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'[EE] Snubber capacitor'
2012\05\12@023235 by Electron

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Hello,

Can I use a 0805 ceramic (rated with enough voltage of course) or must I use a big MKT (polypropylene) and such?

Thanks,
Mario

2012\05\12@135437 by Dwayne Reid

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At 12:29 AM 5/12/2012, Electron wrote:

>Can I use a 0805 ceramic (rated with enough voltage of course) or
>must I use a big MKT (polypropylene) and such?

What's the application?  Triac on AC line voltage or MOSFET driving a DC load?

dwayne

-- Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2012\05\13@042800 by Electron

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At 19.54 2012.05.12, you wrote:
>At 12:29 AM 5/12/2012, Electron wrote:
>
>>Can I use a 0805 ceramic (rated with enough voltage of course) or
>>must I use a big MKT (polypropylene) and such?
>
>What's the application?  Triac on AC line voltage or MOSFET driving a DC load?

Capacitive discharge. It's mostly to reduce EMI and such.

Cheers,
Mario


>
>dwayne
>
>--
>Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
>Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
>(780) 489-3199 voice          (780) 487-6397 fax
>http://www.trinity-electronics.com
>Custom Electronics Design and Manufacturing
>
>

2012\05\13@123216 by Sean Breheny

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There are a few considerations that I can think of:

1) Safety rating - as Dwayne pointed out, capacitors which are going
to be placed between the AC line and ground or AC line and neutral
require special safety ratings against fire and leakage current. This
does not apply to your situation. These ratings are referred to as X1
or X2 or Y1 or Y2 depending on the exact application (voltage, surge
rating, and whether line to GND or line to neutral)
2) capacitor life - when applying high current pulses to capacitors,
there can be internal self-heating of the capacitor as well as
possibly mechanical stress on the dielectric or plates. Some types of
capacitors will not handle this well and will have shortened life.
3) voltage rating - the cap must be able to handle the peak voltage
seen during the spikes
4) ESR and ESL - if the cap has too high an effective series
resistance or inductance (ESR or ESL), then it will not behave like an
ideal capacitor in the snubber.
5) Nonlinear dielectric - some dielectric materials, especially in
ceramic capacitors, have a capacitance which changes with voltage
level. This will alter the behavior as well.
6) Piezoelectricity - ceramic capacitors will microscopically expand
and contract as they are charged and discharged. If the spikes are at
an audible frequency, then the caps will "buzz". Probably not an issue
with an engine running nearby but it could be annoying in office
equipment, for example, or especially audio equipment.

I have definitely used ceramic caps in snubbers and the practice of
doing so is well-established. However, you need to operate the cap
below its max peak current and peak voltage, take ESR and ESL into
account, and probably select a dielectric type which is not very
nonlinear over the voltage range involved. Unless you are using type
NP0 or C0G capacitors, then this means that you will likely have to
restrict the peak voltage on the snubber to less than 25% of the
voltage rating of the capacitor.

Sean


On Sun, May 13, 2012 at 4:27 AM, Electron <electron2k4spamKILLspaminfinito.it> wrote:
{Quote hidden}

>>

2012\05\13@214340 by RussellMc

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> 6) Piezoelectricity - ceramic capacitors will microscopically expand
> and contract as they are charged and discharged. If the spikes are at
> an audible frequency, then the caps will "buzz". Probably not an issue
> with an engine running nearby but it could be annoying in office
> equipment, for example, or especially audio equipment.

A ceramic cap hit with a steep voltage waveform that causes it to ring
piezoelectric can then generate a higher voltage than the input in
response to it's own mechanical effects. This may not be what you want
to happen in a snubber circuit :-).

I've seen reports of LDO regulators with Vin max ratings not much
higher than Vin_usual failing when a ceramic  input capacitor was used
because the ringing spike from the capacitor when input voltage was
applied caused Vin to exceed the LDO Vin ratings.



            Russell McMaho

2012\05\14@023925 by Electron

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Thank You Sean and Russel for the precious insights.

The ESR and ESL are not even rated in the datasheet of these ceramic caps
I own.. I wanna be safe and will go to the polypropylene route, also because
the pulses are quite steep (dv/dt wise), and polypropylene handles it well.
Not worth the risk just to gain some square mm of board space and some cents.

Thanks anyway for the knowledge contribution, I'm sure it has been useful
to many.

Cheers,
Mario


At 18.32 2012.05.13, you wrote:
{Quote hidden}

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