Laptop DC Adapter - Burnt Out

[Sadlercomfort]

Hi everyone, my DC adapter for my laptop made a popping sound yesterday and filled the room with a burn smell very similar to those party poppers. On the underside of the board all solder joints/SMD components seem fine and I can't find any apparent damage to the top. All the top components are intact and free of burns and there are no fuses which could of blown.

The transformer closest to the AC supply smells mostly of burning so I think this is where the fault was but am not too sure. The adapter has a 240V 1.5A input and a 18.5V 4.8A output, I have many spare parts but am not quite electronic savy yet and need help diagnosing the problem.

Thanks,
Ash

 

[duke37]

Ash,
I think you have some die back !

The transformer has almost certainly failed and the unit is scrap. These small transformers often have a thermal fuse embedded in the windings which pops off if the transformer is too hot.

I have tried replacing the fuses but have never had success. You are unlikely to find a new transformer and a rewind would be too expensive.

Duke

 

[Sadlercomfort]

Hi Duke,
Thanks for your reply

I think I might just strip the parts of this then, I do have another adapter but an SMD IC went on that one and it has a different output (15V 8A) so I might have to buy a new one.

As a student can I just ask what type of transformer is this, what's it's purpose and what distinguishes it from other transformers?

 

[duke37]

Ey oop

I have had another look at your pictures and the transformer is not a normal mains to low voltage transformer with a laminated silicon steel core.

There is a 400V 120uF capacitor which indicates that it is a switched mode supply. The mains is rectified (240V * 1.414) to give about 340V, then a transistor (fet ?) is used to 'charge' the transformer. When the transistor is turned off, the charge is fed to the output via rectifiers and the torriod chokes. The whole thing runs at very high frequency so the transformer can be very small. It will be wound on a ferrite core.

Most of the circuitry will be to set the output voltage and limit the output current.

From reading various forums, it appears that some electrolytic capacitors are unreliable, if you replace the transformer, capacitors, transistors and diodes, what do you have left?

Why is it covered in custard?

 

[Sadlercomfort]

Haha, it's a foam-like insulator which looks a real mess.

So the transformer steps-up the voltage to 340V at high frequency in order to rectify the mains voltage efficiently. I will have to read about SMPS, there more complicated than linear regulated supplies.

What are the chances of fixing this, are these transformers still uncommon? I have a few old power supply units for desktop computers which run on SMPS with quite a few transformers in, but the question is are they suitable.

I might be better off repairing my other adapter, or simply buying a new one.

 

[davenn]

So the transformer steps-up the voltage to 340V at high frequency in order to rectify the mains voltage efficiently. I will have to read about SMPS, there more complicated than linear regulated supplies.

no, the 240VAC ( what ever in the UK) gets rectified smoothed ( to around 340VDC) and then switched at high freq that hi frerq gets applied to one of the primary coils ( there are often 2 coils). the low voltage AC secondary(ies) are then rectified to produce you low voltage DC at high current. There is also a feedvack circuit from the LV to the HV side which is used to regulate the frequence ( switching speed) of the transistor on the primary side. This us usually done with PWM, Pulse Width Modulation.

Dave

 

[Sadlercomfort]

Thanks Dave,

I understand the circuit much better now, does anyone have any similar schematics on this type of circuit. I've given up repairing this for now, but from a students perspective I'd like to learn more about this circuit.

Wait think I found a schematic with a input/output power similar to mine.

Thanks,
Ash