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OT - Photo - Are these what bad caps look like?

IIsiGuy

Member
Would someone take a look?

Pic is probably a couple of meg in size ("bad" ? circled in red, "good" ? in blue)

http://img204.imageshack.us/img204/3443/capsuq.jpg

I'm not a printed circuit board level guy. But hanging out here, I learned about various problems related to bulging or leaking caps.

OT though - It's a 7 year old "non-Apple" device (kept for the occasional compatibility problem with those other OSs used at work) turned on for the first time in 8 weeks. (And of course unusable the night before a project is due.)

The symptoms made me think PSU or motherboard. Opened up the PSU and to my inexperienced eye, visually OK.

But on the motherboard to the side of the CPU, caps look suspicious to me. But I've never seen a confirmed bad one before...

Thanks.

 

Byrd

Well-known member
Bulging/leaking/dried out caps as picture = yes, definately faulty! The "blue" caps you have circled, you should also replace.

 

Dennis Nedry

Well-known member
The red circled ones are definitely bad. There is no visual indication to me that the ones circled in blue are bad, but as long as you are replacing some of the capacitors, you should consider replacing them all just in case the others are soon to follow. Capacitors can go bad and not give a visible sign too, so that's another good reason to replace all of them.

 

trag

Well-known member
I concur.

As an aside, or possibly support for the fact that caps go bad. I picked up a 24 port 10/100 D-Link (or was it Linksys?) switch at Goodwill a few months ago. I was suspicious that it might have ended up there for a reliability problem. I opened it up, and sure enough, a bunch of bulging leaking caps. Unfortunately, in values I don't keep on hand.

 

mcdermd

Well-known member
Anecdotally, those "Mercedes Benz" topped caps are the ones that fail. The "K" topped ones are usually okay. That's been my experience in the last dozen or so that I've repaired (iMacs and eMacs).

 

James1095

Well-known member
There was a *huge* rash of bad caps starting in the early 2000's and running through most of the decade. Word is a company producing the electrolyte ripped off the formula, didn't get it right, and the resulting defective stuff made it into tens of millions of capacitors. I started seeing them on Pentium III motherboards but found them also in countless power supplies used in PCs, hubs, switches, routers, DVD players, flat panel monitors and TVs, you name it. 90% of the dead LCD monitors and TV's I've encountered have had a bunch of bulging capacitors, mostly the output smoothing caps in the power supplies When I see one bad one like that, I replace ALL capacitors in similar applications in the device. They don't have to be an exact value, +100%/-20% with an equal or greater voltage rating is almost always fine. I keep a stock of 470uF and 1,000uF in 25V and find that adequate for most things.

 

olePigeon

Well-known member
I don't think so. I'm a newbie at this stuff, but from what I understand, the uF has to be the same. The voltage rating can be higher, though.

So you could use a 25v/47uF capacitor, but not a 25v/470uF. That's 10x more... uh, uFs. :I

I would also suggest that, since you're replacing them, go ahead and use tantalum capacitors. They won't leak, and are only a penny or two more. Well worth it, in my opinion.

 

LOOM

Well-known member
why can't they be higher? They just store voltage for a uF-period of time(?), higher uF=longer period right? So whats the difference?

 

trag

Well-known member
why can't they be higher? They just store voltage for a uF-period of time(?), higher uF=longer period right? So whats the difference?
At high frequencies the response of lower uF caps is faster than the response of higher uF caps. That's the way the math works out.

So to smooth the power to the devices, one uses a mixture of high and low capacitances. The low capacitance caps will respond quickly to small fluctuations, providing a small burst of compensating current, or absorbing a short burst of over-current. The higher capacity caps will have time to respond if the fluctuation lasts longer (lower frequency) and provide or absorb larger amounts of current to compensate.

Changing to a higher capacitance means that the capacitor will respond to fluctuations much more slowly. If there are enough other low capacitance capacitors in the circuit, it might not matter, but it's likely to leave a hole in the frequency smoothing.

