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laptop power supply on the blink. What should I check?

Posted on 2014-12-13
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Last Modified: 2016-11-23
Dell inspiron 1540  power supply not working. Power is reaching from plug to the unit. Measured it with volt meter. However the led light in the unit is not showing so the problem must be in the unit? The unit is sealed so cannot b opened. End of story. Need a new one, but am in a remote part of West Africa where it won't be easy. Anything I might try in the faint hope of prompting it back to action?
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Question by:Michael Murphy
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by:andreas
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AC Voltage at your place meets the requirements of the power supply?
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by:Gerwin Jansen
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This Dell power supply is rated at 100-240V 50/60Hz - output 19.5V /  4.62A- you can check the adapter.

When the LED is not lighting up, it is most certainly defective and there is nothing you can do without opening it up.

Best way to try is opening it at the sides in the length, be careful though to not cut yourself. Best guess is that there is a blown fuse inside.

Finding a different supply that you connect the secondary cable to is also an option, but then you'd have to get some other power supply first.
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by:rindi
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One good thing about Dell Laptops is that most of them use pretty interchangeable Powersupplies. There are maybe only about 4 different models around. Besides that, Dells are fairly common, so I could imagine that it should be easier to get those even in remote regions. Maybe look on ebay or similar for a defective notebook, maybe it's PSU would do fine. Another thing, in remote regions people are often very resourceful, and they may be quite capable of repairing the PSU even if it is sealed. It might not look nice after the repair, but often it will work.

Besides that, really any PSU should work, provided it has the correct output Voltage, and enough Wattage (that can also be higher than what you currently have). You may just need to change the plug so it fits your laptop (which is a simple soldering job, just make sure the poles are correct).
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by:nobus
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>>  Measured it with volt meter  <<   so you measured the output voltage ok
that is one good thing, but NOT conclusive, often the PS outputs the correct power WITHOUT load
the only valid test is when it's properly loaded, eg,if you measure on the input connector contacts of  the laptop - when powered on, you'll probably see the voltage drop

imo - test with another PS - in general, they are difficult to repair , and open; and when opened - even more difficult to close properly
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by:Michael Murphy
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I meant that the lead from the house socket, the other end of which connects to the unit, is transmitting current.
Thanks for other advice's which I will follow up
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by:☠ MASQ ☠
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Dell laptop PSU's have a built in testing circuit to check the correct current can be delivered, if the green LED lights and you can measure the correct voltage and polarity at the laptop end then chances are the PSU is fine.  As has already been posted if you can find another adaptor they are pretty much interchangeable just check the output voltage rating on any replacement is not higher than yours.  Polarity on Dells is the same across the range.

The only laptop based tests you can do with your explicit requirement not to open the case is to try with and without battery in place.  Check the battery is Dell and not generic.  If it's a Dell it will have a self test button to check the voltage capacity.  An exhausted battery could feasibly be causing this but it's unlikely.  Without cracking the case look for looseness around the power input and if anything rattles when you shake it (!).
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by:nobus
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then - as first step, disconnect the AC adapter from the laptop, and measure it's output voltage - between 18 and 20 V DC that is
ssome tips for measuring V and resistance here :  http://www.fonerbooks.com/laptop11.htm
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by:Michael Murphy
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No power is reaching beyond the unit. There is no power in the connection going into the laptop. There is power at the other end. From the pour source to where the cable enters the Unit (it has to be connected by hand). The light on the unit does not show when everything is connected. I have now opened the unit and will trace the circuit to where it breaks down.
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by:☠ MASQ ☠
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Depending on how you are with electronics you might find these schematics for your PA-12 adapter useful
http://320volt.com/en/dell-pa-12-laptop-smps-19v-3-34a-adaptor-semasi/
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by:rindi
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The first thing you may want to check is the fuse (F1) in those schematics masquerade linked to.
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by:andreas
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you might check the big capacitor, fuse and diodes near the input first, also the soldering points of those and of the mosfet. Frequently the capacitors got bad and cause powersupplies to fail.

