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Before I begin, I'd like to note that I am very unexperienced in the electronics field, so some things that may be obvious are completely unknown to me.

I recently was dealing with a laptop that would not charge, boot, or show any signs of life. After some discussion over the phone, I learned that the AC adapter's wire was frayed, and so I immediately ordered a new one. When it arrived, there was still no success in getting any power to the laptop. I proceeded to use a multimeter to test both AC adapters, and like I suspected, the old one was reading 0V. The new one, however, was reading only 15.3V on the multimeter. The adapter says it should be outputting at 19V (both on the adapter itself and on the box it came in). Additionally, the battery is labeled at 14.4V, if that matters at all.

So my question comes down to this: could the fact that the multimeter read 15.3V instead of 19V on the AC adapter be related to why the laptop refuses to show any signs of life? If not, I will probably have to start taking it apart until I get can at the power connector.

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not a direct answer to your question, but multimeters often show lower voltages when their batteries are weak. i'd check the multimeter against a known good power source to be sure. –  Journeyman Geek Mar 12 '11 at 5:31
    
Normally laptop chargers have voltages in range of 15 V to 24 V, so it could be that charger is expecting some sort of signal before it switches to higher voltage. This could have been implemented as a failsafe, in case it gets connected to a laptop with lower voltage requirement. –  AndrejaKo Mar 12 '11 at 9:20
    
I'm not experienced enough to say for sure, but if the laptop power is a switching power supply, wouldn't it not supply full voltage until an amount of current flow is met? –  Earlz Mar 12 '11 at 21:51

2 Answers 2

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At first, your laptop power source may be really weird one (not likely). Some chargers do provide only lower voltage, until device that's going to be charged requests higher voltage. Good thumb of rule: if there's only two connectors (typically round one, outer one is ground), then it should provide correct voltage all the time.

If that's not the case, then either your multimeter is showing wrong readings or your power adapter is broken.

You can check your multimeter with some known voltage, for example new 9V battery.

Typically laptop power adapters provide overvoltage without any load. It's not unusual to get for example 23V from 19V AC adapter, when it's not loaded.

Your laptop charger circuit regulates whatever is coming from AC adapter to more appropriate voltage for charging battery. In the same way, battery voltage is regulated down to 3.3V, 5V and probably also 12V for computer internals (processor, memory, motherboard, display and so on).

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My charger does in fact have an outer ground and inner connector, which is what I tested with the multimeter. I also am fairly sure that my multimeter is correct--it reads 9V and AA batteries fine. To remove all doubt, I called up a few relatives and found one with the same charger and took a trip to test it. My charger worked on their computer and theirs still did not work on mine. Additionally, I used my multimeter to test their charger and got a different value less than 15, so I assume it does in fact have to do with not having power drawn. Thanks for the lengthy explanation. –  Doug Mar 13 '11 at 6:34

One test worth trying is to remove the battery from your laptop and then see if your laptop will run from the power supply. If it does work your battery may be faulty and causing the problem.

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+1 A dead battery can load the power supply. I have seen this before in laptops. –  SteveR Nov 21 '11 at 13:58

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