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Laptop Power Supply outputting wrong voltage. Need some advise.

Posted on 2008-10-05
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Last Modified: 2013-12-11
I have a laptop that wasn't working on AC power but worked fine on battery, i took it apart to see if maybe there was a fault with the DC jack but that was fine i then decided to check the AC power supply and had a reading of 19.8V on the output.  I checked to see on the adapter what the correct output was meant to be and found that it should only be outputting 18.5V.

Now I've ordered a new adapter but until it arrives was looking for some expert opinions and experience on whether the motherboard would have been damaged due to this extra 1.8V, or would the laptop just not take the extra voltage and i should be fine with the new adapter when it arrives?
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Question by:kreeyon
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Expert Comment

by:chops_uk
ID: 22644918
you *may* find that the 'open voltage' i.e. with no load is 19v but under load it drops to 18.5 volts.. so it could still be the notebook's power port.. do you have another transformer / power supply / cord to test with
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Author Comment

by:kreeyon
ID: 22644966
Until the new adapter arrives i don't, i have a few other adapters but none have the correct connection.

I also took readings of the voltage going into the board when it was plugged in before checking to see what the correct output was meant to be and was taking readings of 19V but then the laptop wasn't turning on so i guess that's not really putting it under load.
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Author Comment

by:kreeyon
ID: 22645081
So What's the chances of of it being a case of the voltage being higher before being under load, as all the other adapters i have output the voltage that's displayed on the adapter itself and not higher.

Also IF the adapter is faulty would this be something it would do and output a higher voltage like this? and could this extra 1.8V cause the laptop to not power up on AC but not actually damage the motherboard?

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Expert Comment

by:rid
ID: 22645758
The important factor is voltage under load. Unless the adapter outputs too high voltage when it is powering the laptop, it's highly improbable that it has done damage to your laptop. If it actually has been giving a too hight voltage, the regulator circuit on the mobo may have been damaged. Will the suspect AC adapter charge the battery? Will the laptop run OK with another adapter?
/RID
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Author Comment

by:kreeyon
ID: 22645778
The suspect adapter doesn't do anything when plugged into the laptop.  Laptop works fine on battery alone but on just AC power it doesn't turn on at all.  Don't know if another adapter will work as i have had to order one as i don't have another with the same connection, also in the process of finding out if it's normal for this adapter to output 19.8V when it's not under load.

Would the extra 1.8V stop the laptop from turning on?
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Expert Comment

by:chops_uk
ID: 22645787
hopefully the laptop's charge controller will have prevented the current getting to the rest of the circuitry. certainly lithium ion batteries don't like overvoltage charging so must li-on circuitry should incorporate OVP (overvoltage protection).
In the interim.. i.e. before your replacement arrives, it's a good time to could consider getting a travel adapter for the laptop (12v).. or probably just find someone who has the same machine and ask if you can plug your machine into their power adapter for a few seconds to get a charge ;)
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rid earned 500 total points
ID: 22645793
So:
It runs on battery power? That's good.
It doesn't run on AC adapter only (no battery)? Thats probably a problem with the adapter or the power jack.
Does it run on battery, even if the AC adapter is connected?
Even if the adapter outputs a "too high" voltage when not under load, it may be bad and fold totally when it has to power the laptop.

You will either have to have the adapter tested under load, or try another adapter. A slight overvoltage shouldn't harm anything.
/RID
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Author Comment

by:kreeyon
ID: 22667790
Just received new adapter and it outputs the correct 18.5V.  Tested it in the laptop and the laptop boots up fine and battery is charging, so i guess it was the adapter after all and luckily it did not damage the motherboard.
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