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Re-purposing a Dell laptop PSU - Question

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WimWalther

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Jan 12, 2019
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I'd like to reuse a 19.5V Dell laptop PSU, but ran into something confusing..

The low-voltage cable (plugs into laptop case) turns out to be a "triaxial" design.. that is, starting at the outer jacket and working inward I find:

Outer jacket > shield 1 > inner jacket 1 > shield 2 > inner jacket 2 > center conductor. So a total of 3 conductors..

So what are these used for? Between two of them, don't recall which, I can read 19.5V DC. Between others, I find 0.5VDC. Obviously, I can connect the 19.5V pair, but what to do with the other conductor?

Thanks!
 
Btw, I believe the original plug had a 3 conductor design: Outer cylinder, inner cylinder and center pin.

So it seems all 3 conductors were being brought into the laptop case
 
DeltaDon said:
I believe that is a sense line to let the computer talk to the pwr supply and shut it off upon a short or other error.
Click to expand...
That's what I was thinking, FWIW. But no t knowing what it expects is bugging me.

So do I just ignore the 'extra' conductor, or tie it to +/-?
 
@DeltaDon

Thanks for finding that, though it's a typically info-poor YT video. Really, all points of interest could have been covered in about 20-30 seconds - and he never bothers to tell us WHICH WIRE IS WHICH, though this seems to be critically important.

From the first schematic, it seems that the connections are like: Outer shield = -DC, Inner shield = +DC, Inner conductor = ID line.

For my own (re)purposes, it appears safe to either ignore the ID line entirely, or safely connect it to +DC. It could also safely connect to -DC, though this seems clumsy and pointless.

So how does the ID circuit work? It must be that, internally, the laptop connects ID to +DC through a current-monitoring resistor. And based on current draw (or maybe voltage drop?) it determines the type of charger present and proceeds accordingly.
 
The center pin is the data line that the laptop uses to communicate over a 1 wire protocol to the power brick. It uses this to determine what kind of power adapter it is, and what wattage it can output. If the laptop doesn't get this signal, it will either not power on, or complain on POST and operate in a "limp" mode where the laptop goes into its lowest power mode and won't come out of it to avoid damaging the power brick.

If you plan on using the power brick for something else, just disconnect the data line and leave it floating. Don't tie it to any other voltage rail or you may cause problems.
 
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