The great Wiki says PS/2 port has a maximum of 275mA, so about 1.4W. Anything above would be out of spec, though I think the port is generally fused on the motherboard and I wouldn't be surprised if the fuse was generally set a bit higher than that.

RiverBoa wrote on 2022-09-17, 16:53:

I have a few laptops that have no USB but use add-on devices that need USB power, like an OPL3LPT or external speakers that run off USB power. Anyone know how much power I can safely draw off an external keyboard or mouse port to run those using a PS/2 to USB adapter?

Have you considered connecting a powered USB hub to the PS/2 adapter?

That's how I solved my vintage PS/2 keyboard behaving finicky through an active USB converter.

Lylat1an wrote on 2022-09-17, 19:41:

RiverBoa wrote on 2022-09-17, 16:53:

I have a few laptops that have no USB but use add-on devices that need USB power, like an OPL3LPT or external speakers that run off USB power. Anyone know how much power I can safely draw off an external keyboard or mouse port to run those using a PS/2 to USB adapter?

Have you considered connecting a powered USB hub to the PS/2 adapter?

That's how I solved my vintage PS/2 keyboard behaving finicky through an active USB converter.

No, I haven't. But I'm trying to reduce the amount of AC plugs I need, especially if it's possible to go fully portable on a few laptops. I just don't want to blow up the board.

RiverBoa wrote on 2022-09-17, 16:53:

I have a few laptops that have no USB but use add-on devices that need USB power, like an OPL3LPT or external speakers that run off USB power. Anyone know how much power I can safely draw off an external keyboard or mouse port to run those using a PS/2 to USB adapter?

Good idea just dropping by to compliment your idea.

Calamitylime has a thread that I believe talked about the power of din kb ports.

Re: USB to Serial Mouse using the Pi Pico!

I don’t think they talked about mini din though

Sphere478 wrote on 2022-09-17, 20:19:

Good idea just dropping by to compliment your idea. […]
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RiverBoa wrote on 2022-09-17, 16:53:

I have a few laptops that have no USB but use add-on devices that need USB power, like an OPL3LPT or external speakers that run off USB power. Anyone know how much power I can safely draw off an external keyboard or mouse port to run those using a PS/2 to USB adapter?

Good idea just dropping by to compliment your idea.

Calamitylime has a thread that I believe talked about the power of din kb ports.

Re: USB to Serial Mouse using the Pi Pico!

I don’t think they talked about mini din though

I've actually had reasonable success with an OPL3LPT without any issue. But I'm trying to use some USB speakers (very cheap/low draw) ones, but I'm going to assume they'll use more power no matter what.

My late 1990's Keytronic PS2 keyboard says: Requires 5v @ 500mA max. Not sure about your laptop but many mainboards have a 125v-2A fuse to KBD/Mouse from memory of replacing one.
Just double checked two of my boards and that is what they have, that power though splits to both connectors. So you should be able to draw near 1 amp safely IF the lappy is designed the same.