Higher wattage for a resistor ever bad?

[larry moe 'n curly]

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

 

[John Tserkezis]

larry moe 'n curly wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

Sometimes, ordinary resistors ARE used as fusable.
Anything to save a cent, or a fraction of a cent anyway.
--
Abandon all hope ye who have entered cyberspace.

 

[Jon Kirwan]

On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), "larry moe 'n
curly" <larrymoencurly@my-deja.com> wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

Hmm. Not obvious why there would be a worry. The
differences amount to the following:

1) Longer body, and
2) Larger body diameter, and
3) More weight, and
3) Lower temperature at given power dissipation.

Other than the thought that it does have a fuse function,
which you dispute above, there can be:

A) The longer body or larger body diameter causes the wiring
to be too close to nearby parts, mechanically stresses
something, or blocks some hole that needs to be clear, or
B) The greater weight causes some problem (such as if it
were at the end of a propeller, for example), or
C) The lower temperature affects something else that depends
on the earlier higher temperature.

In short, I can't think of a problem that would not be pretty
obvious when you were replacing it. But then my imagination
ain't what it used to be, either.

Jon

 

[Rich Grise]

larry moe 'n curly wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

No. The first step of repairing this thing is diagnosing what caused
the first one to fail. If it was just a failure of the part itself,
then the replacement only needs to be as big as the original, albeit
one of higher wattage rating can do no harm - the thing is, if you
use that (higher wattage R) without knowing what caused the first
one to burn up, you might be masking a more fundamental problem.

Hope This HElps!
Rich

 

[Lord Valve]

larry moe 'n curly wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

Resistors mostly cook due to two reasons -

1) Under-specked. Runs too hot for the application.
Good idea in this case.

2) (Most common reason) Something else failed
and pulled excessive current through the resistor.
If you replace just the resistor, and use a higher
wattage part, you might cook other (more expensive)
stuff.

Lord Valve
American - so far

 

[Martin Riddle]

"larry moe 'n curly" <larrymoencurly@my-deja.com> wrote in message
news:4765c808-4b10-438c-b419-872be457422e@cf8g2000vbb.googlegroups.com...

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

No, it shouldn't be a problem. Different wattage is usually associated
with size.
If the resistor was 'cooked' then it was running at or beyond it's
rating. Which means there was something else in the circuit that is
defective.

Most resistors are metal film nowadays, carbon comps are hard to come
by.
(But Carbon comps do handle peak currents much better than metal films.)

Cheers

 

On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), "larry moe 'n curly"
<larrymoencurly@my-deja.com> wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

A larger resistor may have different stray inductances and
capacitances, which might affect some RF circuits.

 

[Phil Allison]

"larry moe 'n curly"

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

** Carbon comp resistors are rare and almost obsolete these days - did you
mean carbon film ?

Most resistor burn ups are the result of misuse or OTHER failures in the
equipment, so no upgrade is needed

If a new resistor of the same size gets unusually hot, then an upgrade is
justified.

BTW

2 and 3 watt carbon comp resistors that run hot ( 100C or more ) gradually
fall in value and get even hotter, ie a vicious cycle exists until complete
failure occurs.

Replace them with a wire wound type.



..... Phil

 

[John Fields]

On Tue, 16 Aug 2011 22:41:21 -0400, "Martin Riddle"
<martin_rid@verizon.net> wrote:

"larry moe 'n curly" <larrymoencurly@my-deja.com> wrote in message
news:4765c808-4b10-438c-b419-872be457422e@cf8g2000vbb.googlegroups.com...
When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

No, it shouldn't be a problem. Different wattage is usually associated
with size.
If the resistor was 'cooked' then it was running at or beyond it's
rating. Which means there was something else in the circuit that is
defective.

Most resistors are metal film nowadays, carbon comps are hard to come
by.

http://www.ohmite.com/cgi-bin/showpage.cgi?product=little_demon

http://www.seielect.com/Catalog/SEI-rc.pdf

http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=66690


--
JF

 

[Spehro Pefhany]

On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), the renowned "larry moe 'n
curly" <larrymoencurly@my-deja.com> wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

The safest thing is to use an exact replacement unless you understand
the circuit more than well enough to have designed it in the first
place. Or get an ECO that allows the use of the substitute part.

