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Fixing Laptop Power Problems

I created this site because of the enourmous number of power problems that people have written about to me. I hope it is helpful to you, and I welcome any suggestions. It is mainly based on my experience with HP and Dell laptops, but should be pretty universal as far as I know.

USE THE INFORMATION AND DIRECTIONS ON THIS PAGE AT YOUR OWN RISK. I DO NOT WARRANT THIS INFORMATION AND WILL NOT BE LIABLE FOR THE OUTCOMES OF USING THIS INFORMATION.

Intro

Understanding how laptop power works can be very useful in determining what the problem is. I will first explain how it works, then I will explain how we can troubleshoot.

AC power adapter

How the AC adapter works

The most common type of AC power adapter involves a small box, with one wire to go to the computer and another wire to go to the wall. There is usually an LED light on it, to tell you that it is on. (Is it lit on yours?) By the way, Apple notebook power adapters often have an LED that is actually built into the side of the computer, not into the adapter box, so obviously it won't light up when it's not connected to the computer.

The power adapter box does several things. It lowers the voltage from the wall's 120 volts AC to typically 19 volts DC (it will be labeled with the exact voltage). It includes some power noise filtering. It often includes an automatic circuit breaker or overload detection. If this gets tripped, you can generally reset it if you unplug it from everything for a few minutes.

You can measure the voltages on your AC adapter with a multimeter. When it is unplugged from the computer but still plugged in to the wall, it is normal to find that the voltage may be 1 to 3 volts higher than the printed output rating.

Troubleshooting: AC Power adapter LED is off when you plug the adapter into the wall but not into the computer

Has it been overloaded? Is the power strip turned on / is the outlet working? Is the cord from the adapter to the wall fully plugged in on both ends (try wiggling)? Your adapter may be fried- try borrowing an indentical adapter from a friend and seeing if that one will work in it's place (but do not plug it in to your computer, or you might fry your friend's adapter).

Troubleshooting: Check output voltages with a volt meter/multimeter

For those of you with circular connectors, your task is easy. Measure the voltage between the inside and the outside. A diagram on the adapter's label will tell you which should be positive and negative

For those of you with the 3-pin Dell power connectors, I don't have a diagram for you yet, sorry. If you try measuring each of the three possible pairs of pins, you should get 20 volts between one of the pairs.

Power distribution inside the laptop

How laptop power distribution works

voltage regulator Okay, the power comes into the plug on the back of the computer. This connects it to metal traces inside the mainboard of the computer. These will be connected to voltage regulators which often output 5 volts, 12 volts, 3.3 volts, and the CPU voltage; they sometimes look like the image at right.

These will be connected to cylindrical capacitors distributed throughout the mainboard. Capacitors act like tiny, fast batteries, supplying extra power where needed to maintain a steady voltage during high demand.

Finally, power goes to all the devices that need it.

Power for the LCD display is usually done seperately. Generally, LCD backlights require high voltage to operate. A component that might be called a power inverter will step the voltage up to what is needed for the backlight. Sometimes this inverter is a discrete and replacable component, and sometimes just a chip on your mainboard. It is also involved in the brightness control.

The battery is an important part. It supplys power to the input-side of the voltage regulators, just like the power from the AC adapter. Additionally, there is a charging circuit, which uses the power from the AC adapter prior to the voltage regulators, to charge the battery. (By the way, this is why the AC adapter voltage is always rated higher than the battery's rated volatge-- you need higher voltage for charging.)

There is a separate page exclusively for Laptop Battery Problems.

Short circuits

Anywhere in your laptop, physical or electrical damage can cause a short circuit. A short circuit will consume all available power, causing your laptop to not turn on. If your power adapter LED comes on when you plug it into the wall, but then goes off when you plug in the laptop, you probably have a short circuit.

Troubleshooting short circuits

An ohm-meter / multimeter will read less than ~3 ohms when you measure the resistance between sides of power going into a short circuit. When measureing between then power pins going into the back of your computer, consider the 2 pins where you expect voltage to be applied by the AC adapter. If the resistance indicates a short circuit, this is bad.

A common trick to help isolate possible problems is to try to removing anything that might be a short circuit. Take out the battery, the hard drive, the DVD/CD drive, the floppy drive, PCMCIA cards, USB devices, miniPCI cards. See if the computer will then turn on.

You might have luck with a thermal imager (see story below) to find a hot spot caused by the short circuit.

Spilled liquid

Laptops exposed to small amounts of spilled liquid are repairable in some cases. Liquid can cause the following types of damage:

Immediately when a spill occurs, you typically want to turn off the laptop, turn the laptop upside down (so the liquid goes out the same way it came in), remove the power and battery, and allow to dry at least overnight.

To remove dried liquid residue: Try removing the keyboard and cleaning any components that have dried liquid, by using distilled water and Q-tips.

