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2amp 5v wallplug psu

cjdelphi

Oct 26, 2011
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I had a pretty mc driven addressable led strip going in the bathroom, the arduino board shorted across the hand rail (metal)

And viola!

The bridge rectifier diodes are gone, the 7805 (is the transistor looking one which blew) the power transistor is there as a pass transistor (unsure if it's blown...)

But! - F1 the "fuse" is actually a resistor! I'm about to desolder it and find out..

The molten/ metal sparks was fun until the house fuse blew, a 240ac fuse in place of the low ohm resistor seems like a good modification to make, if the transformer is ok..

Here goes!
 

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cjdelphi

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The fuse reads 1ohm (well resistor lol)
The power transistor is shorted
2 of the 4 diodes maintain a .6v drop
The 7805 is pretty much dead, i can't be certain it is a 7805 all it reads is 805 the rest is destroyed to read from...

Waiting on more diodes (expecting any day now)
 

Harald Kapp

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a 240ac fuse in place of the low ohm resistor seems like a good modification to
The idea may not be as good as it seems in the first place. The resistor may have bee a special fusible resistor which servers a dual purpose:
  • It limits the current (e.g. inrush curent for charging the electrolytic capacitor
  • It fuses when overcurrent occurs.
Using a pure fuse will not limit the current anymore which may have a detrimental effect on the lifetime of the board.
 

cjdelphi

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What about a 2ohm resistor in series with a 2amp fuse then?
 

cjdelphi

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Since it reads 2 ohms, i will pass a 2amp load through it, it may have survived
 

Harald Kapp

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Since it reads 2 ohms, i will pass a 2amp load through it, it may have survived
P=1²*R -> you'll need a 8W resistor! It is unlikely that the original resistor was rated at 8W.
Also, at 2A a 2Ω resistor will drop 4V, leaving you with only 1V remaining from the 7805's output.

Green-Black-Silver looks like an 0.5Ω resistor. Can you confirm these colors (I may have misread them from the photo)?

Draw a schematic diagram of the circuit following the traces on the PCB and the component pins. This will greatly helo understanding the circuit and identifying any issues as well as suitable replacement components.
 

cjdelphi

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Oops yes 240ac input (2amp out)

When i get to the bench i'll take a closer look!
 

cjdelphi

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Green black silver gold i think
 

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cjdelphi

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Ah and yes, obviously my brain was not located in my head today...

It occured to me, what i thought might have been a 7805 is infact a S8050! a 500v cap it's on the hot side before the optoisolator! Duhhh


(Edit) Ok stumped... again it reads something 8050 the s8050 is low voltage high current, guess i'll have to draw a schematic in the morning..

I have 30 s8050's i went mad one day buying all the small 3 pin transistor types... the regulator happens to be a shunt tls4 something, anyway if this is a fusible resistor how do i test if it's good or not?
 
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cjdelphi

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Mystery deepens...

It is a s8050! (20v max ???)
 

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cjdelphi

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Maybe but where's the fun in that :p

Although, getting just the blown parts equals the price of a new 5v psu lol (the fun part still stands :) )
 

cjdelphi

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After some reading, that 1.2ohm fusible resistor looks good

I just need a 400v npn power transistor now
 
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