What to consider when buying replacement caps?

[davexnet]

Hi all,
finding the uf, the correct voltage, diameter, lead spacing, etc,etc.,
is easy enough, but what are the rules regarding things like ripple current
and/or impedance when considering replacements?

Or should an effort be made to actually find the series of the existing
(burned out) caps? Anything else to consider?

Appreciate any guidance.

The motherboard in question is a 12 year old PCchips KM266 pro
version 9.2a. My son is asking me to save his existing system
because he has so much stuff installed.

Five of the bad caps are to the left of the HSF (voltage regulator?) Close to where the 4-pin
CPU wire plugs in (yellow and black leads. The other two are below the HSF.
4 of them are 16v 1500uf and 3 are 6.3v 3300 uf. All are 10mm wide 25 tall.

TIA -
Dave

[mariushm]

Re: What to consider when buying replacement caps?

Try to get the impedance/esr (the value shown in datasheets @ 100kHz) equal or lower than the original capacitors' impedance.
The ripple should be equal or higher than the old capacitors' value.

4-pin connector brings 12v into the motherboard, so that's why you have 16v rated capacitors there. You must stick to 16v rated capacitors, but if you can't find 10mm capacitors and you don't have room on the pcb to fit 12.5mm wide capacitors, you can use 1200uF for example.

The output of the voltage regulator is about 1.5-2v, whatever the cpu uses. They use 6.3v capacitors because that's a standard voltage rating easily available. It will be a bit hard to find 3300uF 6.3v in 10mm diameter, so again if you really can't fit 12.5mm in there, you can go for 2200uF without any real problems. Those old systems aren't that sensitive.
Alternatively, you can use polymer (solid) capacitors which are also available at 2.5v or 4v rating. 2.5v would probably be enough, but just to be safe 4v rated polymers are better.
With polymer capacitors you can probably go with 1500uF 4v ones without any problem. They're a bit expensive though, not really worth it for such old system.

Note that the capacitors on the motherboard can fail due to old age or due to poor manufacturing quality, but they can also fail due to the quality of the electricity provided by the power supply slowly degrading. So it may be a good idea to open the power supply and also check the capacitors in the power supply.

[davexnet]

Re: What to consider when buying replacement caps?

Try to get the impedance/esr (the value shown in datasheets @ 100kHz) equal or lower than the original capacitors' impedance.
The ripple should be equal or higher than the old capacitors' value.

4-pin connector brings 12v into the motherboard, so that's why you have 16v rated capacitors there. You must stick to 16v rated capacitors, but if you can't find 10mm capacitors and you don't have room on the pcb to fit 12.5mm wide capacitors, you can use 1200uF for example.

The output of the voltage regulator is about 1.5-2v, whatever the cpu uses. They use 6.3v capacitors because that's a standard voltage rating easily available. It will be a bit hard to find 3300uF 6.3v in 10mm diameter, so again if you really can't fit 12.5mm in there, you can go for 2200uF without any real problems. Those old systems aren't that sensitive.
Alternatively, you can use polymer (solid) capacitors which are also available at 2.5v or 4v rating. 2.5v would probably be enough, but just to be safe 4v rated polymers are better.
With polymer capacitors you can probably go with 1500uF 4v ones without any problem. They're a bit expensive though, not really worth it for such old system.

Note that the capacitors on the motherboard can fail due to old age or due to poor manufacturing quality, but they can also fail due to the quality of the electricity provided by the power supply slowly degrading. So it may be a good idea to open the power supply and also check the capacitors in the power supply.

Thanks very much for this info. I wanted to order from here,
but I couldn't find a matching set, I wanted to get both types the same
series. Ended up ordering the Nich. HM from somewhere else.

[volumetricsteve]

Re: What to consider when buying replacement caps?

Looking at the first post, I had the exact same questions..though I'm still not clear on Ripple Currents. I've read a lot of explanations on them but ...they sound like bad things, yet the higher values seem to be sought after.

I see a lot that links ESR to Ripple Currents, but is there a mathematical way to relate the two...like if you know the capacitance, voltage and ESR can you find Ripple Current, or is it just a totally different thing?

I'm looking to upgrade all of my electrolytics on a ton of old P3 boards, and I just wanna be sure I don't end up building an elaborate fire hazard

[budm]

Re: What to consider when buying replacement caps?

"I've read a lot of explanations on them but ...they sound like bad things, yet the higher values seem to be sought after." You want cap that can handle high ripple current not lower. You can calculate the circuit ripple current value and then you will find the cap that can handle that ripple current, the cap will have to dissipate that heat.
http://www.designnews.com/video.asp?...Layout=videoBC
http://www.eetimes.com/document.asp?doc_id=1273335

https://cdn.badcaps-static.com/pdfs/...34293019f9.pdf
https://cdn.badcaps-static.com/pdfs/...3127f28bdd.pdf

[volumetricsteve]

Re: What to consider when buying replacement caps?

VERY cool, thanks for the info - that cleared it up a lot.

My only remaining concern is....and sorry if this sounds really dumb I just want to be extra safe here, is there any reason anyone would ever seek out high-esr or low ripple current rated caps?

I was reading that...obviously, there are other properties to caps, resistors, diodes, etc that, in reality, are all non-zero. Things like ESR, ripple current, things other than capacitance. So if I'm understanding correctly, it seems like the goal is to make a capacitor that has the desired voltage and capacitance requirements while minimizing the other factors as much as possible, right?

Based on that assumption, you should be able to swap out caps with the same capacitance and voltage ratings provided that the ESR is lower and Ripple Current rating is higher than the original cap, yes?

Thanks again, sorry for the noob question.

[budm]

Re: What to consider when buying replacement caps?

"Based on that assumption, you should be able to swap out caps with the same capacitance and voltage ratings provided that the ESR is lower and Ripple Current rating is higher than the original cap, yes?" Some circuit may become unstable when ESR is too low.



https://cdn.badcaps-static.com/pdfs/...3565ddcda4.pdf

[volumetricsteve]

Re: What to consider when buying replacement caps?

Ok, I can see that, if you consider that ESR as part of the load the rest of the circuit is meant to handle, that makes sense.

but what about ripple current? is that always better if it's higher, or can it be too high?

[budm]

Re: What to consider when buying replacement caps?

"but what about ripple current? is that always better if it's higher, or can it be too high?" Higher is better but most of the time the can size will be bigger.

[volumetricsteve]

Re: What to consider when buying replacement caps?

Understood! Thanks so much for your help!