 

James1095

Well-known member
That's a complex question, but it's the same as knowing what sort of fittings to use where when plumbing or wiring a house, or what sort of paint, stain, varnish etc to use when refinishing furniture. It starts with having knowledge of the various types of components, their pros, cons and characteristics. Then you have to know what purpose you wish the capacitor to serve. Physical characteristics and price take consideration and are some rules of thumb and intuition involved too. Like any form of engineering, it relies on many layers of knowledge.

For a really basic overview, electrolytic capacitors are used where large values are needed at low cost, like for filtering the outputs of power supplies, longer duration timing and low frequency filters. Small ceramic capacitors with their very low ESR are used for decoupling and high frequency timing and filter circuits. Tantalum and multilayer ceramic capacitors provide high capacitance, extremely low ESR, very compact size and very long life in the same sort of applications that electrolytic types would normally fill. There are of course MANY other types of capacitors out there, but this covers some of the most common.

 

James1095

Well-known member
I should clarify that my mention of 470uF and 1000uF is in regards to electrolytic filter capacitors in the output section of switching power supplies and such. For something originally using a 47uF 16V capacitor, you'd probably be ok to go as high as 100uF and higher voltage is fine (but often physically larger), but 47uF is such a common value there's little reason to sub.

 

techknight

Well-known member
When designing a board, it depends on what your designing.

If your designing a filter network, you have to take in account the rate of charge, and whats known as the "reactance" of X sub C. This is basically the resistance(impedance) of the capacitor over a span of frequencies. Known as the ESR in a way.

So for example, when building a low pass filter, you want a shut to ground at high frequencies. But the roll-off point is calculated in a Resistor-Capacitor filter from an RC formula, there are multiple other formulas for other filters. You have to damn near be a mathematician to engineer electronic circuits. Me, Im not but i have enough experience to know exactly what to do and how to handle it.

Anyway, In SMPS circuitry most of the output is basically whats called a Pi-Filter. The formulas that calculate the proper values of components are all over google, but basically its purpose is to knock out noise over a given frequency or range of frequencies. And how this works is by varying the total reactance (impedance) of the circuit over a frequency spread. This is all calculated out via a set of formulas. Basically in a nutshell components values are always figured out by using math depending on the circuit.

Most pi-filters are based upon 2 caps and an inductor in-between. For example if you design an SMPS with a fundamental frequency of 80Khz, you would want your pi-filter having a reactive value of nearly 0 ohms to ground at that particular frequency. so the switching noises dont make it into the load. This is why ESR is important, as you want as close as 0 ohms as possible at the given frequency.

If its >0 heat goes up inside the capacitor and will cause it to vent/leak. The rate this occurs is in another formula.

So in a nutshell, the filter cap values are important, But there is some wiggle room. But careful, you do NOT want to exceed its original uF value grossly, or it can de-tune the pi-filter enough so switching noise will get through. Some better power supply designs compensate using high frequency ceramics to make sure any electrolytic drift wont affect the output too much.

But at the same time, Capacitors especially electrolytics are cheaper and more commonly available in its regular values. so when engineers design the circuit, they usually use precision resistors/inductors which are available in nearly any value, to compensate for the roughness in the electrolytics. this is what gives you the "leniency". Also voltage can get important in critical circuits, it is very very small, but voltage does change the charge curve a little bit. But again, this is small and in a less critical application such as an SMPS, its almost non-existent to even matter. So i could stick a 1500v cap in place of a 15v and it isnt going to matter.

I could go on and on and on. But this is basics in a nutshell. In SMPS or analog environments you can get away with even the most basic of formulas without much trouble. However, in RF circuits all this stuff becomes highly critical and tolerances have to be tight. Heck, even routing a trace 45degrees to the right instead of 38.5 degrees to the right can make or break an entire RF stage. Yes, it can get that critical. Just saying.

Here is a link that pretty much mathematically explains capacitors:

http://en.wikipedia.org/wiki/Capacitance

 
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