Computer turns off randomly

Your computer can turn off due to: power overload in the AC adapter, overheated processor / clogged fan, overheated battery, pushing the power button, Windows telling it to, the BIOS telling it to, loose wires (especially the power cord), or intermittent short circuits.

The most common problem is cooling. Try going somewhere air conditioned. Notice if the fan is broken or clogged with dust (common older HP problem).

Web links

Story: Finding a Short Circuit (contributed by Chris Wilkinson)

This laptop (HP zt1250) was used for 2.5 years. At 1.5 years it began running the fan constantly and would occasionally shutdown. At 2 years an external desk fan was required while burning CDs. At 2.5 years the laptop external power supply indicator light dimmed when the plug was inserted and was bright when the plug was removed. An electrical short had developed somewhere inside the laptop. The laptop would no longer run off external power.

The small charge left in one battery confirmed that the rest of the laptop was functioning.

Disassembly of the laptop revealed large accumulation of lint on the cooling fins of the heat pipe. See photograph. The cooling fan had slowly but surely suctioned lint and dust from the users bedroom, some of which was unable to pass between the cooling fins. Over time a huge blockage developed.

(Blowing air into the computer would push this clump backward towards the fan. This lint would likely stay clumped together and not leave the machine chassis.)

Fan Lint Picture
(Click photo to enlarge)

Removal of the lint did not remove the suspected electrical short. Further disassembly revealed a bad solder joint on the plus + pin of the power connector. Resoldering did not help. Further inspection did not reveal any abnormalities in the connector or surrounding printed circuit board. A shorted resistor, capacitor, or diode was now suspected but without schematics it was impossible to proceed.

The local fire department (where I volunteer) carries a thermal imager camera to see heat (infrared wavelengths) through smoke and walls. The camera was used to observe the circuit board while power was applied. The area near the power connector immediately began to show heat - thus confirming the connector or circuit card traces as the location of the electrical short. (A good thing I did not pry up any surface mounted components!).

Dissection of the connector did not reveal the short. (Dissection meaning nibbling with diagonal cutters.) I still measured zero ohms resistance from the plus side to the minus side. Removal of the plus connector pin and resoldering of a small wire in its place somehow cleared the short.

The laptop works fine but I am disappointed not to know the exact source of the problem. The circuit board is likely a multi-layer board. One can see the top and bottom outer layers of copper foil on fiberglass epoxy board, and see shadows of the inner layer (often a copper ground plane). Surface mount components attach to the surface, through-hole components pass from top to bottom through a hole drilled through. The inner layer ground plane (by design) should not get too close to this drilled hole. The "leg" of the +19VDC power connector passed through - one cannot observe what may have happened in that hole. (The short may re-appear.)

A loose power connector repaired

Thanks to Hana for this picture of a broken power connection on a HP laptop, which was successfully repaired afterwards:

Circuit board picture
(Click photo to enlarge)

Letters and Answers

Names removed.

Burnt power pins

>Hello Greg,
>
>I was looking at your webpage for reference on fixing laptop 
>power problems. I picked apart my HP ze4101 laptop and i notice that there was 
>a bad solder joint for the power pin. It was also darkened like it was burned 
>out or something. I've tried to solder it back but still no power coming to 
>the laptop. I know for a fact that the computer works itself. My AC adapter 
>works as well, it lights up when i plug it into the outlet. Also, when i 
>gently touch the pin, i can tell it is very lose.
>
>Any suggestions? Are there any local computer stores that i can purchase new 
>power pins so i can try to solder it again? Thank you.


I do not know of anywhere to actually buy replacement power pins, but there 
might be places on the Internet like http://partsurfer.hp.com. Any blackened 
material will act as an insulator and would need to be cleaned off before it 
could be used.

If it was me, the next step I might try is:
   1. Verify the voltage coming out the end of the wire with a multimeter
   2. Plug in the adapter, with the laptop still apart, and verify what 
voltages are being conducted to the mainboard correctly
   3. Once you find the exact problem spot, maybe to you could try again to 
repair it.

Good luck,

Greg

Wrong power adapter

>Hi Greg,
>
>I've got a HP pavilion laptop and it has been plugged
>with a 30V ACDC adaptor instead of 19V. The polarity
>was the same.
>After a while, it stopped and there was smoke near
>the power connector. And now it doesn't work at all...
>even the LED when I plug it in.
>
>Do you know if this kind of laptop has a separated
>power card or if it is on the mother board?
>
>I hope that there is only one component which has
>burnt but which one and is it easy to change it?
>
>Many thanks

No, the power circuit is part of the mother board.
Your motherboard will probably be dead, although you could see if the laptop 
can still be powered by battery. Also, you could look inside to see which 
parts have burn marks, if any. 

It is possible that some other component burnt up and you can replace it, but 
it is not likely. I suspect you have melted a voltage regulator, because these 
guys take the brunt of any extra voltage you give them.

Sorry it is not easy to fix,

Greg


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