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'[EE] Overdriving power resistors'
2011\10\01@214443 by PICdude

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I have a few chassis-mount resistors mounted to an aluminum block  specifically for the purpose of heating the aluminum block (3D printer  extruder head).  Can't I run much more power (than rated) through the  resistors, as long as I don't exceed the max operating temp?

Specifically, the resistors are 5-ohm 10W, and if I power it from 12V  (lead-acid battery) I get 2.4A per resistor, and 28.8W.  I would PWM  it to reduce the average power, but with feedback.  So it would run at  28.8W until it comes up to temp (I need <150 deg-C), then reduce  average power.  The resistors are rated at 250 deg-C max, so I'd be  well within that.

Cheers,
-Neil.

2011\10\01@233328 by doug metzler

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I don't know the answer to your question, but I note that with the
latest extruder head even the Makerbot people have abandoned using
resistors as heaters.  they've gone to the little canister elements
that everyone else is using.

Somewhere before I blew up my computer I had a link to a great website
that showed pictures of the resistor cores migrating out of the
aluminum heat sinks and destroying themselves.

I personally had problems because every time I fired the thing up the
solder would soften up and the resistors would disconnect themselves.
You're probably not running them that hot.

but generally I would (and did) use nichrome wire for heating up large
flat things, even though it's a royal pain to wire.

Thanks,

DougM

On Sat, Oct 1, 2011 at 6:44 PM, PICdude <spam_OUTpicdude3TakeThisOuTspamnarwani.org> wrote:
{Quote hidden}

>

2011\10\02@003515 by Bob Blick

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On Saturday, October 01, 2011 6:44 PM, "PICdude" wrote:
> I have a few chassis-mount resistors mounted to an aluminum block  
> specifically for the purpose of heating the aluminum block (3D printer  
> extruder head).  Can't I run much more power (than rated) through the  
> resistors, as long as I don't exceed the max operating temp?
>
> Specifically, the resistors are 5-ohm 10W, and if I power it from 12V  
> (lead-acid battery) I get 2.4A per resistor, and 28.8W.  I would PWM  
> it to reduce the average power, but with feedback.  So it would run at  
> 28.8W until it comes up to temp (I need <150 deg-C), then reduce  
> average power.  The resistors are rated at 250 deg-C max, so I'd be  
> well within that.

Hi Neil,

I have some experience using cheap ceramic resistors as heaters in a
commercial product. To sum up my experience, you will experience a
higher-than-desirable failure rate if you significantly overdrive them,
even if the temperature is low, the duration is short, and feedback
limits the temperature. I was using 5 watt resistors. 6.4 watts under
worst-case seemed to be the sweet spot for me. More than that and I
could expect some failures.

So 28.8 watts from a 10 watt resistor is about 15 watts more than I
would allow in a volume product.

You could probably hand-select resistors or use "qualilty" brands and do
OK. But don't expect cheap ones to all survive.

Cheerful regards,

Bob

-- http://www.fastmail.fm - Access all of your messages and folders
                         wherever you are

2011\10\02@090813 by PICdude

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part 1 3022 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

Background: I set my high-school engineering mentees off to design an  extruder head, by researching the DIY (makerbot, reprap, etc) units  out there that work, and the resulting design looks like this ...  http://wiki.makerbot.com/plastruder-mk5 .  The nichrome-wire wrapped  was discussed, but finding high-temp cement was difficult, so they  went with the resistor block type.  Attaching a pic of ours, which I  hope will make it through to the list.  Adding kapton tape helped a  lot (not shown in that pic).

My engineering mentees are off for a bit, so I'm on it for now.  I've  seen those resistor-type units work well, so I should be able to tweak  it into submission.  Ours melts the ABS plastic, though just not  enough to flow freely.  Currently, the resistors are two-in-series,  paralleled with the other two in series, so 7.2W of heat is generated  by each resistor (each resistor is rated at 10W.  They're get no where  near hot enough to melt solder currently.  IIRC ABS melts in the low  200's (deg-F) range.

