Electronics > Beginners
Ceramic capacitor is of too low voltage?...som,etimes blows up at switch on.
ocset:
We have an LR8 linear regulator downstream of our mains diode bridge (240vac). The capacitor at the bridge output (at the input to the LR8) is just a 100nf, 1812, x7r, 630vdc capacitor. It sometimes blows up at switch on. (one in 50 times)
It surely cannot be blowing up because of over voltage above 630v, because the sensitive LR8 would surely have blown up first.
What we suspect is that our board stuffer has accidentally placed a 250V capacitor there, and most often these are surviving in circuit, but every now and again the product is switched on at mains peak, and this 250v part blows up. What do you think?
Do you think this is likely?...ie the board stuffer has accidentally placed a 250v capacitor here, instead of the required 630vdc capacitor?
Surely it’s the only explanation?
Strangely, the capacitor , when blown up, has a hole in it and there’s tarnish on the PCB near it, but when desoldered the capacitor actually still measures 100nf.
LR8
https://www.microchip.com/wwwproducts/en/LR8
Giaime:
Maybe too strong of an inrush current?
rs20:
--- Quote from: Giaime on July 05, 2018, 06:46:09 am ---Maybe too strong of an inrush current?
--- End quote ---
^ This. Inductance in the wiring leading up to the capacitor can create an LC circuit; an LC circuit can (ideally) see a hard switch-on event leading to the voltage peaking to twice the steady state. Interestingly, and perhaps unintuitively, simply adding a capacitor with ESR (i.e., a bog standard electrolytic capacitor) in parallel to the ceramic cap can alleviate this a lot; the ESR damps the LC tank.
Delta:
Don't forget that the mains voltage in the UK regularly reaches 292vAC RMS for periods of several minutes.
Giaime:
--- Quote from: blueskull on July 05, 2018, 06:53:49 am ---
--- Quote from: rs20 on July 05, 2018, 06:51:12 am ---^ This. Inductance in the wiring leading up to the capacitor can create an LC circuit; an LC circuit can (ideally) see a hard switch-on event leading to the voltage peaking to twice the steady state. Interestingly, and perhaps unintuitively, simply adding a capacitor with ESR (i.e., a bog standard electrolytic capacitor) in parallel to the ceramic cap can alleviate this a lot; the ESR damps the LC tank.
--- End quote ---
What makes you think the OP doesn't have an electrolytic in parallel? He can't be that stupid.
--- End quote ---
Yep we don't know. Maybe that's the input of a boost PFC stage? In that case, I would still expect a plastic X2 capacitor for EMI suppression :-//
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