Replacing a current-sense resistor with the wrong type (too inductive)
could cause a whole lot of pain, for example.

If you actually are talking about carbon composition resistors (fairly
uncommon and relatively expensive these days, as others have said),
then there isn't much problem with a higher wattage other than getting
it to fit, but most resistors these days are metal film (with some
carbon film still around in through-hole applications, and wirewound
used in power applications). There's also metal foil, fusible, high
voltage rating, metal-oxide-film (MOF), tight tolerance, low tempco,
positive TCR, low inductance, and lots of other types of resistors.

Wattage rating is only one parameter (and it really isn't a simple
parameter- it might be lower in a given application than the big print
value if you limit how hot the body of the resistor can get or limit
the ambient temperature or have pulses of current that yield high peak
power).


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

 

[Lord Valve]

Martin Riddle wrote:

Most resistors are metal film nowadays, carbon comps are hard to come
by.

....

Nah, all the major parts houses in the USA carry
'em...Mouser, Digi-Key, etc. Hell, you can even
get 'em as SMDs.

LV

 

[Bill Sloman]

On Aug 17, 12:41 pm, "Martin Riddle" <martin_...@verizon.net> wrote:

"larry moe 'n curly" <larrymoencu...@my-deja.com> wrote in messagenews:4765c808-4b10-438c-b419-872be457422e@cf8g2000vbb.googlegroups.com...

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that.  Why?  I'm not referring to fuse resistors
but ordinary carbon composition resistors.

No, it shouldn't be a problem. Different wattage is usually associated
with size.
If the resistor was 'cooked' then it was running at or beyond it's
rating. Which means there was something else in the circuit that is
defective.

Most resistors are metal film nowadays, carbon comps are hard to come
by.
(But Carbon comps do handle peak currents much better than metal films.)

Carbon composition resistors have a negative temperature coefficient
of resistance - put enough current through them and you form a "hot
channel" with almost all the current being carried by thin filament
that is almost incandescent, and presents a very low resistance.

Back in 1974, my then boss amazed me - and several other engineers -
by persuading a nominally 10k carbon film resistor to pass something
like an amp with a voltage drop of about 100mV for about half an hour.
There was a very narrow stripe of discoloured paint above the hot
channel, but that was the only evidence of what was going on.

He been having trouble with an "intrinsic safety" standards committee,
and this demonstration (and a couple of repeat performances) managed
to persuade them that carbon film and carbon composition resistors had
to be banned in intrinsicly safe devices.

--
Bill Sloman, Nijmegen

 

[Spehro Pefhany]

On Wed, 17 Aug 2011 06:31:29 -0600, Lord Valve
<detritus@ix.netcom.com> wrote:

Martin Riddle wrote:

Most resistors are metal film nowadays, carbon comps are hard to come
by.

...

Nah, all the major parts houses in the USA carry
'em...Mouser, Digi-Key, etc. Hell, you can even
get 'em as SMDs.

LV

"Most" (by far) by quantity used, but even by types offered, fewer
than 3% of the resistor part numbers that Digikey has in stock are
carbon comp.

 

[Don Lancaster]

On 8/16/2011 7:21 PM, Jon Kirwan wrote:

On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), "larry moe 'n
curly"<larrymoencurly@my-deja.com> wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

Hmm. Not obvious why there would be a worry. The
differences amount to the following:

1) Longer body, and
2) Larger body diameter, and
3) More weight, and
3) Lower temperature at given power dissipation.

Other than the thought that it does have a fuse function,
which you dispute above, there can be:

A) The longer body or larger body diameter causes the wiring
to be too close to nearby parts, mechanically stresses
something, or blocks some hole that needs to be clear, or
B) The greater weight causes some problem (such as if it
were at the end of a propeller, for example), or
C) The lower temperature affects something else that depends
on the earlier higher temperature.

In short, I can't think of a problem that would not be pretty
obvious when you were replacing it. But then my imagination
ain't what it used to be, either.

