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I recently bought a new laptop that came with a USB-C charger, leaving me with a few universal chargers from my old laptop that do not have a USB-C tip, but have the power capacity to supply the 20V/5A = 100W that the included charger supplies.

I assumed there would be another tip created for the universal chargers that I could use to adapt the old chargers to the USB-C plug, but in the research I've done to find something of that nature, it looks like there's something about the PD interface that makes it necessary to have a chip between the raw power and the USB-C input.

Is this correct? Or is it feasible to simply chop off the end of my universal charger and splice on a USB-C lead?

Thanks!

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  • \$\begingroup\$ Many folks have made "adapters" to USB C PD. Connection must first start assuming 5V, and then negotiate. This requires circuitry, the cheapest of which is the relatively unreliable SW2303 (add lots of heat sinks to improve maybe?). There are other devices or projects that supply USB C PD from DC input... \$\endgroup\$
    – Abel
    Commented Sep 25, 2023 at 22:59
  • \$\begingroup\$ The other direction is easier. A small device called a PD Decoy (sometimes trigger) can ask a PD capable power supply to give a specific voltage. \$\endgroup\$
    – Abel
    Commented Sep 25, 2023 at 23:06
  • \$\begingroup\$ I take these comments to say the answer is "no" to simply splicing a USB-C lead into the end of my 20V supply. I just hate for all the chargers I already own to be wasted. \$\endgroup\$
    – Lowraxe
    Commented Sep 25, 2023 at 23:36
  • \$\begingroup\$ Got it. Find a cigarette lighter USB-C plug that can take 12-24 volts and supports fast charging, PD, etc. You can get 45 watt chargers for well under $20, and 30watt ones for under $10. \$\endgroup\$
    – dandavis
    Commented Sep 25, 2023 at 23:36
  • \$\begingroup\$ Also, is the logic that's built into these PD chargers the reason why they're all so thick? Some of my universal chargers are a considerably thinner profile. They fit so much better in my computer bag than this thick block that was included with my laptop. \$\endgroup\$
    – Lowraxe
    Commented Sep 25, 2023 at 23:38

2 Answers 2

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A compliant USB C power supply must start at 5V. Higher voltages can be negotiated later through the "power delivery" protocol but 5V is always the starting point. This makes the connection safe for equipment that may be unable to tolerate the higher voltages.

I have heard of some cheap off-brand laptops shipping with fixed voltage supplies with USB C connectors. I strongly recommend against such supplies, they are very likely to lead to equipment (particularly smartphones) getting destroyed.

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  • \$\begingroup\$ This answer clarifies why anything special is needed beyond the simple voltage/current combination offered by a "dumb" laptop charger. Using the idea from @dandavis above, I think I have a good solution. Thanks! \$\endgroup\$
    – Lowraxe
    Commented Sep 26, 2023 at 8:06
  • \$\begingroup\$ Technically speaking, a compliant USB C power supply must start at 0V, and source the +5V_SAFE VBUS only after it sees a connect state on CC lines of a sinking device. \$\endgroup\$ Commented Oct 3, 2023 at 3:25
  • \$\begingroup\$ OK, I got it all set up, though not at full power, and I question how long it'll hold out for temperature. I bought a car charger adapter that claimed to be 100W, but it's actually 35W on the USB-A output and 65W on the PD USB-C output. Nonetheless, I bought a cigarette lighter adapter with two leads, and wired that up to a barrel connector. That worked, but it maxes out at 65W. Cont'd below due to character limit... \$\endgroup\$
    – Lowraxe
    Commented Nov 28, 2023 at 0:01
  • \$\begingroup\$ I then bought another of that same car adapter and chopped it up. The USB-C part is distinctly separate of the USB-A piece inside, so it was easy to de-solder and break into two. I then fit the USB-C chip into a small aluminum tube that came with a solar light I bought for my yard (lucky coincedence), and 3D printed some end pieces to tidy up everything into a nice package. The laptop I'm writing this on is chugging along with this setup at the moment. \$\endgroup\$
    – Lowraxe
    Commented Nov 28, 2023 at 0:06
  • \$\begingroup\$ The tidy little package on the other hand is running quite warm. So now my question becomes, how do I keep this cool, and where do I get a board with just the USB-C that can handle the full 100W or more? \$\endgroup\$
    – Lowraxe
    Commented Nov 28, 2023 at 0:07
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USB3.1 power delivery requires that the supply vary the voltage up from 5V on command from the device.

So if your universal supply does not support 5V output, or cannot have the voltage controlled from the tip of the DC cable it's a non-starter

If it can meet those requirments it may be possible to retrofit a power delivery tip (containing the aprpriate interface chips) to the universal power supply.

If it cannot meet thos reqirements the only option is to get a power delivery DC-DC converter and use the universal supply to power that.

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  • \$\begingroup\$ To my knowledge, "power delivery tip" never was used to control voltage of a AC-DC power adapters. The tip (if present) is used solely for identification purpose. \$\endgroup\$ Commented Oct 3, 2023 at 3:19
  • \$\begingroup\$ I have seem some universal supplies that have bias dividers in the tip that return a control signal to the brick to set the voltage. theoretically this signal can be made by the power delivery source controller chip. \$\endgroup\$ Commented Oct 9, 2023 at 20:32

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