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need help tracing power circuit on laptop
(railroadrum) wrote:
I'm attempting to fix a laptop for a friend. The power plug from
the power supply has been broken off and lost. Rather than spend
$80 for a new supply I am trying to hard wire (via a quick
disconnect) the power wire to the jack on the computer. I'm
skilled with a soldering iron and I know I can make the necessary
connections. My problem is I don't know how to find out which
pins in the jack are positive, etc. The wire is a four conductor.
Ground is easy to find by ohming to the case but I need to know
how to trace the circuit for the other wires. Can someone please
intruct me a bit on this? Many thanks.
Tracing the pin polarities on the laptop motherboard is probably the
last thing you want to try to do. First you should try to get specs
for the charger and power plug off the web. Next, you can probably
order a replacement plug from someplace like DigiKey and solder it
onto the damaged power cord from the original power supply. Third,
you may be able to pick up a replacement power supply on the cheap
from Ebay or at a local hamfest.
Finally, if you really can't get this solved any other way, you just
need to identify the power supply components on the laptop motherboard.
There are only a few companies that make most of the analog/powersupply
chips in common use (Dallas/Maxim, National Semi., etc). If you can
identify the power regulator portion of the laptop PCB, you can probably
make a good guess as to the regulator's topology and associated parts.
With that, you should be able to trace directly back to the socket.
Assuming that there is nothing more than simple linear regulation on
the laptop PCB (all the heavy lifting, 120VAC - ??VDC, having been
done by the external power adapter) it shouldn't be too hard to find
the power regulator chips and look them up on-line. If all there is
is linear regulation, the Vin lines to the regulators should be, more
or less, directly wired to the line power socket pins. Now, just find
the linear regulator chips on the laptop PCB, look up the chips' data
sheets on-line, and find continuity from the Vin pin on the regulator
to the power pins in the socket.
If the laptop uses a switching regulator on the PCB, this could be a
bit more involved. Again, you need to identify the power regulation
portion of the PCB, look up the power regulator chip on-line, and then
figure out how it was used on the PCB (what topology was used). Next,
you should be able to guess at what are the rest of the power regulator
components on the PCB. Finally, you should be able to discern where the
input voltage enters the power regulatioin circuit and trace continuity
back to the power pins in the socket.
- Jeff Dutky
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