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'[EE] Dead Mystery Component'
2008\01\22@135624 by Brooke Clarke

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Hi:

A computer monitor stopped working and had that smell.  After getting it apart
enough to have a look found this circular component cracked almost into two
parts and the PCB is charred on one leg.  It's at the line input section of the
mains power supply right next to a bridge rectifier and large filter cap.  On
the pcb below the part is what looks like a schematic symbol in the form of a
"Z".  Photo at:   http://www.prc68.com/I/HaT.shtml#CM

What is it?  Do they fail, or is it likely something down stream is shorted?

--
Have Fun,

Brooke Clarke
http://www.prc68.com/Alpha.shtml  All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
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2008\01\22@140941 by peter green

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Brooke Clarke wrote:
> Hi:
>
> A computer monitor stopped working and had that smell.  After getting it apart
> enough to have a look found this circular component cracked almost into two
> parts and the PCB is charred on one leg.  It's at the line input section of the
> mains power supply right next to a bridge rectifier and large filter cap.  On
> the pcb below the part is what looks like a schematic symbol in the form of a
> "Z".  Photo at:   http://www.prc68.com/I/HaT.shtml#CM
>
> What is it?  Do they fail, or is it likely something down stream is shorted?
>
>  

I suspect it is a surge supressor.

2008\01\22@141746 by PAUL James

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Usually called a surgistor.

-----Original Message-----
From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu] On Behalf
Of peter green
Sent: Tuesday, January 22, 2008 1:10 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Dead Mystery Component

Brooke Clarke wrote:
> Hi:
>
> A computer monitor stopped working and had that smell.  After getting
> it apart enough to have a look found this circular component cracked
> almost into two parts and the PCB is charred on one leg.  It's at the
> line input section of the mains power supply right next to a bridge
> rectifier and large filter cap.  On the pcb below the part is what
looks like a schematic symbol in the form of a
> "Z".  Photo at:   http://www.prc68.com/I/HaT.shtml#CM
>
> What is it?  Do they fail, or is it likely something down stream is
shorted?
>
>  

I suspect it is a surge supressor.

2008\01\22@142353 by Brian Kraut

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I believe it is a transorb or MOV.  Either way, it is a surge supressor.  It
is likely just after the input fuse and in parallel accross the AC input.
Sometimes after repeated surges they permanently short.  Most likely you
don't have a problem down the line that caused the problem.

Brian Kraut
Engineering Alternatives, Inc.
http://www.engalt.com

{Original Message removed}

2008\01\22@142434 by Moses McKnight
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Lollipop?  It looks like a surge protector to me.  I forget the real
name for them.

Brooke Clarke wrote:
{Quote hidden}

2008\01\22@143640 by Ruben Jönsson

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Hi,

It looks lika a varistor. See:

<http://en.wikipedia.org/wiki/Varistor>

They are supposed to take the heat if there is a short transient on the line
voltage.

BUT - Its clamping voltage gets degraded by each transient. Too many transients
and you may get this result. It could also be that the transient surge is too
long or has too much energy for the varistor to handle. It should not be
damaged by failed components down stream in the circuit.

/Ruben

{Quote hidden}

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
rubenspamKILLspampp.sbbs.se
==============================

2008\01\22@144330 by Spehro Pefhany

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Quoting Brooke Clarke <.....brookeKILLspamspam.....pacific.net>:

> Hi:
>
> A computer monitor stopped working and had that smell.  After  
> getting it apart
> enough to have a look found this circular component cracked almost into two
> parts and the PCB is charred on one leg.  It's at the line input  
> section of the
> mains power supply right next to a bridge rectifier and large filter cap.  On
> the pcb below the part is what looks like a schematic symbol in the form of a
> "Z".  Photo at:   http://www.prc68.com/I/HaT.shtml#CM
>
> What is it?  Do they fail, or is it likely something down stream is shorted?

Almost 100% it is a Metal Oxide Varistor (MOV). They typically fail  
shorted, and likely nothing else is bad behind it. You can replace it  
with another of the
same physical diameter and with appropriate voltage rating. Digikey and others
carry these parts, even Radio Shack used to have some.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
EraseMEs...spam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


2008\01\22@145726 by Dwayne Reid

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Hi there, Brooke.

Couple of possibilities: its either a surge suppressor (MOV) **or**
its a NTC surge-limiting thermistor.

How do you tell?

If its a MOV, it will be across the line.  If its a surge-limit
thermistor, it will be in series with the line.

One additional clue: NTC surge-limiting thermistors often have a
rubber coating to act as a thermal barrier.  The idea is that they
are initially high resistance, thus limiting the peak current into
the filter caps.  As they get hot, the resistance drops.  But,
because the resistance drops, they want to cool down.

Having an insulating coating allows them to remain hot and thus have
a lower ON resistance.

They used to be common in TV sets and I've also seen them in older
switch-mode power supplies.

dwayne


At 11:56 AM 1/22/2008, Brooke Clarke wrote:
{Quote hidden}

--
Dwayne Reid   <dwaynerspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2008\01\22@171012 by Charles Craft

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I've seen them with a notched package.
Is this done by the MOV manufacturer or the device manufacturer?
I assume it's to let the magic smoke out when they go shorted?

{Original Message removed}

2008\01\22@173649 by Marcel Duchamp

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Charles Craft wrote:
> I've seen them with a notched package.
> Is this done by the MOV manufacturer or the device manufacturer?
> I assume it's to let the magic smoke out when they go shorted?
>

On some ceramic disc through hole capacitors, this is done as a
spark-gap.  These are (or were) often used in the H.V. section of
television sets.

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