,

  • a downstream USB port that you attach your device to,

  • a pair of 2mm holes to stick your multimeter probes in for measuring current.

  • a jumper to either short the two test points when not making measurements, or attach a cable or oscilloscope probe

  • a place for a shunt resistor

  • a lot of LEDs. These are not only there to make it look nice - they also have a function. When powering a low-current USB device from a USB power bank, the current draw needs to be above 50mA or the power bank will shut down. By having 50mA or more used by the LEDs you can ensure the power stays on so you can measure even very low current devices.


You will need:

J1 - micro-USB connector, P/N 0103592-0001LF

J2 - USB-A connector, P/N 87583-3010RPALF

D1-D7 0603 LED, any color you like, but see note on resistors below

D8-D9 2835 LED, any color you like, but see note on resistors below

R10 - optional shunt resistor in case you are using an oscilloscope probe to measure current

9pcs 0603 resistors. You need to select the value for those depending on the forward voltage of your LEDs

Here is how to calculate the resistor values:

You need to have a total current draw of 50mA. This is split across 9 LEDs. If all the LEDs have the same current, you need 5.56mA per LED. Each LED has a forward voltage. The USB supply voltage is 5V. The LED drops some amount of that. Let's say the forward voltage is 3V (typical for a blue or white LED). That leaves 2V across the resistor. Then we can use Ohm's law to calculate the resistor value - Volts divided by Amps equals Ohms. 2V/0.00556A = 360Ohm. So that's how you select your resistor values. If your LEDs have different forward voltages, decide how you will spread the 50mA across them, and calculate the matching resistor values.


It's best to use a stencil for assembly - use the stencil to spread solder paste on the pads on the board and then use tweezers for assembly. You can then bake the board on a hotplate, using hot air, or with a reflow oven. When placing the 0603 LEDs place the vertical ones with cathode pointing down, and the horizontal ones with cathode pointing right.

Dec 07,2018
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