Are oil capacitors suitable for snubber circuits

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Are they low ESR, etc?

i


Re: Are oil capacitors suitable for snubber circuits


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Probably not.  I haven't found any data on current.

And I've been looking since obviously, I'm also interested in cheap
alternatives for concentrated, low ESR capacitors...

You'll have to go with snubber-grade capacitors (a revelation).  Digikey has
some nice polypropylene stuff.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: Are oil capacitors suitable for snubber circuits


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So, I only should look for polypropylene caps? Is that right? I do not
really mind that, I just want to understand.

i


Re: Are oil capacitors suitable for snubber circuits


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Yeah, pretty much.  Excuse my brevity.. polypropylene has the lowest loss,
so they don't really make low ESR, ESL caps out of anything else.  Generic
film (mylar, etc.) comes close but it's not really what you're looking for.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: Are oil capacitors suitable for snubber circuits


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I got it. Thanks a lot.

i


Re: Are oil capacitors suitable for snubber circuits




Ignoramus6607 wrote:

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Polypropylene caps have the largest dV/dt capability of any dielectric I know.

Graham


Re: Are oil capacitors suitable for snubber circuits


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Their internal construction (as with most types) can vary quite a bit,
though generally they have a pretty high current rating for their
capacitance, compared to something like metalized mylar.  But the good
ones are sold specifically as snubber or commutation capacitors.

There are also high current polypropylene foil capacitors that are
made specifically for high peak current applications with heavy
connections.  Though there are also metalized film versions that
emphasize self healing instead of high current ratings.  Don't assume
any particular type meets your peak and RMS current requirements
unless you read that it does in a data sheet.

Re: Are oil capacitors suitable for snubber circuits


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Thanks. Another person also mentioned polypropylene.

What is low enough ESR for my purposes, how would I arrive at a
number?

i
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Re: Are oil capacitors suitable for snubber circuits


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You start with the RMS current and peak current the cap must handle.
the instantaneous waste heat will be peak current squared times ESR
and the average heat will be RMS current squared times ESR.  Many
capacitors do not dissipate heat at all well.  The peak current rating
(often listed as I^2 * time) is based on something melting inside the
cap and the RMS rating is based on overall internal temperature rise.

You will be ready to shop for a capacitor when you have the design
well enough defined to at least estimate the peak current and duration
and RMS current.  Then you dive into data sheets.  Or you just spend
what ever it takes to get the best caps you can find and hope they are
good enough.

I think an industrial distributor like Newark may have a few more high
current choices than Digikey does.

Here are some to study:
http://rocky.digikey.com/WebLib/BC%20Components/Web%20Data/2222%20383%20Metal%20Poly%20Film%20Cap.pdf
http://rocky.digikey.com/WebLib/BC%20Components/Web%20Data/2222%20338%202%20Interf%20Sup%20Film%20Cap.pdf
http://www.cornell-dubilier.com/framesnubber.htm

Re: Are oil capacitors suitable for snubber circuits


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I see. Thanks.

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Thank you, I will read these datasheets.

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http://rocky.digikey.com/WebLib/BC%20Components/Web%20Data/2222%20383%20Metal%20Poly%20Film%20Cap.pdf
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http://rocky.digikey.com/WebLib/BC%20Components/Web%20Data/2222%20338%202%20Interf%20Sup%20Film%20Cap.pdf
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Thanks. I found a digikey part number 338-1142-ND, that I think fits
what I need 100%.

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Re: Are oil capacitors suitable for snubber circuits


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Yes. I've used the CDE oil-filled caps in 500 kVA inverters. Look at the
snubber capacitors at:

http://www.cde.com /

They don't *have* to be oil-filled. Their chemistry depends on the application.

Cheers,
John

Re: Are oil capacitors suitable for snubber circuits


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BTW, on the opening page at http://www.cde.com/ in the "Site Features" box,
click on "Technical Papers". That will produce a list of documents one of
which is titled "Design of Snubbers for Power Circuits".

Just thought you'd like to know.

John

Re: Are oil capacitors suitable for snubber circuits


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Thank you, that's a great paper.

i


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