Testing for Short Circuit in Laptop Power Jack

Discussion in 'General Electronics Chat' started by gdgrana, Apr 6, 2010.

  1. gdgrana

    Thread Starter New Member

    Apr 6, 2010
    I have a Toshiba laptop that does not receive power from the power adapter (Targus brand with interchangeable ends). My suspicion is in the end-piece that comes with the power adapter; all the other parts show an acceptable voltage difference between positive and ground, but the one for my laptop shows zero (or very close to).

    However, I want to rule out any problems with the power jack on the computer itself. I recently swapped out the factory jack for a sturdier one from Radio Shack. At the time, all the connections were soldered properly, and it worked without incident until a few weeks ago, when all of a sudden it stopped receiving power. I have taken the computer apart and, thus far, nothing looks out of the ordinary; I notice no scorch marks on the MOBO, and the soldering still looks solid (excellent conductivity between the wires and the actual contact points on the jack).

    My concern is in actually testing for a short using the jack. With the battery in place, I read around 12kOhms at the jack... but this is using the ground lead from the multimeter on the positive pin in the jack, and the positive lead on the ground portion of the jack. If I try it the correct way, I get 0 (or very small) Ohms. When I try to read the voltage, the correct polarity is preserved; approximately 8 V when read with the positive lead on the pin and the negative lead on the ground, and -8 V oppositely. Removing the battery brings the resistance and voltage difference to close to zero regardless of where I place the leads.

    Any help with further attempting to diagnose the problems would be most welcome. I will provide any needed clarifications if they are required. Thank you.
  2. roadey_carl

    Senior Member

    Jun 5, 2009
    Don't take offence but it seems to me like you might be doing this the hard way. The best thing to do with computers is to break it down logicly...

    1- visual check, is the laptop plugged in? is there any lights on the psu? is there any lights on the laptop?

    2- does the power suppy work? - test the fuse is the plug, then test at the end of the jack with a multimeter and depending on what laptop you have the voltage should match ( most tosh's are around 18-20v)

    after these 2 stages you will know wether the problem is a psu or laptop component problem!

    let us know when you try that out!
  3. kkazem

    Active Member

    Jul 23, 2009
    I think you're on the right track, but I want to caution you, the battery in your laptop can cause severe burns with the extremely high output current it can source (several 10's of amps at least). Also, in many laptops between the power supply jack and the battery, there is often a) a fuse and b) a series diode. Now, due to the series diode, you won't be able to measure the battery voltage at the jack. And if the battery has an internal fuse and it was blown by your Targus charger, you may have to replace the battery. Really, in order to tell, you ideally need a lab power supply with both adjustable constant voltage and constant-current that you can use to check if the battery is taking a charge thru the charging jack. But of course, be very careful to get the polarity correct. It may or may not be on the computer, but they are always (almost always) marked with the polarity on the charger. Most often, the center conductor is positive, but not always. If you don't have a lab supply, and you are very careful, you can make-up a cable to go between your Targus charger and the jack on the laptop. You'll need a DMM, which you should connect on the amps scale, but I don't recommend it unless your DMM has a 10A DC Amps scale. This will tell you if the battery is taking a charge or not thru the jack. If not, you can remove the battery and carefully and ensuring the correct polarity, connect the charger thru your ammeter directly into the battery to see if it takes a charge and if not, you very likely blew an internal safety fuse inside the battery casing.
    Good luck.
    Kamran Kazem