Why do they use a 10V cap instead of a 6.3V cap?

[Rulycat]

I see this a lot on PSUs that are cheap. Why do they do this? Surely 6.3V caps are enough. I am theorising that in fact they use a 10V capacitor on areas only needing a 6.3V cap to extend the life of the cap?

[tom66]

Re: Why do they use a 10V cap instead of a 6.3V cap?

10V caps are larger, so they have lower impedance and higher ripple current. Often economies of scale can make certain values cheaper, too. So they might use 10V caps on 3.3V as well.

Applying a voltage up to the maximum rated does not damage the capacitor, nor will it measurably shorten the life. Exceeding it, however, will damage it and shorten the life.

[larrymoencurly]

Re: Why do they use a 10V cap instead of a 6.3V cap?

Could it be voltage spikes? My Powmax/Leadman PSU put out 50V pulses across some of its 1uF or 10uF 16V caps. The spikes were still pretty high after the caps were replaced.

[aurgathor]

Re: Why do they use a 10V cap instead of a 6.3V cap?

While I'm not sure where exactly those 50V pulses come from, rectified AC before filtering can have peaks I think about 1.414 times higher than the nominal voltage, and that's about 7V peak for 5V nominal, and the cap should be rated for that.

[tom66]

Re: Why do they use a 10V cap instead of a 6.3V cap?

^Only applies for sine waves. For SMPS, peaks can be arbitrarily high, depending on how the transformer is constructed. The positive peaks are generally the same amplitude as the output, negative can be anything.

[Rulycat]

Re: Why do they use a 10V cap instead of a 6.3V cap?

How does the ESR change when you use a 10V cap instead of a 6.3V cap? Does this allow you to get away with a more "general purpose" 10V cap instead of a Low ESR 6.3V cap?

[Topcat]

Re: Why do they use a 10V cap instead of a 6.3V cap?

I disagree with the power spiking theory for using 10v caps on a motherboard. Any 6.3v cap will never see more than 5v, most never see even half of 5v....so if any 5v circuit in a motherboard being spiked to the 6.3v limitation, any SCR or other IC will be instantly destroyed....forget about ever needing a 10v, if it ever spiked that much, there will be smoke! Motherboard VR's were not intended to clean up filthy power, they were intended to regulate clean power. A lot of manufacturers use whatever they sweep off the floor a lot of times when it comes to caps in hardware today (especially in the entry to mid-level grade hardware). So many over-estimate the engineering, and have no clue how much 'slop' you can get away with when recapping such devices. I could cite tons of examples of this, but I'm sure you all get the idea.

Use of 10v caps in a PSU is feasible for spikes on the 3.3v and 5v rails, but any half way decent PSU won't be spiking like that...and if a crap one is, the least of your worries would be the caps....its everything else the spiking has fried.

[mockingbird]

Re: Why do they use a 10V cap instead of a 6.3V cap?

How does the ESR change when you use a 10V cap instead of a 6.3V cap? Does this allow you to get away with a more "general purpose" 10V cap instead of a Low ESR 6.3V cap?

Voltage in an electrolytic capacitor is dictated by the width of the etching in the can. Due to the self-healing nature of electrolytic caps, a high-volt cap should, in theory, adopt the voltage rating equal to the amount that is pulsing through it. That's why people here say to never re-use motherboard VRM caps for applications that require them to perform at their original voltage rating.

[tom66]

Re: Why do they use a 10V cap instead of a 6.3V cap?

Self healing applies to polypropylene caps and other similar mains rated/safety rated caps. It refers to the ability for the cap to prevent internal short circuits forming between metallised layers. AFAIK, it is "forming" for electrolytic caps. Another thing that can happen is with aging or damage to the cap, capacitance can go up, but since there's no free lunch, voltage rating falls, and this is likely to bring an increase in ESR.

[Shocker]

Re: Why do they use a 10V cap instead of a 6.3V cap?

Use of 10v caps in a PSU is feasible for spikes on the 3.3v and 5v rails, but any half way decent PSU won't be spiking like that...and if a crap one is, the least of your worries would be the caps....its everything else the spiking has fried.

Plus if that was the case they'd also have to put 25V caps on the 12V rail

[Rulycat]

Re: Why do they use a 10V cap instead of a 6.3V cap?

So the general idea then is that some of these companies e.g SunPro Powmax etc will use a 10V cap for its higher ripple specs?