Looking for tweaks to get some more oomph out of it, but starting with  the electrical things, before going to mechanical things (reducing  thermal mass, etc).

Cheers,
-Neil.


Quoting doug metzler <.....doug.metzlerKILLspamspam@spam@gmail.com>:

{Quote hidden}

>> --

2011\10\02@093044 by PICdude

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How do these things fail?  This is not a volume or commercial product,  but a one-off 3D printer head.  Resistors are these ones from Yageo...
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=810F5R0E-ND

So I have some options now...
(a) Use an 18V source (perhaps add a 6V battery in series with the 12V  battery, so each resistor would generate 10.8W of heat),
(b) Build a PWM switcher now and put all 4 resistors in parallel, then  adjust for 10W to each resistor,
(c) Swap the resistors to 15-ohm units (same physical size and 10W),  which in parallel will be 9.6W per resistor,
(d) Run 28.8W through each resistor and see if I can get it to work  for some time before it fails.

(a) is complicated cause I'll have to buy a 6V battery to experiment.   (b) will take time to get components for a PWM switcher, but probably  makes most sense.  (c) is pointless, cause I might as well try (d)  first and if they fail I'd then do (c).

Think I'm leaning to see if I can dig up a power mosfet to build a  PWM-switcher temporarily using a function generator.

Cheers,
-Neil.



Quoting Bob Blick <.....bobblickKILLspamspam.....ftml.net>:
{Quote hidden}

>

2011\10\02@211539 by RussellMc

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> but generally I would (and did) use nichrome wire for heating up large
> flat things, even though it's a royal pain to wire.

I'll second the use of Nichrome for high power levels. As I noted
yesterday, I had occasion to dissipate up to 500 Watts (not for long)
or 100 - 200 Watts for longish periods, and suitably thick Nichrome is
very abuse resistant. It is run at red hot temperatures by design in
eg toaster and radiant heater elements and seems to take such
treatment well. I ran air cooled coils at below visbly hot (just
you-really-don't-want-to-touch-this hot) and reliability  was never a
problem.

As Doug says - connection is "annoying". There are a number of
webpages that provide soldering advice. I used mechanical clamps
(connector block principle but heavier duty screws. )





      Russell McMaho

2011\10\02@212416 by Bob Blick

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Hi Neil,

They fail open. The voids in the molding process and looseness at the
weld point between nichrome and lead-wire are the fail points. Remember
I am using ceramic-sand-and-concrete resistors. But in my past I have
seen resistors like you are using fail in similar ways.

If this is not a commercial product, I'd say "just go for it". If you
get a dud once in a while, you'll fix it. My application is on a diesel
truck with serious vibration and operation down to -40 degrees, so the
thermal shock is considerable.

Cheerful regards,

Bob


On Sunday, October 02, 2011 6:30 AM, "PICdude"  wrote:
{Quote hidden}

> > --

2011\10\02@231527 by RussellMc

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On 3 October 2011 02:30, PICdude <picdude3spamspam_OUTnarwani.org> wrote:
> How do these things fail?  This is not a volume or commercial product,

I used commercial white ceramic 5W and 10W resistors to "power" a
Fluidyne Stirling Engine (small and for fun). These were run at about
rate wattage with the aim of being "as hot as possible". From memory
these were placed in contact with a metal tube, possibly layered over
with high temperature silicone rubber and wrapped in fibreglass
insulation (intended for in-wall insulation). These lasted "for a
while". Given that the engine only got occasional use I'd suspect they
lasted 10's of hours of run time. This was OK for the application.
Temperature unknown. Failure O/C.

If you used Nichrome and brought the ends out to a cooler location
you'd not have the problems that Bob mentions.


  Russell

2011\10\03@000701 by Antonio Benci

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part 1 1572 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

We've done similar things with 20W and 100W resistors.

