[Linux-aus] USB-C chargers

Paul Wayper paulway at mabula.net
Mon Nov 28 19:38:15 AEDT 2022

On 28/11/22 6:53 pm, Adam Nielsen via linux-aus wrote:
> As for the charger itself, if you look at the specs they list the
> voltages and currents supported for USB PD.  I think the Thinkpad wants
> 20 volts (although it may accept less, I don't know), and both chargers
> you have linked to claim to output 20 volts in their specs, so in theory
> they would work.
> However without knowing the specs on the 30 W charger you have, it's
> hard to say whether this is the problem.  If the 30 W charger can't
> output above 17 V then I'd suspect that could be why it doesn't work,
> given a known good USB-C cable.

The other problem here is that the USB 3 PD spec has the charger and the 
device talk to eachother to find out what power modes each supports.  At 20V 
there is one mode for 45-60W and another for 60-100W; a device capable of 
delivering 30W may well be in the 15V range (27-45W).  If the battery charging 
hardware requires greater than 18V (the usual reason AFAIK for laptops to need 
20V), then it's just not going to turn on for a 15V charger.


If the new 30W charger does actually deliver 20V, then it's under-spec - the 
minimum should be 45W.

Caveat: I am not a USB 3 expert :-)

> As more devices start to use USB PD, you may want to consider getting a
> USB power monitor that supports PD, so you can plug it in between the
> device and charger and actually see what's going on.  These devices
> will show the actual voltage and current on a small screen, so you can
> see exactly what's going on and more easily compare different devices
> and chargers.  Here is an example I picked randomly from Aliexpress:

+1 for these devices, very handy.  Make sure you also get one that's 
bidirectional - I mistakenly got one that only shows charging stats if it's 
plugged into the charger (i.e. the charger has to have a socket, not a plug).  
Very annoying.

So why were we trying to get a low power charger?  I don't find the Thinkpad 
one very bulky... is it?

Have fun,

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