Jon

If the original resistor burnt, something is wrong and needs fixed.

Replacing the resistor with a bigger one is not in any manner a solution.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Jon Kirwan]

On Wed, 17 Aug 2011 08:13:16 -0700, Don Lancaster
<don@tinaja.com> wrote:

On 8/16/2011 7:21 PM, Jon Kirwan wrote:
On Tue, 16 Aug 2011 19:11:18 -0700 (PDT), "larry moe 'n
curly"<larrymoencurly@my-deja.com> wrote:

When I come across a burnt resistor, I usually replace it with one
rated for twice the wattage as the original, but I was told it's
sometimes bad to do that. Why? I'm not referring to fuse resistors
but ordinary carbon composition resistors.

Hmm. Not obvious why there would be a worry. The
differences amount to the following:

1) Longer body, and
2) Larger body diameter, and
3) More weight, and
3) Lower temperature at given power dissipation.

Other than the thought that it does have a fuse function,
which you dispute above, there can be:

A) The longer body or larger body diameter causes the wiring
to be too close to nearby parts, mechanically stresses
something, or blocks some hole that needs to be clear, or
B) The greater weight causes some problem (such as if it
were at the end of a propeller, for example), or
C) The lower temperature affects something else that depends
on the earlier higher temperature.

In short, I can't think of a problem that would not be pretty
obvious when you were replacing it. But then my imagination
ain't what it used to be, either.

Jon

If the original resistor burnt, something is wrong and needs
fixed.

That _may_ be the case. You don't know and neither do I. But
the OP is asking about whether or not it could be bad, so I
think it's good you point this out -- others already have,
though, so there's no need to do it twice. And I already
know, so it doesn't help telling me.

Replacing the resistor with a bigger one is not in any
manner a solution.

Designers aren't always perfect. Some of them are even
hobbyists or worse. These are carbon and the devices may be
old and just gradually failed -- I've seen that happen. They
do develop fissures and fail, sometimes just from vibration
sometimes from just aging or part variability that wasn't
accounted for. It _may_ be a problem elsewhere in the
circuit. It _may_ just be a failed part, itself.

I'm just glad the OP is getting all the options laid out.

Jon

 

[Spehro Pefhany]

On Wed, 17 Aug 2011 13:34:16 -0700, the renowned Jon Kirwan
<jonk@infinitefactors.org> wrote:

I'm just glad the OP is getting all the options laid out.

Jon

S/he should keep in mind that all absolute statements made on Usenet
are false.


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

 

[John Fields]

On Wed, 17 Aug 2011 20:26:20 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Wed, 17 Aug 2011 13:34:16 -0700, the renowned Jon Kirwan
jonk@infinitefactors.org> wrote:


I'm just glad the OP is getting all the options laid out.

Jon

S/he should keep in mind that all absolute statements made on Usenet
are false.

---
How firmly do you believe that?

--
JF

 

[Tim Wescott]

On Wed, 17 Aug 2011 20:26:20 -0400, Spehro Pefhany wrote:

On Wed, 17 Aug 2011 13:34:16 -0700, the renowned Jon Kirwan
jonk@infinitefactors.org> wrote:


I'm just glad the OP is getting all the options laid out.

Jon

S/he should keep in mind that all absolute statements made on Usenet are
false.

Absolutely.


--
www.wescottdesign.com

 

[Jon Kirwan]

On Wed, 17 Aug 2011 20:26:20 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

On Wed, 17 Aug 2011 13:34:16 -0700, I wrote:

I'm just glad the OP is getting all the options laid out.

Jon

S/he should keep in mind that all absolute statements made on Usenet
are false.

Jon

 

[George Herold]

On Aug 17, 8:26 pm, Spehro Pefhany <speffS...@interlogDOTyou.knowwhat>
wrote:

On Wed, 17 Aug 2011 13:34:16 -0700, the renowned Jon Kirwan

j...@infinitefactors.org> wrote:

I'm just glad the OP is getting all the options laid out.

Jon

S/he should keep in mind that all absolute statements made on Usenet
are false.

That's totally wrong.

George H.

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