Do you know that feeling when a device you really care about, that you put a lot of effort into and that you love suddenly starts to misbehave? That it actually feels broken? And that you have admit that this fail has probably something to do with what you did last?

When something like this happens my blood pressure rises, my mood crashes and I curse myself for playing around and trying to improve things that already worked quite nicely! Exactly this is it that usually breaks things 🙂 But without this drive to make things better, where would I be otherwise?

So it happened to me recently with the Teclast F5 laptop I described here and here. I bought myself a portable 13.3″ fullHD screen that gets attached via USB type-C – pretty neat! Just one plug for power supply and video signal. The F5 also uses type-C Power Delivery (PD) for charging and power supply, the screen has a tye-C PD input and a type-C input for the video signal, the PD power supply is then used for the screen and to drive the video source, so in my case the laptop. Cool!

So I connected the PD charger to the screen, the screen to the laptop and *flicker*flicker*flicker* a picture! Oh wait, gone it was… oh there it is again! And gone and there until it finally became stable. I thought some cold joint of some plug, damn, I got a Monday model. I worked with it a little until I notice that the battery of the laptop got drained, though I had connected the PD charger. What is going on here? I connected a type-C power meter in between charger and screen and was shocked to see that the power intermittently came on, went off, came on again, stayed for a bit, went off etc. Even best case it got up to 5V and just some few mA. WTH? I instantly thought the screen killed my laptop’s type-C PD input, F*CK!!!

I rapidly disconnected the screen from the laptop and connected the charger + meter to the laptop alone – and SHOCK! Also here power went on and off and on and off – F*CK! Did I really kill my laptop!? Just by trying this stupid screen? I already started to curse myself… I tried different PD power supplies hoping that just the power supply went bad but no, two others showed the same. I rebooted the laptop and see there, as long as the laptop was powering up and kept in BIOS, it kept power and charged. Did I do something bad in Linux? Some kernel update I did not notice? So I booted older kernels, no improvement. In total despair I dug out the Win10 SSD, built it in and booted Windows *yuck*. The same… I was already thinking about where I could get a replacement laptop…

And then it slowly started to occur to me… I tried one more power supply, one that can supply just a little more juice – hooray! It worked! I was soooo relieved.

So what happened? Well, something that should not happen with type-C but can happen with crappy implementations. USB type-C PD can negotiate the power, i.e. voltage and current. The sources and sink should negotiate it in a way so that the total can be actually supplied. So the screen needs to negotiate something with the power supply that is enough to power itself _plus_ the attached device. The attached device must negotiate enough with the screen to power itself (and charge a battery if necessary, or not charge the battery but just power itself). But what happened here is that just the voltage was negotiated (I could see it going up to +12V) but current was ignored. So the screen thought it was OK and then the laptop thought it was OK and then both started to draw current – the screen, the laptop plus charging the laptop battery which then was too much for the power supply, so it went into over current protection and switched off, the load dropped, it switched on again, the devices started to draw, it switched off and so on. This was also the cause of the flicker.

Attaching a proper tyep-C PD supply with >30W resolved it.

Also note with type-C and video you will need proper full function type-C cables. I have a couple of type-C to type-C cables which work nicely for power supply (PD) and USB data but not for video alt mode. You will notice full function cables with their thickness, they are usually thicker and stiffer than the USB data only type-C cables.