Repair Power Supply for fun

[smashrc]

Thank you for reading this post

I know nothing about electronic but I am very interested to repair my borken power supply. This power supply is a 2 in 1 design that means it has 2 power supplies built in the same case. You can see the picture, left and right sides are the same. The output of this power supply is 12v 22A each channel.

One day it stopped working. So, I opened the case and see which part is broken. I found the 3 parts are broken

300v 460uf Capacitor is puffed
5D-15 NTC Thermistor is broken
Fuse is blowed

Then I ordered those 3 parts online and replaced myself. Turned on the power supply with no loading, the screen (yes it has lcd screen) said one channel is over load and one is normal. After a few seconds, I hear some clicking sound inside. Then I off the power supply and opened the case for a check up. This time same thing happen, the Capacitor is puffed, the 5D-15 NTC Thermistor was very hot, the fuse almost blowed. It means that the problem is not sloved, it must be something else got wrong to make those 3 parts keep blowing again. As I have no electronic knowledge, I would like to ask what parts should I replace\check to make this power supply back to life. I understand I may not provide enough information here asking for help. So, please feel free to ask me any information if needed.

Last, thanks so much for your help.

 

[jrwebsterco]

I'm not sure this is going to be very "fun" for you.
This appears to be a switched-mode power-supply, which can be quite a complex design.

Short of obtaining some kind of servicing manual (assuming such a thing exists, since the manufacturer may assume total unit replacement due to economic reasons), you might want to try the following ...
-> Examine the PCB for any obvious shorts (e.g. due to solder bridging or debris or corrosion) or opens (e.g. due to fatigued or aged/poor/oddly-shaped solder bumps), and remove or re-wet the problem spots with an iron
-> Examine the inductors/transformers for damaged insulation enamel (shorting) and also use a DMM to check that two windings are not shorted together, nor individually open
-> Check all diodes for proper unidirectional operation (e.g. using the diode-test setting of a DMM's ohmmeter function)
-> Do the same for all BJTs and/or FETs (they usually look shorted when they fail, but might also be open)
-> Check all caps to not be shorted, and/or to see that they basically will ramp a charge up and down using a DMM's ohmmeter and reversing the leads back-and-forth
-> Check all resistors for correct ohms

Some of the above can be done roughly and quickly (like checking resistors).
But as you see, this is like a "where's Waldo" problem, and could start to drag on for a much longer time than you expected, unless you get lucky.
Moreso, the practical problem with all this is that components are interconnected together, and could confuse your ability to test any single component as if it was in isolation.
A schematic (or tracing out the PCB if no hidden layers) might help anticipate this.

I'd be concerned that after spending time and more money on replacement parts, you still may not finally fix it.

 

[Darkcobra]

Capacitor failure may also damage the bridge rectifier, which is the black rectangular four-pinned component to the left of the blown cap.

A damaged bridge may pass AC, instead of DC as intended, to the replacement cap; quickly resulting in failure of it and the associated components you listed. In my case, the lid of the capacitor blew clean off and rolled out of sight down the hall. My boss, who had convinced me to attempt repair on the $50K machine with no test equipment and outside my job description (I'm a programmer), was watching; and laughed so hard he fell on his arse.

Fortunately I lucked up and correctly guessed the bridge failure, resulting in a successful repair on the second try. A damaged bridge often doesn't show any physical signs of damage, but can be tested using the diode check function on many multimeters.

Perhaps you may be lucky too. Otherwise, Jrwebsterco's advice is sage.

 

[smashrc]

Thank you jrwebsterco, Darkcobra for helping me grasshopper.

I have inspected those mosftets, diodes, hi-speed diodes by my eyes. There is no holes, chips or smokey residue. Also no short circult on the board.

For the bridge rectifier (still no holes, chips or smokey residue), I don't have ohmmeter. If I buy the ohmmeter and how can I check to tell if the bridge rectifier is good or bad? The bridge rectifier is really low price maybe I just replace it. The cost is way lower then buying the ohmmeter I believe.

For the Mosfets (still no holes, chips or smokey residue), how can I check if they are good? I only have a voltmeter here.

The capacitor on the left, the top area already puffed a little. I want to know should I replace it or I can still use?

 

[Rhine]

Hi Smashrc

I'm the manufacturer of power Supply. It's good to see some want to learn new things.But according to me you should not try by your self until u are not qualified.
It's very dangerous to repair such kind of supply by your self. Sometime Capacitors with high volts explodes if are not properly installed and can cause serious injuries.
There is also risk of electric shock too.But still if you want to repair this i can help you but at your own risk. Kindly post new clear photograph from each angle and c it clearly which parts are damaged in power supply.I cannot tell you to repair until i c the picture clearly.

 

[jrwebsterco]

I concur on the dangerous nature of potential installation of incorrectly rated or installed replacement components (not to mention fire hazards).
Also, I have replaced many blown transistors where there was absolutely no externally visible sign of any damage - that would be an example of getting lucky.

If you ever intend to do more stuff like this in the future, then you should invest in something better than what you have.
(you can get a digital multimeter for e.g. $25 at a local Radio Shack, or on sale for only $5 at a Harbor Freight, or else try Amazon)
Most all digital ones have a diode check function - analog ones are not easy and intuitive for that.

But maybe you first need to come up to speed on how a basic diode works - and how to detect a functioning simple diode using a meter - before testing a diode bridge?
I respectfully say this only because your responses seem to indicate you might not be aware of how to do this.
There are intro articles on the web about such basic methods for testing circuit components - have you tried looking into this yet?
Here is a good example: https://learn.sparkfun.com/tutorials/diodes/real-diode-characteristics
You might also want to look up "diode bridge" on WikiPedia.
Transistors are also able to be basically tested, but I'm out of time to go into that here ... again I believe there are already plenty of sources already on the web to introduce and explain much of this stuff!