# Blown capacitor in power supply

#### Welshhobo

Sep 7, 2012
4
Hi, after a power surge yesterday my PSU from my computer died. I opened it up and found a blown 2200 microfarad capacitor (i think it was the biggest capacitor in there). I've removed it but before i go out and find another to replace it, what would happen if powered the PSU up without it?

#### Jotto

Aug 24, 2012
120
They are not cost effective to work on. If you lost the largest cap, which in experience with power supplies your better off just buying a new one. More than likely this is a SMPS, and if you lost that on the earth ground side you have another problem, these caps almost never go bad.

I won't even take one apart.

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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,613
Without the cap the PSU will not operate properly. The cap has the function to provide a more or less smooth DC to the electronics.
However, unless you have a spare cap lying around to give it a try, I agree with Jotto. Chances are that a few more components have been damaged, though not necessarily visibly.
If you try to repair it anyway, I suggest you remove it from the computer and add some dummy load (e,g, 12 V lamp) on the secondary (some computer PSUs don't like being turned on without load). That way you can at least save the PC from a premature ride to the landfill.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
I would think it unlikely that a surge would "blow" a capacitor.

It is far more likely that the particular capacitor has been slowly deteriorating for some time.

The problem is almost certain to be unrelated. I would look at the main switching transistor and check it for a short.

The next most common causes of failure include a shorted diode on one of the outputs, or dead output capacitors. The symptoms are different though.

Uf you're seeing the input fuse blow immediately (and with obvious *huge* overload) as soon as you turn it on, then the switching transistor would be a thing to check (although there are others). As Harald points out, some faults take out a number of other components and this can be one of them.

A failure in the low voltage side of these power supplies is generally quite repairable, but if replacement of the switching transistor (if fount to have failed) does not immediately restore operation, there may be a long trail of pain ahead.

Commercially, you're likely not going to find anyone to repair them as the cost of a new one is so low.

So, it's a DIY thing, operating on a piece of equipment that can easily kill you.

It might be useful to see *how* the 2200uF capacitor failed. And if you're really keen, we can point out what you need to measure to confirm failure in some of the more commonly failed components. Sharp pictures of both sides of the board are needed for this.

#### CocaCola

Apr 7, 2012
3,635
Commercially, you're likely not going to find anyone to repair them as the cost of a new one is so low.

So, it's a DIY thing, operating on a piece of equipment that can easily kill you.

IMO it's not even cost effective for the DIY unless it's an obvious failure that you know how to fix already or you have time to waste... You can easily drop replacement cost in just new components by trial and error... OEM rated power supplies can be had for about $15 high end name brand ones only costing$35 or so... It's not until you get into the high end or high Watt gaming power supplies that you really see the cost add up...

Every time one of my computer supplies toast, I gut it for the fan, switch (if it has one) and wiring harness/connectors and toss the rest...

#### Welshhobo

Sep 7, 2012
4
I'm getting a new PSU anyway, but want to tinker with it. I've completely earthed the device as far as i know and have only been touching it while wearing rubber gloves. I repair computers as a side-job and something like this could be extremely profitable as it would cost under 10 pence to repair, but I could charge customers a lot for it.

I'm pretty sure its a smoothing capacitor on the 12v rail, so was going to attach a paper clip over the pins and measure the current with a multimeter. If there is a 12v charge running through there ill solder a new capacitor in place and test it on a test rig i have (bunch on ancient worthless computer parts cobbled together). I was just wondering what would happen if i tried to power it up without the capacitor or hot wiring the pins together.

Edit: Its not the filter cap, theres no visible damage to that but we'll find out soon if there is. And the capacitor is definatley the issue - the top as quite obviously blown, and so has the paper inside. The power supply can work with a damaged smoothing capacitor in it, so long as the circuit isn't broken, its just unstable and can shut down at any time which is what happened i think. I could smell burning for about 30 minutes until we had another power surge in the area and then it shut itself off.

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#### Welshhobo

Sep 7, 2012
4
Pic of the psu + capacitor

#### Attachments

• IMAG0372.jpg
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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
For all that's good, DO NOT ATTACH PAPER CLIPS TO THE LEADS!

Do you see that 400V 220uF capacitor in the picture? Touch that and you'll be sorry.

Do you see the metal heatsinks? One, and possibly both are at mains potential when the PSU is operating and probably connected to that 400V capacitor.

If you replace this capacitor, ensure you get a high quality low ESR capacitor. it will cost you more than 10p.

#### CocaCola

Apr 7, 2012
3,635
I repair computers as a side-job and something like this could be extremely profitable as it would cost under 10 pence to repair, but I could charge customers a lot for it.

Greed can make you blind and foolish... Yes, it could cost you 10 pence and presto it works but might fail tomorrow because you missed the real fault, or you could spend dollars and hours and it still doesn't work, and worst case it could numb/burn your hand/fingers or kill yourself...

Why not just pick up a brand new $14 power supply, charge the customer$30 for it, and charge $30 to install it? The customer gets a new power supply and that is a selling bonus with a warranty and you also bank over 400%... That fact that you even imply jamming a paperclip in there makes me cringe and suggest that IMO you have no business poking around in there until you educate yourself further... #### alfa88 Dec 1, 2010 349 Get a new PSU. Harvest the heatsinks, fan, bridge, socket and thermister for future use. That way it wont be a total loss. By the way it looks like the toroid next to the bad cap got pretty hot. Last edited: #### Welshhobo Sep 7, 2012 4 I thought this forum would encourage fixing electronics rather than just buying new everything. For the record, i'd never put a$14 PSU into any computer. That thing would probably blow up within the hour.

I've got it working without any problems i can see. I used an adjustable power supply to power it, and wore an earthing belt and rubber gloves. I attached a paperclip over the pins and switched the thing on and tested my the current going through the clip with my multimeter (idea is if theres current going through, then there aren't problems with the circuit elsewhere), and there was a 12v current going through. I soldered the new capacitor in place and run it on its own for about 30 minutes without any problems. I plugged in into my test rig and run it off mains and has been running for about 4 hours now without any problems. I am still buying a new PSU because i don't trust it - i only wanted to see if i could fix it. Gonna use it full time on my test rig now.

#### CocaCola

Apr 7, 2012
3,635
I thought this forum would encourage fixing electronics rather than just buying new everything.
Depends upon the circumstances... Just because you 'can' fix it doesn't mean that is the route that should always be taken...

For the record, i'd never put a $14 PSU into any computer. That thing would probably blow up within the hour. But you are willing to swap out a 10 pence part and charge the customer big bucks for the repair, and hope it won't blow up in short? In my past I have purchased high end power supplies and the cheap OEM ones for replacement...$ for $I have found the cheap ones to be just fine unless you are pushing them hard... All the computers in my house run 24/7... From many years of experience I can say that you will get much more than an hour out of the cheap power supplies, I can't say I have had one fail in any short time, more like several years down the road... In many cases you get very good warranties with the cheap ones, look here a$15 power supply with a 3 year warranty, parts and labor... A local retail pickup for me...

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4365849&CatId=106

Here is one on sale for \$15, with a lifetime warranty...

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1147662&CatId=106

if theres current going through, then there aren't problems with the circuit elsewhere

That is a bold leap of faith...

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