I have several "old" Thinkpads. Recently I came across that Type-C PD mod to replace charge port with Type-C one. Description said:

Please pay attention what you order because normally your laptop identifies the charger by an ID pin.

Also mentioned video explains some details of making that mode and pays attention to ID pin proper resistance "simulation".

I wondered before what that pin in the middle of a plug was for. I've fixed several of my old chargers worn plugs with plugs from aliexpress, I've found they have only two wires so I soldered them, no extra wire for ID pin - and they work fine charging my old Thinkpads, chargers differ by wattage only, volts are same for them as far as I saw. What does that pin really add for laptops charging? Why chargers work w/out it when video explained its importance?

Added 1:

I've tried to measure resistance with multimeter as explained in the video (ground to pin). For 65w with original round (older) plug it: ~10 kOhm (video claimed for 65w resistance is ~ 286 Ohm, I was surprised - hypothesis is that video was about newer square one), for 45w newer square one ~110 Ohm, for 90w round to my surprise nothing (broken wire?). For power supply extended with aliexplress cord/plug resistance was infinite - my guess pin in the plug is just not connected to anything. Also I have round-to-square adapter - separated it measured ~ 550 Ohm - that could mean it has resistor and by comparing with other square ones: video ~286 for 65w, ~110 of 45w mine, I guess 550 Ohm supposed to signal high power available (not safe?).

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    \$\begingroup\$ The pin tells the laptop how much power the charger can supply, similar to the function of the USB-PD controller with a type-c charger. If you don't connect the pin, probably the laptop assumes the lowest speed charger. \$\endgroup\$ Nov 19, 2021 at 1:15
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    \$\begingroup\$ For your questions on the specific resistors, your readings are right. See the sections on this page. thinkwiki.org/wiki/Power_Connector \$\endgroup\$
    – Passerby
    Nov 19, 2021 at 3:07
  • \$\begingroup\$ @Passerby, good that the link for newer plugs has power-resistance tables! I've found all, including 90w - disconnected. My adapter indicates 90 W, I use it with 65w, so could be theoretical overheating, but I have not noticed, hypothesis is that I do not own power-hungry laptop(s). How correct could my hypothesis be? \$\endgroup\$ Nov 19, 2021 at 3:16
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    \$\begingroup\$ Could be. The laptop will only pull what it needs and with the complicated power management ICs involved in the laptop and the chargers, plus "tolerances", quality etc, you could be lucking out. If the laptop needed to pull 90W but couldn't you'd may also get an error or pop-up saying that. Overall though you should just avoid miscommunating the capacity if you can help it. \$\endgroup\$
    – Passerby
    Nov 19, 2021 at 3:21

2 Answers 2


While people often call laptop power bricks "chargers", they are really just power supplies. The actual power management and charge control circuitry is inside the laptop itself.

To operate correctly the charge controller needs to know how much current it can safely draw from the power supply. If the charge controller thinks the power supply is smaller than it really is then charging will be slower than expected as much of the power brick's capacity is left unutilized.

If the charge controller thinks the power supply is larger than it really is then the charge controller will try to draw more current than the power supply can safely supply. What happens next depends on the design of the power supply, it might shut down on overload protection, it might get hotter than it should reducing it's lifetime, it might lower it's output voltage and therefore crudely limit the charge current (especially if the battery voltage is already near the top end of it's range) it might burn itself up.

As to the connectors you bought from Aliexpress it's difficult to comment without having one on the bench and making measurements, but my guess is that the connectors incorporate a fixed resistor, probably coded to claim to be one of the smaller power supplies.


Thinkwiki has a page describing Thinkpad power connectors. The page describes a whole bunch of connectors, most of which are either very old or used only on a handful of thinkpads.

There are two thinkpad power connectors* you are likely to encounter on machines still in use today. The "big barrel advanced connector" (round, older) and the "slim tip" (rectangular, newer). The two connector types use different resistor values. Interestingly on the older big barrel connector both open circuit and short circuit are valid codes. I've combined the resistor values for the two connector types in a table below.

       Big barrel    Slim tip
36W                   7.3kΩ
45w                   120 Ω
65W     10kΩ          280 Ω
90W     Open          550 Ω
135W    0Ω            1 kΩ
170W    1.5kΩ         1.9 kΩ
230W                  4.6 kΩ 

So it does seem a bit risky that your adapter is coded for 90W when it could be used with a 65W power supply. It seems an official adapter did exist but was fairly rare, so I suspect your adapter was made by some third party.

It's also quite possible that you will never notice problems with an incorrectly coded charger either because the charger crudely limits the current or because the laptop you have in the way that you use it simply doesn't want any more power.

* On more modern thinkpad models you can also use USB C for charging.

  • \$\begingroup\$ Please see Added 1. I've measured several power supplies, looks some lack ID pin resistance, still work. Your guess about resistor looks to be to the point for adapter, but not soldered plug. What does laptop "usually" do "discovering" infinite resistance of the pin? \$\endgroup\$ Nov 19, 2021 at 2:26

Some laptops or other devices refuse to charge from "unauthorized" chargers.

Some will behave different depending on what the charger identifies as. So a laptop that normally wants a 60W charger may charge slower from a 45W charger, charge faster from a 75W charger, not charge but still power a laptop from a 30W charger and refuse to use a 15W or 90W charger. A laptop expecting a 75W may ignore the 30W charger. Depends on manufacturer.

Some laptops use a simple resistor method for the ID pin. Others use One Wire or other protocols with an IC like an eeprom or microcontroller for iding the charger speed.

Usb C laptops do away with that and either support standard USB C power delivery communication or they use extended modes.

By telling your laptop you have a 60W charger but connecting it to a 45W charger, it could try to pull more current which can lead to overheating or other bad things.


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