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'[PIC]: Pin impedance SOLVED'
2001\04\27@011009 by David VanHorn

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Turns out, it was a 0.1uF leaded monolithic cap, that had developed an
internal "short" of 11.2k.

I've seen them fail shorted (near zero R) before, and open, but never 10k!

It's a more complicated failure though, as I wasn't able to pull it up to
5V, even with a 1k resistor.
47 ohms got it there though.  Sort of like a bad zener, and definitely not
what you expect from a capacitor!

FWIW, there's the culprit.
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2001\04\27@013347 by David Duffy

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At 11:57 PM 26/04/01 -0500, you wrote:
>Turns out, it was a 0.1uF leaded monolithic cap, that had developed an
>internal "short" of 11.2k.
>
>I've seen them fail shorted (near zero R) before, and open, but never 10k!
>
>It's a more complicated failure though, as I wasn't able to pull it up to
>5V, even with a 1k resistor.
>47 ohms got it there though.  Sort of like a bad zener, and definitely not
>what you expect from a capacitor!
>
>FWIW, there's the culprit.

I always use MKT's for audio and timing applications. The mono's seem
to curl up and die when used for other than PSU de-coupling purposes.
Maybe it shouldn't be so, but repair experience has shown it to be true.
Regards...

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2001\04\27@020409 by David VanHorn

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At 03:31 PM 4/27/01 +1000, David Duffy wrote:
>At 11:57 PM 26/04/01 -0500, you wrote:
>>Turns out, it was a 0.1uF leaded monolithic cap, that had developed an
>>internal "short" of 11.2k.
>>
>>I've seen them fail shorted (near zero R) before, and open, but never 10k!
>>
>>It's a more complicated failure though, as I wasn't able to pull it up to
>>5V, even with a 1k resistor.
>>47 ohms got it there though.  Sort of like a bad zener, and definitely not
>>what you expect from a capacitor!
>>
>>FWIW, there's the culprit.
>
>I always use MKT's for audio and timing applications. The mono's seem
>to curl up and die when used for other than PSU de-coupling purposes.
>Maybe it shouldn't be so, but repair experience has shown it to be true.
>Regards...


All this one had to do was get charged and discharged through 10k and
10-150k resistors to 5V.
Hardly a stressful app..

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2001\04\27@020923 by David Duffy

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At 12:51 AM 27/04/01 -0500, you wrote:
{Quote hidden}

All I can say is that's exactly how I've seen them fail.  :-)
Can't explain it - it just happens!!
Regards...

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2001\04\27@041600 by Alan B. Pearce

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>Turns out, it was a 0.1uF leaded monolithic cap, that had developed an
>internal "short" of 11.2k.

>I've seen them fail shorted (near zero R) before, and open, but never 10k!

>It's a more complicated failure though, as I wasn't able to pull it up to
>5V, even with a 1k resistor.
>47 ohms got it there though.  Sort of like a bad zener, and definitely not
>what you expect from a capacitor!

I have seen them go short in various ways. The worst one was on a brand new
mini computer system which (luckily) we had left to do a burn in before
commissioning. Someone suddenly noticed that smoke was issuing forth from
the machine and hit the power off button rapidly. The culprit was one of
these capacitors, which had gone low enough in impedance that the 300A 5V
supply figured more current was needed and the capacitor glowed red hot
until a hole a bit bigger than a quarter was burnt in the circuit board.

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2001\04\27@162052 by D. Schouten

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I've experienced the same problem too once in a while, in a mass
produced
unit. Sometimes a 1uF/50V 1210 SMD ceramic cap fails resulting in an
almost short. This cap is charged from a car battery via a 6800 Ohm
or 16kOhm resistor. Perhaps 1uF/50V in a 1210 shape is pushing
capacitor technology resulting in weaker parts. Anyway, I won't
place this cap type in newer designs anymore. Smaller values don't
fail.

Daniel...


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