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USB data blockers are small adapters that have a male and female USB connector, but inside the data lines are disconnected.

They can be used to safely charge your devices on public USB ports without the possibility of malicious software being installed on your device. Unfortunately, most standards that offer higher charging power (BC 1.2, Qualcomm QuickCharge etc.) use the data lines to apply certain voltage levels on these data lines to communicate certain voltage and current levels.

As these (from my understanding) don't use high frequency signals, but just static voltage levels, would it be possible to just connect the data lines to ground with large (1uF) capacitors?

This would effectively prevent data transmission while still allowing power negotiation. Would this work? If yes, what would be a good size for the capacitors?

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  • \$\begingroup\$ I'm not familiar with the standards, but I'd wonder if you could add a small uController to such a device to do any communication and request the appropriate power on behalf of the device you're protecting. \$\endgroup\$
    – LordTeddy
    Nov 13, 2023 at 12:32
  • \$\begingroup\$ Can you clarify type of USB ports this is for? E.g. USB-C has the CC1 and CC2 pins on the connector used to establish and manage the Source-to-Sink connection. There are USB-C PD controllers designed to interface to the CC pins to allow the negotiation of a power delivery contract with a source without MCU support (auto-run mode). E.g. the STUSB4500 \$\endgroup\$ Nov 13, 2023 at 13:08
  • \$\begingroup\$ @LordTeddy: The MCU would have to communicate in both directions to properly handle this job. I am looking for a much simpler solution. \$\endgroup\$
    – Spatz
    Nov 13, 2023 at 15:29
  • \$\begingroup\$ @ChesterGillon: Sorry, I could have been more precise... I am talking about good old USB A. USB C PD is far too complex for a simple solution, and in my understanding also uses real data transmission for negotiations. \$\endgroup\$
    – Spatz
    Nov 13, 2023 at 15:32
  • \$\begingroup\$ sounds like it would work and not hurt anything, let me know if you try it and it works. \$\endgroup\$
    – dandavis
    Nov 15, 2023 at 7:53

1 Answer 1

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If the phone uses the data lines for detecting how much current is available, you need to replicate thr exact same circuitry and resistance to the phone that it requires, not capacitors.

You can of course leave the data lines between phone and charger disconnected, but then the phone cannot communicate with charger about higher voltages and how much current the phone will take.

If you need to charge a phone in public, charge power bank from the public charger first.

Large enough capacitors may prevent power level negotiations as well, not just high speed data.

Also a malicious charger may be used to fry your phone charging or data wires so it is not only the data communications you should worry about.

Nothing prevents from making a data isolator that supports BC and other protocols as well.

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  • \$\begingroup\$ My idea is to modify the data lines in a way that makes them intransparent to high speed signals, but transparent to low speed signals. If this works, I would not have to recreate any circuitry, as the phone and charger could still communicate for power negotiations, but transmitting data would be physically impossible. \$\endgroup\$
    – Spatz
    Nov 13, 2023 at 15:33
  • \$\begingroup\$ @Spatz How low speed are we talking? I'm pretty sure you can get data off a phone at USB Low Speed speeds, which is 1.5 Mbps--which is pretty slow as far as I'm concerned. \$\endgroup\$
    – Hearth
    Nov 14, 2023 at 4:48
  • \$\begingroup\$ @Hearth: It seems like all charging standards (BC 1.2, Apple, Samsung, QuickCharge) are just applying certain voltage levels to the data pins. For QuickCharge, the voltage must be held for at least 1.25 seconds. So we are talking about sloooooow speeds. I was thinking about two 15R resistors in series, with a 1uF to ground in between. This gives a cut off frequency of about 10kHz which should be enough for any charging standard, but should degrade any data transmission strong enough to be totally unusable. \$\endgroup\$
    – Spatz
    Nov 14, 2023 at 11:29

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