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'[EE] resistor failure'
2006\04\06@165212 by Wouter van Ooijen

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I customer returned a motor controller to me. It contained two parallel
1 Ohm carbon resistors, which both had failed open. Nothing to be seen
on the outside, just infinite (> 5 MOhm) and high (~ 500kOhm)
resistance. Is this a known failure mode? Possible causes?

Wouter van Ooijen

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consultancy, development, PICmicro products
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2006\04\06@170654 by Steve Smith

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face
Loose end caps
Seen it once years ago. The resistor was intact but the end caps weren't
connecting properly. The one I found had spent its life being heat cycled.
When the cap was removed the resistor was still ok inside. Cant explain two
in parallel going o/c over current maybe

Steve..

{Original Message removed}

2006\04\06@171056 by andrew kelley

picon face
Vibration caused leads to separate from the resistor internally?
Overheating could cause them to raise resistance, but should be
visible.

andrew

2006\04\06@172158 by Bob Axtell

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Wouter van Ooijen wrote:

>I customer returned a motor controller to me. It contained two parallel
>1 Ohm carbon resistors, which both had failed open. Nothing to be seen
>on the outside, just infinite (> 5 MOhm) and high (~ 500kOhm)
>resistance. Is this a known failure mode? Possible causes?
>
>  
>
YES. It is a common failure. Usually caused by high starting current.
You need to
use wirewound or metal film types, not carbon.

--Bob

{Quote hidden}

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2006\04\06@203418 by Spehro Pefhany

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At 10:52 PM 4/6/2006 +0200, you wrote:
>I customer returned a motor controller to me. It contained two parallel
>1 Ohm carbon resistors, which both had failed open. Nothing to be seen
>on the outside, just infinite (> 5 MOhm) and high (~ 500kOhm)
>resistance. Is this a known failure mode? Possible causes?
>
>Wouter van Ooijen

Hi, Wouter:-

*Metal* film resistors will fail open like that under severe overload, with
nothing to be seen on the outside except perhaps a little puncture (which
might be on the bottom of the part). Carbon film tends to arc and burn.

Often "carbon film" resistors of that value (under 5-10 ohms) are *actually*
metal film...

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
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2006\04\06@221252 by Dwayne Reid

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face
At 02:52 PM 4/6/2006, Wouter van Ooijen wrote:
>I customer returned a motor controller to me. It contained two parallel
>1 Ohm carbon resistors, which both had failed open. Nothing to be seen
>on the outside, just infinite (> 5 MOhm) and high (~ 500kOhm)
>resistance. Is this a known failure mode? Possible causes?

Seen if often.  Metal film and carbon film both have lousy peak power
ratings.  I tend to use wire-wound or metal oxide resistors in those
applications.

Watch out for just which metal oxide resistors you get.  The ones I
have had the best luck with have a continuous film over the entire
surface of the resistor.  Those with a spiral cut do not have as good
a peak power rating as those without.

dwayne

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2006\04\07@023742 by Wouter van Ooijen

face picon face
> Loose end caps
> Seen it once years ago. The resistor was intact but the end
> caps weren't
> connecting properly. The one I found had spent its life being
> heat cycled.
> When the cap was removed the resistor was still ok inside.
> Cant explain two
> in parallel going o/c over current maybe

These units were tested a few days ago, and I can't imgaine having
missed this.

Moderate overcurrent (or rather: overheating) can be spotted easily by
clor, wrinkling and smell. I would expect severe overcurrent to have
even more spectacular results. But these resistors looked perfectly OK.

> YES. It is a common failure. Usually caused by high starting current.
> You need to use wirewound or metal film types, not carbon.

OK, I'll switch to wire-wound.

> This is a common failure mode for metal film resdistors. Low-value
'carbon' resistors
> are often in fact metal-film.

OK, I get the message :) I'll definitely switch to wire-wound!

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\04\07@042959 by Alan B. Pearce

face picon face
>I customer returned a motor controller to me. It
>contained two parallel 1 Ohm carbon resistors,
>which both had failed open. Nothing to be seen
>on the outside, just infinite (> 5 MOhm) and high
>(~ 500kOhm) resistance. Is this a known failure
>mode? Possible causes?

Yes it is a known failure mode with devices like the Philips CR25 resistors.
They will do exactly as you describe with a high current pulse, such as a
short on a power supply. No sign of damage externally, but the carbon layer
is thin enough to go open without heating the paint hot enough to show.

When I was in the last stages of my first employment we used these resistors
in the supply lead of the final stages of VHF landmobile transmitters for
the reason they would act as fuses. In normal operation they were used as a
current shunt to put a voltmeter in circuit to tune the transmitter, and had
the extra bonus of playing fuse if the transistor decided to go short
circuit. It "just happened" that they did not show when they fused open.

2006\04\07@090814 by Spehro Pefhany

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At 08:37 AM 4/7/2006 +0200, you wrote:

>OK, I get the message :) I'll definitely switch to wire-wound!
>
>Wouter van Ooijen

Be very careful that the added inductance of wirewound resistors does not
cause you serious problems in applications such as current-sense resistors.

>Best regards,

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




2006\04\07@094104 by Paul James E.

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Wouter,

It wouldn't hurt to use a resistor that has a higher power rating than
the one you are using, providing you have the room to put it in the
circuit.   This will give you a little more headroom in the worst case
power dissipation area.   You may not know into what the end user will be
installing these units.  And therefore, there may not be enough airflow to
properly carry away the dissipated heat.  So going to a higher power
resistor will alleaivate this problem somewhat by being able to retain
more heat so that failure doesn't occur.

Just a thought.


                                                    Regards,

                                                      Jim



{Quote hidden}

> --

2006\04\07@095159 by Wouter van Ooijen

face picon face
> Be very careful that the added inductance of wirewound
> resistors does not
> cause you serious problems in applications such as
> current-sense resistors.

It is indeed a current sense resistoir, but I think that won't hurt for
me. I will keep an eye.

The load is a motor, so it is 'a bit' inductive anyway. And the voltage
is RC filtered.

Wouter van Ooijen

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Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
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2006\04\07@100435 by Wouter van Ooijen

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> It wouldn't hurt to use a resistor that has a higher power
> rating than the one you are using

I used two 1/4 'carbon' resistors. The lowest-watt wirewound I can get
from my preferred supplier is 5W (I'll use one), so that should give
'some' margin.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\04\07@131545 by David VanHorn

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On 4/6/06, Wouter van Ooijen <EraseMEwouterspam_OUTspamTakeThisOuTvoti.nl> wrote:
>
> I customer returned a motor controller to me. It contained two parallel
> 1 Ohm carbon resistors, which both had failed open. Nothing to be seen
> on the outside, just infinite (> 5 MOhm) and high (~ 500kOhm)
> resistance. Is this a known failure mode? Possible causes?


In high voltage applications, you see this sort of failure with bleeder
resistors.
The values are different, but in this case, the resistors are occasionally
hit with high dissipation pulses, and I think it's the thermal stresses that
cause the failures.

NEVER trust a bleeder!  Carry a JESUS! stick.


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> Feel the power of the dark side!  Atmel AVR

2006\04\09@115737 by dr. Imre Bartfai

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face
Hi,

to be on the sure side, it is a worth of effort to replace resistors with
same value/power rating and check temperature by IR meter if available. I
am 50 and I wonder the decreasing size of the same-rated resistors, which
seems to me as somewhat misterious (or people tends to gamble).

Regards,
Imre

On Thu, 6 Apr 2006, Dwayne Reid wrote:

{Quote hidden}

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