Be careful, the capacitor might hold a big charge of a high voltage for a longer time, even after disconnecting the PSU from AC.

Some of the heat sinks might also be live or on the potential of the capacitor.

Another easy test is to see if the there is a DC voltage after the rectifier. ITs round about 320V DC if input is 230V AC. If yes Fuse and things up and including the rectifier are ok.

Furthermore you only should carry out this work if you have some knowlege about electronics and electricity. A small mistake could be fatyl and lead to injury or even death. Also a improper repair cause a fire risk or risk of electric shock.
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by:Michael Murphy
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I have experience and will b cautious. Thanks for diagram.
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by:Michael Murphy
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Just looking at diagram. Where is the fuse located. Have good idea but ..
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by:andreas
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Are you serious? You ask where the fuse is in the schematics? And say you have experience? ITs one of the most easy things to find the fuse in the diagram...

In the given diagram its the part labeled F1. Its a slow 3.15Amp fuse.
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by:Michael Murphy
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Of course I see it in the diagram. Just checking location on board
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by:rindi
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It would be close to the Mains plug.
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by:☠ MASQ ☠
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Dell PAs usually have ceramic fuses (The kind they put in automobiles) not cartridges. If you look at the picture in that link it should be the red square in the opposite corner to the big capacitor.

TBH if that goes you really have a problem & despite the hassle you should look at trying to import a replacement.  On the PA-12 the most likely components to fail are the capacitors at the DC end of the transformer. Look for burn marks on the PCB, corrosion or bulging of the caps.
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viki2000 earned 125 total points
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Do you really intend to repair that power supply?
I think is too much trouble for nothing.
It is cheaper and easier to buy another compatible one as was suggested several times above.

