Laptop charger 3 core cable

[flester]

Planning to use 19.5v laptop charger as a power supply for amps and other projects. It has a 3-core cable, red, blue and black. I only expected '+' and '-', whats the 3rd for?

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[Jazz P Bass]

The 3rd wire is an identifying wire for the laptop.

[phatt]

Just a warning , some of those lappy supplies are noisy due to the switchmode circuits used.
it's a lottery but you can get lucky so don't commit until you have tested in the real world.
I use a SMode 19Volt laptop PSU and it's noise free, I added a 9volt regulator circuit for the lower voltage pedals.
Ground return paths can cause issues so I have added a ground back to the mains plug which is built into the pedal board.
If you don't understand how to do that then get advice when working on a mains setup.

The cases of those Lappy supplies are plastic so they can radiate a lot of noise/switching hash if close to audio circuits so some shielding maybe needed.
Phil.

[flester]

Yes I know switched mode can be noisy but have been gathering old supplies and finding quiet ones by trial  and error. Do you think I can just leave the 3rd wire disconnected?

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[phatt]

Good on you for collecting them as they are often sold very cheap. I pick them up from op shops for $2~$5.

As to the 3rd wire,, well if it powers up and you have DC voltage at the other 2 wires then obviously you won't need to connect that 3rd wire. (others here might have more experience with those)

Some come with 2 wire mains input but some may have a 3 wire IEC Mains input socket. With those you may find the Mains Earth is connected to the Negative of the secondary wire, in which case you already have a ground return path to Mains Earth which in my experience has helped with interference.
Phil.

[flester]

Will just try it anyway. If it works and theres no smoke or sparks then its OK.

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[joecool85]

Will just try it anyway. If it works and theres no smoke or sparks then its OK.

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That's pretty much it.  For what it's worth, I believe the Dell branded ones are quiet.  I'm sure others are as well though.

[Jazz P Bass]

Quote: "Do you think I can just leave the 3rd wire disconnected?".

Again. The third wire is used by the laptop computer itself.

If you simply want to use the supply as an external supply then the third wire is not used.