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WTF? This computer's CPU fan is powered on by only a HDMI monitor (streamable.com)

submitted 1 year ago by JustARegularNerd@aussie.zone to c/technology@beehaw.org

10 comments fedilink hide all child comments

I actually intended to post this to Reddit but I thought I would contribute content to here instead to get the ball rolling here and do my part.

Anyway, this is a Windows XP-era machine I have at work for testing, and I had just this monitor plugged into it and saw the CPU fan trying to spin. I spun it a bit myself and it just kept going. I disconnected the HDMI cable and it stopped.

The monitor is actually DisplayPort, with a passive adapter to HDMI which then goes to the HDMI cable connected to this PC. The GPU is just PCI-E. The computer has some old ~2007 AMD CPU in it. The GPU actually doesn't seem to work anyway, the PC posts normally but there's no image from either the GPU or onboard, but when putting either another GPU or no GPU, there's an image from the appropriate output.

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[–] themoonisacheese@sh.itjust.works 11 points 1 year ago

HDMI has a 5V power rail (your adapter isn't passive, it's just parasitic on that rail). I'm guessing that this old hardware isn't equipped with diodes everywhere, and the fan 5V is connected directly to the PCIe 5V rail, and the card is also not shielded and so the 5V rail ends up directly connected to the monitor.

If i was to guess I'd say that the adapter itself also has a part in this. I think it is sending back power when it shouldn't.

[–] JustARegularNerd@aussie.zone 4 points 1 year ago (1 children)

Oh, the video was recorded on an iPod Touch 4th gen because I had it on hand. That's why the quality is so.. crusty.

[–] Curtains@beehaw.org 2 points 1 year ago (1 children)

Windows XP machine and an ipod touch. You are in a time machine.

[–] Cowbob45@beehaw.org 3 points 1 year ago

Next he'll say that this all happened while he was playing on his GameCube.

[–] Hexorg@beehaw.org 3 points 1 year ago (1 children)

Could it be something like CPU generated just enough heat to be passively cooled and having hdmi plugged in causes use of pci and the CPU heats up ever so slightly to trigger the need for cooling?

[–] Oinks@feddit.de 2 points 1 year ago

The PC isn't connected to power though, so somehow the fan has to be powered from the HDMI cable...

[–] edent@lemmy.one 2 points 1 year ago

It's my understanding that HDMI can supply a small amount of electricity - but it is from the computer to the display. See https://web.archive.org/web/20150319112324/http://www.hdmi.org/learningcenter/kb.aspx?c=13

[–] lowleveldata@programming.dev 2 points 1 year ago (1 children)

Is this why my HDMI cable shocks me from time to time?

[–] UsualMap@fedia.io 1 points 1 year ago

I'd imagine if it shocks you it's down to a grounding issue. The power in the HDMI spec wouldn't be transmitted through the shielding (at least, I hope not...).

[–] mikehunt@lemmy.world 0 points 1 year ago

Uh-oh electricity flowing through the wrong paths is always a worrying sign.