Input Capacitors for noise filtering- failing caps

What kind of device ?

Low ESR electrolytics presumably ?

What's the failure mode ?

Not without more info.

Graham

The surge current through a low-ESR cap at power-on can be HUGE. e.g. a

1000uF 35V low-ESR cap might have 0.1 ohms ESR, and a lot of amps will be flowing into it at power on. Look at past posts here about tantalum caps exploding.

Tim.

Depends on dV/dt of course.

That's tantalum for you ! Aluminium ones should be ok.

Graham

With power-on at the top of the mains cycle dV/dt is infinite (in theory)

Regards

Klaus

You mean zero surely ?

Graham

Hello Graham,

I guess Klaus meant to write dI/dt. But since power comes from a linear supply that is unlikely infinite. Except if that power source has some large low ESR output caps and the unit is hard-switched onto its rail. That wouldn't be a good idea anyway since it'll fry the switch over time.

Anyway, Matthew should look at two things: Can that turn-on be muffled? Are those caps of questionable quality? I have seen some rather deplorable electrolytics lately. On one of them the legs had rotted (!) off. The thing just fell off the board, sans legs. Another had a fluffy white layer like Christmas ornaments.

Regards, Joerg

Surely not that either ?

Ouch . Indeed. lack of info as ever.

What makes(s) were these ?

Graham

Hello Graham,

The "Christmas cap" was Nichicon, the other I don't remember.

Regards, Joerg

I spoke (wrote) before thoroughly reading the post. I assumed (assumptions is the mother of all F*** ups) that he was talking about a DC link capacitor directly on the bridge rectifier sitting on the mains voltage

Sorry for the bother :-)

Regards

Klaus

Hello Klaus,

ROFL! That is a true word. Reminds me of an event at a machine shop.

"I thought you meant mils"

"Yes, but mils as in millimeters" (British accent)

"You've got to be kidding..."

About $800 later a lesson was learned :-)

Regards, Joerg

Americans really should learn that the right name for a thousandth of an inch is a 'thou' !

Graham

It's situational. Machinists say "thou", and electronics types say "mil".

Google: "volts per mil" 13,100 hits "volts per thou" 2 hits

Best regards, Spehro Pefhany

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