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I have a Raspberry Pi 3 system that I need to be small, light, and portable. It uses a USB Wi-Fi adapter that plugs into one of the ports on the Pi. The adapter is quite particular about its supply voltage, and will not initialize if its supply voltage is below 5.0V.

The Pi and adapter draws about 400mA. I've found that my standard 5V 2A power supply for the Pi puts out 5.0V, which results in about 4.85 at the USB port. I have another 6-port USB "high power" USB charger, but it is also well-regulated to 5.0 volts output.

By powering from a variable bench supply, I've found that a supply of 5.3V results in 5.1V measured at the Pi's power connector, and 5.0V available at the USB ports on the Pi. This is acceptable to the adapter but not very portable.

Rather than having to create some sort of customized supply to provide the 5.3V, I'd prefer to use a commodity "high power" "smart" USB charger (Anker, etc) and provide whatever type of circuitry is required to cause the supply to provide 5.3V.

This is a problem that was solved decades ago (in the general case) with remote sensing power supplies, but USB chargers approach it differently - more about current limiting than voltage control.

Is there any smart charging variant that enables output voltage regulation based on resistor values on D+ and D-?

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  • \$\begingroup\$ You are on the verge of asking for a recommendation for a particular product which is frowned upon here. A good question would be how do I change the output voltage on a usb charger? \$\endgroup\$ – RoyC Jan 1 '17 at 21:44
  • \$\begingroup\$ I think you should power the Wi-Fi adapter separately \$\endgroup\$ – PkP Jan 1 '17 at 21:45
  • \$\begingroup\$ PkP - I tried that, and built a USB cable assembly like an H to provide power to the adapter from the supply directly. With the 5.0 V supplies, there is still too much voltage drop for adapter. That seems strange, so I've also ordered some other adapters from other vendors, hoping they are less sensitive. I'll report back in a few weeks on how that goes. \$\endgroup\$ – tim11g Jan 1 '17 at 21:51
  • \$\begingroup\$ use better cables, or change wifi adapter. It is quite odd that it is designed to power on at exactly 5V, not a smart move precisely because there's a bunch of bad cables out there. \$\endgroup\$ – Vladimir Cravero Jan 1 '17 at 23:07
  • \$\begingroup\$ Vladimir - the custom cable I assembled was cut from high quality USB cables (26 ga, which turns out to be heavy gauge in the USB world), and shortened to 10-20cm length. Agree the adapter is either defective or a bad design. Even stranger is that under 5.0V it works fine on 2.4 GHz, but reports 5 GHz as unsupported. Above 5V, 5 GHz is available. Strange...Thus I have ordered another. There are very few dual band USB adapters with Linux support, though. \$\endgroup\$ – tim11g Jan 1 '17 at 23:19
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There is no "smart charger" that would allow you to negotiate output voltage to a fraction of volts, to 5.3V as you need. At most you can have 5V, 9v, 15V, and 20V. There could be an optional mode for custom voltages, but this is likely beyond normal reach and requires implementation of full Power Delivery protocol. You don't want to do this. Even in the simplest case of QUALCOMM QuickCharger you would need to provide certain sequence of voltages on D+/D- to get it into 9V mode.

Power delivery across Raspberry Pi 3B is kind of under-engineered, so better cables cannot possibly solve the voltage droop problem across the Pi.

Your best option is to take a standard 5V adapter, open it up, and adjust the feedback circuit for 5.5V output - this level is the official top for USB VBUS power, and it will give you some margins for your overly-sensitive WiFi dongle.

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A 0.2V drop at 400 mA means you have a 0.5 ohm drop across your usb cable. This seems pretty bad. Buy higher quality or gauge cable. Otherwise buy a power supply with a higher base voltage. Most modern Samsung supplies output 5.25 Volts before any High speed BC handshake.

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