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25V vs 35V capacitor?

24Volts

Mar 21, 2010
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hello

I am using the following power supply along with the external devices shown in the EMC recommended circuit section.

The problem is the recommended C2 capacitor’s diameter is too large. It’s 8 mm but I would need something like 6 mm.

In reference to power supply part# : PBO-5-S3.3, under the «EMC RECOMMENDED CIRCUIT » section… at the following link:

http://www.cui.com/product/resource/pbo-5.pdf

The suggested C2 capacitor with manufacturer part number : UWT1V221MNL1GS
is unfortunately too large in diameter size.

I asked CUI support if Its it ok if I go ahead and replace it with the following capacitor :

Digikey part #: PCE5009CT-ND

Their recommended one is rated 35VDC, where as the one I want to use is 25VDC, but shouldn’t it be ok since the secondary output of this supply is 3.3VDC ??

Here’s the reply from tech support:

“We do not have any specific recommendations regarding this alternative component as we have not used it in our testing. You could try it to see if it works in your application.”

So much for that....

What do you guys think?

Thanks all for your help!
 

MasterT

Feb 2, 2017
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I can't see any issue with replacing a cap, anything from 6.3VDC and up will be fine. Leakage current and ESR should be taken into consideration as well.
 

AnalogKid

Jun 10, 2015
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Generally speaking with aluminum electrolytic capacitors (all other things being equal for each parameter):
The smaller the case diameter, the higher the ESR
The higher the voltage, the higher the ESR
The higher the temperature rating, the higher the ESR
The higher the life rating, the higher the ESR

Most of the time, none of that matters. But with switching power supplies, it does. A higher ESR can increase the output ripple voltage and switching noise. My vote is to keep the value at 220 uF and decrease the rated voltage until you get a case size that will fit. Then compare that cap's ESR with the recommended one.

ak
 

Terry01

Jul 5, 2017
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As far as I can see they are the same spec apart from the ripple current. The one you want to use is half of the one the manufacturer suggests so should be better. So long as the one you use is the same or a bit higher max charging V it'll be fine.

That's what I think anyway. What's the worst that can happen??? LOL. :cool:
 

Harald Kapp

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The one you want to use is half of the one the manufacturer suggests so should be better.
Slight correction: max. ripple currrent is not half but twice (190 mA for PCE5009CT-ND vs. 190 mA for UWT1V221MNL1GS). The other way round would not be suitable.
 

Terry01

Jul 5, 2017
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Thanks for the correction Harald. That's what I ment but I said it back to front. I knew it would be ok but said what I wasn't thinking. :confused:
It's a max rating right so like you corrected half would have not worked. Are you able to go over this rating any or is it fatal to the cap if the ripple is even a little more?

Thanks for the help
 

Harald Kapp

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A higher ripple than rated will heat up the capacitor and shorten lifetime. When the rated ripple is twice the one that is allowed for the originally recommended capacitor, then that's absolutely fine.
 

Terry01

Jul 5, 2017
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Thanks for the help. That's why I like reading these posts...i pick bits and pieces up along the way that I would normally never be aware of. It's cool too having people there who can confirm or correct the stuff I think I've learned. As with almost everything I'm learning just now a question about 1 thing leads to learning about a few things. It's great! Very interesting and a lot of stuff I can get a breadboard out and try it there and then.

Thanks again Harald.
 

24Volts

Mar 21, 2010
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Hi fellows,

So if I understand you all, the one I want to use which better fits in my project in terms of size
(digikey part # PCE5009TR-ND) is more suitable for my application since the ripple current is greater capacity than the the first one (digikey part# 493-2205-2-ND).

So the 25VDC cap (390 ma ripple current) is better than the 35VDC cap (190 ma ripple current) ?

In any case, I found a third one of 22uF @ 35VDC (as per spec) with a 6.3 mm diameter but ripple current is much higher .... 300 ma low frequency and 600 ma high frequency !!!!

Mouser part#: 647-UCV1V221MCL1GS
Would this be the best choice?

Thanks all for your help!
24V
 
Last edited:

Harald Kapp

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Would this be the best choice?
sounds good, except the design calls for a 220 µF capacitor, not 22 µF!
For 3.3 V output even a 10 V capacitor is suitable with a good safety margin. The recommended design in the datasheet doesn't seem to take into account the real output voltage. Instead it seems to be designed for the max. output voltage of 24 V.


Note that you should provide capacitor Y0 for suppression of switching noise from the regulator. This capacitor is a special class Y capacitor. You should by no means use any other type of capacitor as in case of a failure of this capacitor lethal mains voltage may be present on the low voltage secondary side.

Also make sure your design is safe and complies with the safety regulations applicable. Especially, but not limited to:
  • Keep primary and secondary wiring at least > 6 mm apart for safe insulation (air gap and creepage).
  • Provide a protective earth connection when using a conductive (metallic) housing.
  • Use a strain relief for the mains cables to prevent accidental loosening.
 

24Volts

Mar 21, 2010
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Hi Harald Kapp,

the UCV1V221MCL1GS is a 220 uF cap! .... see here:

https://www.mouser.com/ProductDetail/Nichicon/UCV1V221MCL1GS/?qs=/ha2pyFaduhPs5slRZPWzLPyUAa0Q93akRoIZSGAjIfWnH3h2fuTZw==

The power supply I am using is the PBO-5-S3.3 which is a 120VAC to 3.3VDC output at 1A.
It is possible they are using an overkill on component sizes so they can all work with the whole series of power supplies.

As for the rest of the components I had tech support at CUI verify my list and I am doing it exactly as drawn. And yes a separate ground will be supplied.

Thanks so much for your help!
 

24Volts

Mar 21, 2010
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Also on another note... Harald, at the risk of getting scolded for introducing a new question within a question!!!! :)

you say and I quote:

"Keep primary and secondary wiring at least > 6 mm apart for safe insulation (air gap and creepage)"

Then why do electronic distributors sell header connectors at 2mm pitch good for up to 500VAC????

See datasheet drawing:
https://www.digikey.ca/products/en?keywords=M22-5330505

Just asking!!!

Thanks
 
Last edited:

Harald Kapp

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Then why do electronic distributors sell header connectors at 2mm pitch good for up to 500VAC????
Because there is a difference between the absolute limits of the component (500 V) and the limits and regulations for safe operation of the component.
The max. limits are for a new and clean component. Safe operation takes into account wear, humidity, pollution etc.
 
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