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Old 14th November 2011, 03:34 PM   #1
PauloPT is offline PauloPT  Portugal
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Default Small 5mm 0.1uF caps - what's best?

Hi,

I have a board to fit with small 5mm pitch 0.1uF caps. These caps bypass decoupling electrolytic caps. Is there a better choice than Wima MKS2 ?

Thanks,
Paulo.
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Old 14th November 2011, 07:51 PM   #2
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by PauloPT View Post
Hi,

I have a board to fit with small 5mm pitch 0.1uF caps. These caps bypass decoupling electrolytic caps. Is there a better choice than Wima MKS2 ?
There is a better choice: thin air. As is shown in this thread, amongst others is that paralleling capacitors results most of the times in degradation.
If paralleling is actually needed (doubtful), it is a complex job, and requires multiple elements with scaled damping resistors.
If you're not prepared to go to such lengths, the simplest option is to bypass it (!)
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Old 14th November 2011, 07:56 PM   #3
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Quote:
Originally Posted by PauloPT View Post
Hi,

I have a board to fit with small 5mm pitch 0.1uF caps. These caps bypass decoupling electrolytic caps. Is there a better choice than Wima MKS2 ?

Thanks,
Paulo.
good choice
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Old 15th November 2011, 01:57 AM   #4
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I understand paralleling small film caps with large electrolytics is not the best idea, but with the common rail to rail bypass caps, I suppose those wouldn't be of any issue since there are usually no lytics across the rails?
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Old 15th November 2011, 11:59 AM   #5
marce is offline marce  United Kingdom
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Just out of curiosity what are you going to bypass with these caps, as I would have thought by the time you take into account parasitic inductances they would be useless for decoupling high frequency noise.
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Old 15th November 2011, 02:33 PM   #6
Elvee is offline Elvee  Belgium
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Originally Posted by nereis View Post
I understand paralleling small film caps with large electrolytics is not the best idea, but with the common rail to rail bypass caps, I suppose those wouldn't be of any issue since there are usually no lytics across the rails?
No but there are two in series ==> double the inductance.
If a low-impedance path (one or more capacitors) exists between two nodes, adding a capacitor will close the loop and create resonances.
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Old 15th November 2011, 05:10 PM   #7
PauloPT is offline PauloPT  Portugal
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Quote:
As is shown in this thread, amongst others is that paralleling capacitors results most of the times in degradation.
Thank you for sharing the link. It´s a very interesting thread with scientific data to back it up!

But now I'm confused about this: In PSU I use to put two 100nF just after the rectifier (1 per rail) with the purpose of filtering noise and 2 more after the capacitor bank. I'm I correct to assume it's better to get rid of all the 4 caps?
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Old 15th November 2011, 05:22 PM   #8
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russian teflonS! )
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Old 15th November 2011, 05:32 PM   #9
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Originally Posted by PauloPT View Post
Thank you for sharing the link. It´s a very interesting thread with scientific data to back it up!

But now I'm confused about this: In PSU I use to put two 100nF just after the rectifier (1 per rail) with the purpose of filtering noise and 2 more after the capacitor bank. I'm I correct to assume it's better to get rid of all the 4 caps?
No. There is enough loss in the PC traces to spread out the potential resonances in all but badly designed cases.
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Old 15th November 2011, 05:36 PM   #10
AndrewT is offline AndrewT  Scotland
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I have seen recommendations to use the higher esr MKS rather than the lower esr of MKP.
I have also read that the lossy non C0G caps can better be used due to the damping of the lossy element.
Certainly use of resistor damping is fundamental to Hagerman's method.
Who's the digital/switching expert on the Forum? He did a long and detailed posting of test results for oscillation due to paralleled capacitances.

But absolutely not teflons alone.
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