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Choosing Capacitor (Ceramic)

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tiwari.sachin

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I am using the connections as attached.

The part is a printer and it works on 8V (Consider) and the current drawn depends on what we print. If its all black print, it will draw more current and less current if its only text so on (Depending on strobe activated each time)


Maximum it can take is 2.5Amps/stobe and to acheive max we activate 3 strobes (example) so the worst case maximum current will be 7.5A. (there could be surges etc while printing which is very difficult to see on a CRO I have and I am infact not even sure if it exists)


The ceramic capacitors works fine but after a period of time (Few days or months), the capacitor is short and we need to replace.


What could be the possible reasons for this? Am I selecting a wrong capacitor or is it something else that can cause this.


The capacitor used is X5R
 

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Hi,

possible reasons for ceramics capacitor failures:
* overvoltage - even spikes. Maybe at turn ON or turn OFF, or other situations. --> use overvoltage protection devices, or capacitors with higher voltage rating
* mechanical stress. Either vibrations or during soldering. Thermal stress --> keep on soldering recommendations and temperature chart
* chemical problems, like flux residuals --> try with usual cleaning. If problem persists use ultrasonic cleaning. (Take care that all other parts are allowed for ultrasonic cleaning)

Klaus
 

You can observe the capacitor temperature in continuous operation to be sure that it's no overcurrent problem. But maximum of about 2 Arms per capacitor sounds fair. I'm rather sure that you won't notice capacitor heating. Mechanical stress and inappropriate solder processing seems more likely.
 

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