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 becaus= e 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 cent= s. 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: >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 wrote: >> 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=20 >a DC load? >> >> Capacitive discharge. It's mostly to reduce EMI and such. >> >> Cheers, >> Mario >> >> >>> >>>dwayne >>> >>>-- >>>Dwayne Reid >>>Trinity Electronics Systems Ltd Edmonton, AB, CANADA >>>(780) 489-3199 voice (780) 487-6397 fax >>>www.trinity-electronics.com >>>Custom Electronics Design and Manufacturing >>> >>>-- >>>http://www.piclist.com PIC/SX FAQ & list archive >>>View/change your membership options at >>>http://mailman.mit.edu/mailman/listinfo/piclist >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > >--=20 >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .