Fixing a wall-wart?

Hi,

I have a wall-wart that is working but I'm having some problems with it. It coverts 220AC to 10VDC with 1,3A current. I've been using it mainly to power up 8cm PC fans. Recently I opened it up and saw that below the diodes,board has turned black and it seems fried, that can be from excessive heat generated by diodes, and part of the transformer that was facing this diodes is melted (as you can see in pictures). I think diodes are 1n4005.
1- my first questions is, why these diodes got this hot ? should I change them with diodes that can pass more current?  I have RL205 will that work?
2- I've read somewhere that after rectifier bridge, if capacitor's capacity is high, it will put pressure on diodes and transformer, is this true ? or we can use any high capacitor we can find ?
3- Is there a law by which we figure out how high our capacitor should be after rectifier bridge in wall-warts ? for example If I had a wall-wart with 1000mF capacitor, can I change it to 2200mF ?
4- I am going to use this wall-wart to power up a 7805 regulator, so that I can feed a lithium-ion battery charger module (4056). This module uses 1A. should I put a high value capacitor like 470mF or more, at regulator's input, or is it unnecessary cause of my wall-wart having a high capacitor ?
5- Are they any ways that I can improve this wall-wart ? like if I keep the transformer cool ,when it;s under load ...maybe add another capacitor ... ?

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-max-2 years ago

What you need is a REAL power supply, like this one! ;P

200VA transformer, a huge 25,000uF capacitor can, and a few reasonably sized diodes!

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ehsan_zt (author)  -max-2 years ago

Wow that's huge ! I never knew capacitors that size existed, cool.
If they overcharge and explode, I wonder if there will be a mushroom cloud !!! :D

-max- ehsan_zt2 years ago
LOL I would not want to find out! Large capacitors like these generally have vents to let off pressure if too much builds up inside. You can see it in the first picture.
Yonatan242 years ago

You might just want to buy a new power supply, They'e supposed to be pretty cheap on eBay...

ehsan_zt (author)  Yonatan242 years ago

I know but it belonged to a SEGA Mega Drive, so I'd like to use it someday to play old arcade games [feeling nostalgic] :D

-max-2 years ago

Yes, too large a capacitor with too low a ESR could put stress on the diodes. The capacitor acts like a storage bank for charge. during a AC cycle peak, it charges up, and draws a LOT of current while doing so. However, as the instantaneous voltage on the transformer gets lower, the capacitor supplements the power of your fan to keep them powered. The more this capacitor depletes its charge, the more current it will need for the next AC peak.

-max- -max-2 years ago

That cheap supply is probably not worth fixing. Especially if you do not have spare replacement parts on-hand. You would probably end up paying more to fix it that to just buy a cheap wallwart. Do that and keep the transformer and the wire on it for future projects!

-max- -max-2 years ago

And if you are just feeding the output of that wall adaptor into a 7805, why not just use a 5V phone charger or something similar? I swear those things will randomly spawn into existence in any old tech drawers!

ehsan_zt (author)  -max-2 years ago

thanks for the answers, I have some spare parts like the diodes I mentioned (RL205) , I will replace rectifier bridge and see how it performs under load.
You are right phone chargers are good, and normally provide a very stable voltage, but the ones I have are outputting 700mA unfortunately.

-max- ehsan_zt2 years ago

I really do recommend just getting a small 5V power supply. http://www.amazon.com/s/ref=nb_sb_noss_2?url=searc...

That transformer looks pretty toasted, I would not be so sure that the transformer is still functional. It is clear the tape on the transformers has melted, and that is special high temperature tape too. Hopefully the enamel coating on the transformer wire is OK. Even if the transformer works, I probably would not trust it considering that people will even give perfectly good wall warts away! Just as family or friends.

If you plug it in while open and CAREFULLY measure the AC output of the transformer with a good multimeter, or better yet, an oscilloscope, you can see if the transformer is outputting the rated voltage. If it is not, then definitely toss it.

ehsan_zt (author)  -max-2 years ago

thanks, I think it's outputting the correct voltage (DC), but as you said I will measure it's AC output,good point.

iceng2 years ago

I don't worry about the diodes as each diode is only used for half of the time and is not carrying more then 1.3 / 2 = o.65 A average current.

The winding's on the transformer were poorly varnish encapsulated and some turns were rubbing together causing a hot short circuit that starts a vicious series of events destroying your wall-wart.

How else can the manufacturer sell more stock if they don't fail on schedule...

ehsan_zt (author)  iceng2 years ago

Yes, it's not the best transformer unfortunately.
about the diodes, there was a comment in some other site : "

On top of that, rectifier-capacitor setups force high pulse currents
from the transformer. A full wave bridge may force pulse currents as
much as 10X the DC average current to flow in the transformer. That
current goes through the transformer resistances and the voltage sags
even more than you think the RMS or DC currents would make it. The
bigger the capacitor, the worse this is. And the bigger the cap, the
lower the ripple , so to fairly compare rectifer-capacitor circuits you
have to compare ripple as a % of the output voltage and capitance per
ampere of output." it's in the 5th comment on this link:

http://www.diystompboxes.com/smfforum/index.php?to...
That's why I asked how do we figure out which capacitor to choose,and if rectifier bridge is making things worth...

iceng ehsan_zt2 years ago

True.... but only at the moment you plug it in no more then then 100 milliseconds at start-up. So it is not to worry about.

It is not a problem of the diodes but of the use(r).

The images clearly show that not just the diodes but also the transformer overheated.
This is a clear indication of drawing far too much power from that little thing.
At the rated current I see no chance that the sticky tape would melt like that.
I would not bother with it or with using it for other projects.

ehsan_zt (author)  Downunder35m2 years ago

I forgot to post a picture from the other side of the transformer (the one not facing the diodes), it's not melted and the tapes on the transformer wire are in place. And I have mostly used this wall-wart for PC fan (it's rated 0.5A I think ), that's why don't think its the load. Besides, this wall-wart belonged to a SEGA Genesis II, so I think the output ratings of it (being able to produce 1.3A current) should be accurate, as I have used it for many years.

The diodes are under size 1N4005 are 1 amp and the max output is 1.3 amp so the diodes are probably over heating.

The RL205 diodes are 2 amp and should not over heat as easily.

Upping the capacitor to 2200 just will reduce ripple.

With a 7805 the ripple should be above 7 volts for the voltage regulator to work right.

And the best way to keep the transformer cool add a fan.

ehsan_zt (author)  Josehf Murchison2 years ago

Thanks for your reply. I will do as you instructed .