Author Topic: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?  (Read 740 times)

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Offline PKTKS

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Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« on: May 23, 2018, 03:42:29 pm »

EXPLODED in DARK MAGIC SMOKE .. while just replacing a new 9V DMM BAT.

Anyone can tell  WHAT VOLTAGE THIS SMD CAP is rated for ?
(what is left of it..)
 

Online RoGeorge

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #1 on: May 23, 2018, 03:52:45 pm »
Don't know. Maybe removing the leftovers, power up the board and measure the voltage on the remaining pads will tell you some info about the expected voltage.

Offline PKTKS

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #2 on: May 23, 2018, 04:03:33 pm »

That is the LDO filter  CAP  for the 9V DMM BAT.

It exploded by just inserting a brand new charged 9V bat.

May be 16V tantalum?  or this tiny thing is 35V eletrolitic ?

For the LDO anything above 16V "should" work ..

but not  explode like this...
Paul
 

Offline LateLesley

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #3 on: May 23, 2018, 09:11:05 pm »
Just to think outside the box a little, can't you get the LDO datasheet, and look at the example circuit for a rough idea of the expected value of filter capacitor? It may not get you the exact value, but it may get you in the ballpark?
 

Offline PKTKS

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #4 on: May 24, 2018, 05:49:21 pm »

The LDO is the  standard well known NJU7201U30

Already tested is fine.  The CAP VAPORIZED just with 9V from BAT.

There is a color code still visible but hard to say if that is just "NEG"
or some odd color code to indicate the voltage rating...

Paul
 

Offline SeanB

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #5 on: May 24, 2018, 07:32:59 pm »
Well, that was a tantalum capacitor, and a good replacement would be a 22uF 25v one, though you could also use a 47uF 16V SMD electrolytic there if there is enough height, as they will have similar ESR.  Pretty much all LDO regulators require more than 10uF on the input and output for stability, and 22uF or 47uF is good enough to make them stable. I would go for the SMD electrolytic though, as the chances of it going bang again from ESD damage is pretty high for tantalum.

Either it was one that was 16V rated but had latent damage from ESD, or it was a 10V unit that got unhappy running at rated voltage.
 

Offline PKTKS

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #6 on: May 24, 2018, 07:42:15 pm »

For now i replaced it with a trusty TANTALUM 10uF 35V

but NO SMD. I have managed to craft a regular decent sized CAP
more proper to support spikes than that tiny fragile smd thing.

Tested the LDO and is fine spotted 3V just fine.

35V seems good enough - but imagine if using "MODERN"  DMMs  like this
and a crappy SMD CAP just dead short like this?

SMD fault ( too small for ESD or spikes?)  or plain BAD manufact. ?

dead shorting with a new 9V bat is not a good thing
Paul
 

Offline Twoflower

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #7 on: May 24, 2018, 07:57:53 pm »
That has nothing to do with the physical size of the devices. Tantalum can behave exotherm if the internal insulation layer isn't perfect. There are some special tantalum caps that has lower failure rates. But I'm never checked the availability and additional cost of such parts.

You might find the video in this thread interesting as it explains also the problems with tantal caps (starting at about 22:10, but I recommend to spend the 50 minutes): Capacitors explained by James Lewis of Kemet
 

Offline LateLesley

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Re: Anyone can tell WHAT VOLTAGE THIS SMD CAP is rated for ?
« Reply #8 on: May 25, 2018, 07:43:04 am »
Looking at the LDO datasheet, the test circuit had a 0.1uF capacitor on the input and output. and the max voltage the LDO can handle is 12V. SO a 16V 0.1uf cap is all that's needed. You could put a bigger cap on, like a 1uf, but any higher a voltage is overkill. a 16V will do.
 


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