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1574 posts

Uber Geek
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Topic # 93097 14-Nov-2011 11:59
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Hey there,

I have an HP laptop, and the HP laptop charger appears to have 3-pins on the plug. I believe one pin is ground/common, one pin is +18.5V, and the other pin is some sort of communication channel between the laptop and the charger.

I also have a 12 volt laptop car power adaptor that I am keen to use to charge my laptop on occasion. It is capable of outputting 19V, which the laptop will accept. But it only has two wires, common/ground, and positive.

What I want to do is cut into the existing 18.5V power cable on the original mains charger and join the cable onto the 12volt car charger. I am unable to do this from the standard "multi-adaptor" end that was originally on the 12volt adaptor because it didn't come with the appropriate connector for my laptop, hence why I want to use the existing cable.

My question is really: the middle core in the cable which is used to communicate between the laptop and the charger; is it advisable to leave that wire out? The reason I ask is that 12volt car/laptop charger obviously doesn't have this communication wire.

What would happen if I didn't have this 'middle pin/communication wire' connected to anything? Would my laptop blow up, would it over charge?

Any info/help would be greatly appreciated! :-)


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1032 posts

Uber Geek
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  Reply # 545106 14-Nov-2011 12:29
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I seem to recall that on a Dell laptop it will power the laptop, but not charge the battery if it cannot communicate with the charger. Parhaps the HP laptop will behave the same way.

1574 posts

Uber Geek
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  Reply # 545294 14-Nov-2011 18:03
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Thanks Skolink. That sounds about right.
I could probably give it a go, but it might be more effective just to get a good quality 150-350W pure sine wave inverter.

Probably a fairly expensive way of doing it, but I guess buying an inverter means I could run other things off it too, as opposed to a single function 12volt car laptop adaptor.

Once again, thanks for the info. Much appreciated.


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  Reply # 545335 14-Nov-2011 20:11
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You might be able to get away with a modified sine wave inverter, given that modern laptop chargers have a fair bit of power cleaning/regulating gear inside them

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Ultimate Geek
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  Reply # 545355 14-Nov-2011 20:52
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Below is some background information about HP "Smart-pin".

I encountered this on an HP Mini 5102. The signal voltage on the (3rd) "Smart-pin" probably varies by model and for the 5102 I think indicated to the laptop whether the power supply was an HP power supply (by having the live "Smart-pin") and also whether the particular power supply was the version capable of supplying sufficient current for a "fast-charge" or not. 

Smart-pin power connector HP 6715s

HP laptop go-slow caused by power supply see comment dated 6 Jan 2010 re resistor.

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Uber Geek
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  Reply # 545486 15-Nov-2011 09:35
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Don't bother with a pure sine wave inverter if you plan to use it only with a laptop PSU. They don't draw a sinusoidal current due to the rectifier on the front end, a modified square wave inverter will be fine.
You should be able to get a suitable connector on eBay for a couple of dollars from one of the chinese dealers.

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  Reply # 545743 15-Nov-2011 20:40
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Thanks for all the helpful advice guys!

Good to know most laptop chargers deal with modified sine wave inverters adequately.
In saying that, I think I will eventually grab a pure sine wave inverter just the same. Will probably end up using it for other things, so I might as well.

Thanks lapimate for those links.
Had a skim over them and some people seem to be having some success by putting a 1K resistor from the +19v to the centre smart pin.
Those threads still don't give great clarity on what the purpose on the centre pin actually is, but it goes some way to better understanding how to hack around it.

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  Reply # 545789 15-Nov-2011 22:06
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On our Dell laptop it is used to identify a 65W vs 90W power supply, so the battery can be charged at the appropriate rate without overheating the PSU. Possibly to protect Dell's accessory sales revenue too. If it's as simple as a resistor, then it is just for practical purposes, not to prevent 3rd party PSU being manufactured. I had assumed it would be some sort of handshaking communication, like Nokia chargers used to (before USB connectors)

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