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I'm trying to appropriate the 5v that comes into an LCD monitor on pin #9 of a VGA connector to power a microcontroller with some sensors, and LEDs.

I've found a discussion on this topic on a cached forum thread from EEEforums. While there are references to standards it only discusses a monitor on an external power source and not a locally powered monitor.

How much current can I draw from this pin reliably?

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  • \$\begingroup\$ A word of warning: I can't remember the exact value, but I remember that at a company I worked at for a while, we put non-automatic SMD (non-poly) fuses on the system board for both VGA and PS/2. This meant that once you blew the fuse, the system board needed component level repair. Things may have changed past 19 years though ;o) \$\endgroup\$ – jippie Jul 11 '12 at 9:25
  • \$\begingroup\$ Thanks for the heads up @jippie. I didn't even know that they made non-automatic SMD fuses. Honestly, that sounds like a truly annoying component. \$\endgroup\$ – nelsonda Jul 16 '12 at 5:33
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It's hard to find out, since you have to pay for the required standard documents from VESA.
It's possible that there is no fixed value, and it's different for different graphics cards to implement as they choose. Since (I think) it's required to power the EEPROM in monitors for PnP, I'd say there may be a minimum value of ~10mA ("covering" all versions).

However, looking around I think up to around 100mA will probably be safe. Here are some links that support this:

http://www.vesa.org/vesa-standards/standards-faq/ - See bottom 3 questions.
Discussion of USB power from VGA port specifically this quote:

I found this from http://www.circuitprotection.com/04Databook/C17_video_(133).pdf :

“Devices that comply with the DDC host system standard typically provide supply voltage on pin #9 of the standard 15-pin VGA connector. The voltage is 5V ±5% and supplies a minimum of 300mA to a maximum of 1A.”

but for DVI:

Per the DVI spec R1.0, the “+5V signal is required in a DVI compliant system… the power pin must be able to supply a miniumum of 55mA and the monitor may not draw more than 50mA.”

To be on the safeside, don’t do this hack on your DVI port.

Maxim demo board that runs from VGA power (using ~10mA)

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    \$\begingroup\$ I believe you're right that the 5V is typically used in a monitor to supply power to its EDID, which is an I2C EEPROM that stores the monitors resolution capabilities and timing parameters, among other metadata. \$\endgroup\$ – vicatcu Jul 11 '12 at 2:42
  • \$\begingroup\$ From the actual VESA EDDC 1.2 (which is now freely available from VESA): "All host devices, including battery powered devices, must supply +5 volts (+/- 5%) on pin # 9 whenever the video port is active. The required current capability is 50mA with over current protection to limit the maximum current to a maximum of one ampere." So seems circuitprotection was wrong about the minimum of 300mA. \$\endgroup\$ – poizan42 Oct 16 '20 at 12:53
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VESA have made a lot of their standards freely available, so we can finally find out what the standard actually says. The following is from "VESA Enhanced Display Data Channel (E-DDC) Standard Version 1.3" section 7.1.3.1:

All host devices, including battery-powered devices, shall supply +5V ±5% on pin 9 whenever the video port is active. Host devices shall supply a minimum of 50mA, with overcurrent protection to limit the maximum supplied current to 1A (1000mA).

So no guarantee that you can draw more than 50mA.

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    \$\begingroup\$ Kudos for the necromancy :) but no, seriously, it's nice that a definite, well-sourced answer can be now put forward, as you did! \$\endgroup\$ – anrieff Oct 16 '20 at 13:56

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