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I have this charger: http://www.shintaro.com.au/product/shintaro-multi-device-charger/

It supports QC3.0 and has:

Output
USB-A DC 3.6-6.5V/9V/12V/14.5V/16V3A/19V/20V3.5A QC3.0
USB-C DC 5V/9V/12V/15V/18V/20V3A

i connected a 2pin 12v computer fan to one of its USB-A port and the multi-meter suggested that the charger is outputting 30v

obviously, a 2 pin fan does not support any kind of charging protocol, why did it still getting a > 5v voltage?

How does the charger decide its output voltage for old device like old iphones(5V 1A), or when D+/D- pins are not connected(like my fan)?

I plan to drive a WS2812B RGB LEDs Strip (60 leds, DC 5V, 18W) with the charger, how can i control its output voltage?

Update:

i think its defaulting at 20V output on USB-A, while everyone is suggesting that the default output should 5V when there is not any negotiation

not connected to any device

Imgur

connected to a fan

Imgur

connected to arduino

Imgur

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    \$\begingroup\$ it's negotiated between charger and device. See a lot of other questions, too, that you'll find when searching for "USB-C voltage" on this site. \$\endgroup\$ – Marcus Müller Aug 30 at 13:16
  • \$\begingroup\$ (also, your multimeter is probably confused by the non-constant voltage that the back-EMF of your fan might induce.) \$\endgroup\$ – Marcus Müller Aug 30 at 13:17
  • \$\begingroup\$ I think your link answered part of my question. Thanks for that! But I'm using the USB-A port, and what if my device does not support charging protocol like PD and QC3.0? \$\endgroup\$ – Eric Yu Aug 30 at 13:29
  • \$\begingroup\$ then you get 5V, at a limited current, if your charger is standards compliant. \$\endgroup\$ – Marcus Müller Aug 30 at 13:32
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    \$\begingroup\$ If it's supply anything other than 5v when a load unable to negotiate is connected, it's simply broken and should not be used. For your purposes you should probably just buy a 5 amp 5v switching power supply. \$\endgroup\$ – Chris Stratton Aug 30 at 14:40
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Since your fan did not negotiate any higher voltage, it was supplied 5V. That's the default for all chargers. It's probably a measurement artifact that you have seen higher voltages.

For your LED project: The QC standard allows to negotiate higher voltages but – as far as I know – does not allow to negotiate higher currents. Since the initial voltage is 5V and since the charger is rated for 5V at 3A, you might be lucky. It will likely deliver 5V at 3A without further negotiation. Just try it. That way you get 15W (5V x 3A).

At 5V, a higher current than 3A is not possible, neither with QC 3 nor with USB PD. At least not officially. If you can and want to work with higher voltages, you can go two routes:

  1. Implement the Quick Charge protocol with a microcontroller. It's not too difficult. See Turning a Quick Charge 3.0 charger into a variable voltage power supply for a good example.

  2. Get a so called trigger board, e.g. this module from AliExpress. It implements the rather complex USB PD protocol. It can be programmed to switch to a certain voltage once plugged in.

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