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I have a USB LED light (for plants) that draws 5 V @ 1.16 A when I plug it in, as measured by my USB volt/amp meter.

EDIT: The problem was caused by the specific USB tester model KWS-MX18L. See answer below.

However, when I use the VBUS and GND of a custom USB port (not USB-C, the simple one with 4 lanes) to power the LEDs of this USB light directly (circumventing the circuit board that is between the LEDs and the light's own USB port) the USB meter that I have at this custom USB port shows 0 A constantly.

I have read that devices can negotiate the power they need via USB. I have also read that by default USB 2.0 gives you 500 mA max (unless you tell it otherwise), however the LED light is shining at full capacity, indicating that it uses more than that (1.16 A).

How is it that the USB meter does not show any amps when there is significant current flowing? Is there an easy circuit to fix this or do I need a complex USB controller chip?

And since the current is not properly detected, is it possible to unintentionally break the powerbank or power adapter that I'm using as power source because I might draw too much power without proper communication (say if I add another 5 lights)?

Note: I am just an amateur hobbyist, simple terms would be greatly appreciated.

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  • \$\begingroup\$ Please provide a data sheet for the meter. \$\endgroup\$
    – Andy aka
    Commented Jan 17, 2023 at 21:19
  • \$\begingroup\$ This is the most data-sheet like document I could find for this model. All of this information is also in the aliexpress link from the original post. \$\endgroup\$
    – ZeroMax
    Commented Jan 17, 2023 at 21:24
  • \$\begingroup\$ I'd say if you have a measurement device that does not measure something significant, it's a broken device. Get in contact with your vendor – they'll need to replace it. \$\endgroup\$ Commented Jan 17, 2023 at 21:36
  • \$\begingroup\$ It works properly on all manufactured USB devices. Just not on my own most basic circuit. I'm sure it's related to the fact that I'm just using VBUS and GND plainly without power negotiation. Any tips on this? \$\endgroup\$
    – ZeroMax
    Commented Jan 17, 2023 at 21:38
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    \$\begingroup\$ It is unclear how you are powering and measuring the LEDs when they light up and meter shows 0A. Please explain more. Maybe you are bypassing the meter or measuring incorrectly, or the measured current exceeds what the meter can show. \$\endgroup\$
    – Justme
    Commented Jan 17, 2023 at 21:40

1 Answer 1

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I fixed my own problem by re-calibrating the USB tester. If you are using the KWS-MX18L USB tester from Keweisi (or K0wsi as shown on the display), press its button and then plug it in your USB power source (powerbank or usb adapter) while keeping the button pressed (the button needs to be pressed before the tester receives power). This will reset it to factory settings and perform a re-calibration. It was not obvious in the manual how to do it but afterwards the USB tester behaves as expected.

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