Sometimes the technical knowledge is not enough.
Here are some arguments to discourage you.
Keep in mind that most of the switchable power supplies at low form factor have higher density of the components and most of them have the main IC PWM as SMD type. That requires some good tools and skills to de-solder and replace it. You have a good chance to destroy the PCB tracks by overheating. You may try as hobby, but don't be angry if you don't succeed., rather be prepared for a new power supply.
There are situations when only the wires inside, from mains to PCB are broken, or right inside the plastic isolation at the entry of the power supply.
If that is your case, then you are a lucky guy.
To check that, remove the power, use a cutter and take 1kg of patience with you. Cut slowly the sides of the power supply as there is no way to take out the glue. You have to repeat the slowly cut many times, maybe 20 times on one track. In the end, eventually the plastic case is cut. Make sure the blade does not go inside. The good power supplies have a aluminum protection and that helps for mistakes of the blade, but do not rely on that.
After the hard work of sisyphus and the power supply is opened, eventually de-solder the pins between aluminum case and PCB, then you may measure with an ohmmeter the connection between PCB input of the power supply and the plug side, of course without power, just the continuity. Maybe you are indeed lucky, as I was several times, and you need to replace the cable or to re-solder only.
Then you go on the output side for the same checks.
In my case, several times, there was the problem.
All these power supplies have a protection mode. The output is protected against short-circuit and the LED of the power supply is off and no output voltage. If there is a short-circuit at the output, as in my case twisted wires in the output cable, plastic isolation broken and wires touched, well, then you may short them a bit and remake the connection on the PCB at the output side. If the power supply stays in short-circuit mode too long time then is a good chance to have some components inside damaged.
Then you need to close the power supply and its broken ugly case. Of course you may use some nice glue by sides, but in the past I just used some isolation tape all around and across it. Ugly but work and in case of trouble again, was easy to open it.
I did that ugly work for Toshiba laptops, because the new cheap power supplies that I bought did not last long.
So, if you buy a new power supply, buy a good one, if possible original, even if it is more expensive, eventually higher power so does not run so hot. The cheap that I bough were higher power, but not good made.
Here stops your fun part.
If everything is fine with the cables, then the electronic hobby starts and you have to know more than the schematic.
Fuse blown?
Maybe, but most of the time no. You have to check it, but I do not think is the case. If it is only the fuse, then you are still lucky. But if it is the fuse, then you have a big chance that other components failed.
Most common is the MOSFET switching transistor and additional, many times, the PWM controller IC.
From case to case the feedback couplers fail too and depending how do you use/played with the power supply, are situations when the bulk capacitor rated 400V or more is dried or in short-circuit.
The money that you must to pay on components, the time to replace, in the end not being sure if there is one more component with failure will make you grey hair.
As you mention that is difficult to buy another power supply in your region, then how can you buy the replacement components for repairing?
Did you replace SMD IC before?
Let's say you replaced the MOSFET and the PWM controller. Only by having DC output is not enough to say that the power supply is fully repaired. You should be able to measure all the input/output parameters.
I do not know if the provided schematic is exactly for your case, which is quite simple, as you had difficulties to identify the fuse, but some of the switching  power supplies have also PFC controller. Then you should be able to measure that too.
My opinion: open it for fun, lucky if it is only the cable, eventuality fuse and then stop there.
Buy a new one.
On the other hand, if you are a really hobbyist with electronics then is a good start to spend some money, time and gain some experience with simple switching power supply.
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by:Michael Murphy
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Brilliant post. thanks for taking the trouble. Much appreciated.
 Working on importing a replacement, but will see what might be done in the meantime. Give me a day or two. I have a good idea why it failed. I am in Northern Nigeria where the power supply NEPA is a bit of a disaster. It comes on and off at least 4 times a day, staying on for an hour or two at a time. I survive with generator, solar and inverter. But there is a lot of variable voltage, surges and so forth. Should manage a replacement in a day or two. But out of pure curiosity I will also want to investigate the broken supply.
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by:viki2000
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Based on your power lines description, as bad quality, then you should know that each time when you power on a switchable power supply , most of them but in the end depends by design, if they were off, sometimes even for a small time, then the bulk capacitor starts to discharge and at the power on moment you get inrush currents due to that high-voltage capacitor which now is charging fast. It is a pulse, usually 30-70A and some hundreds of microseconds, max. 1ms. Sometimes you can see that when you touch with your plug the socket and the contact is not firm/good made.
That pulse of inrush current, if it is often repeated, will  destroy the of high voltage bulk capacitor at input side. That’s why it is not recommended to make on/off fast with any device which has a switchable power supply inside, as for instance your TV set.
(Just an aside thought. For instance I have seen that lately with LED drivers, which most of them are switchable power supply. There are people using them together with a motion or presence detection sensor and they connect the on/off contact of the senor at the input side of the power supply, probably to avoid power consumption when the light is off. Then the power supply is dead sooner than expected. The right way is to connect at the output side or at the dimming wires. And that’s why is now in EU the quest for less than 0.5W standby and no-load power consumption for different lighting devices.)
If in top of that you have additional electrical noise on power lines, then the lifetime of the bulk capacitor is shorter. A decent power supply has inside a varistor (MOV) to protect against all the unwanted pulses at the input side. You should check that too, many times is physically exploded or in short-circuit. In normal conditions has a low resistance, but is not short-circuit.
So now you know what is waiting for you if you buy a cheap replacement for your power supply: a very short lifetime under the given stress conditions of a dirty and unstable power line.
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by:Michael Murphy
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thanks and fascinating, you explain it with great clarity. The replacement arrived last night and is working. Am treating it with kid gloves. I will still investigate the redundant adapter, but from your analysis above  it is likely to be the large capacitor.
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by:viki2000
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Here is a nice poster from Texas Instruments with switchable power supplies topologies:
http://www.ti.com/lit/sg/sluw001e/sluw001e.pdf
At the bottom are names of the Application Notes files for details.
It is a start.
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by:Michael Murphy
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I took the advice and eventually sourced a replacement. However the answer contains advice on  repairing the broken adapter  and  I will have a go at this some time taking care not to electrocute myself!!!.
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