The most consistent failure was the soldered joint from resistor tabs to wires. At any elevated temperature above 80degC we noticed that the joints, over time, would fail and become open cct. This was cured by either bolting or crimping wires to the resistor tabs.

We've run 20W resistors (ceramic casing) up to 100degC for up to 10+ hrs before element failure. 100W up to 160degC for close to 40+ hrs before element failure (ceramic/aluminium casing).

Nino.

On 3/10/2011 2:14 PM, RussellMc wrote:
{Quote hidden}


part 2 823 bytes content-type:text/x-vcard; name="Nino_Benci.vcf"
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fn:Antonio L. Benci
n:Benci;Antonio L.
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email;internet:KILLspamnino.benciKILLspamspammonash.edu
title:Professional Officer / E&IS Manager / OHS Representative
tel;work:+613 9905 3649
tel;fax:+613 9905 3637
note;quoted-printable:Albert Einstein=0D=0A=
       =E2=80=9CIf a cluttered desk is a sign of a cluttered mind, then what are=
       we to think of an empty desk?=E2=80=9D=0D=0A=
       =0D=0A=
       Rick Cook=0D=0A=
       "Programming today is a race between software engineers striving to build=
       bigger and better idiot-proof programs, and the universe trying to buildb=
       igger and better idiots. So far the universe is winning"
url:http://www.physics.monash.edu.au/
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part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2011\10\03@050138 by alan.b.pearce

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I have had failures on that style of resistor where the magic smoke came out. I think there was an impulse current not too much higher than the current required for maximum dissipation.

But looking at the photo of your head, I suspect you could get it a lot hotter by simply putting some Rockwool or similar insulation around it. You are going to be losing most of your therms to the atmosphere, and some simple, but high temperature insulation will make a world of difference, especially as you say the temperature is 'almost there'. Personally I would try this before attempting to get more heat out of the resistors.

-- Scanned by iCritical.

2011\10\03@091745 by PICdude

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Here's what's confusing me... I tend to think of the wattage rating as  the ability (or rate) of the device to dissipate heat (given it's  package size, surface area, materials, etc), which is why I imagine  higher wattage being no problem as long as the heat is being sucked  away.  For your situation and anyone else who has had power resistors  fail, were they within specified max operating temp (but higher than  rated wattage)?  Or did the temps go over the specified max operating  temp?

Cheers,
-Neil.


Quoting Bob Blick <RemoveMEbobblickTakeThisOuTspamftml.net>:

{Quote hidden}

2011\10\03@093045 by PICdude

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Quoting RussellMc <TakeThisOuTapptechnzEraseMEspamspam_OUTgmail.com>:

> If you used Nichrome and brought the ends out to a cooler location
> you'd not have the problems that Bob mentions.

Was trying to tweak what I have already, and not redesign/rebuild  (which I don't have the time for right now).  Since what I have is  already/mostly done, and I have no other purpose for those parts, at  this point I will probably just try to overdrive it a bit, and if it  fails I'll prob just buy an existing extruder head kit.

Cheers,
-Neil.

2011\10\03@093118 by Wouter van Ooijen

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> For your situation and anyone else who has had power resistors
> fail, were they within specified max operating temp (but higher than
> rated wattage)?  Or did the temps go over the specified max operating

I think the problem will be that the temperature will *locally* go over the maximum temperature.

--
Wouter van Ooijen

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

2011\10\03@094031 by Dave Tweed

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PICdude wrote:
> Here's what's confusing me... I tend to think of the wattage rating as the
> ability (or rate) of the device to dissipate heat (given it's package size,
> surface area, materials, etc), which is why I imagine higher wattage being
> no problem as long as the heat is being sucked away.

Yes, but it isn't as simple as you think. The manufacturer comes up with
the overall steady-state power rating with full knowledge of the internal
structure and materials in the resistor. They know that if the external
surface is at the specified temperature, the internal parts won't be stressed
to failure at the specified power level.

When you overdrive the resistor, the internal temperature distribution
changes, and some spots may become hotter much faster than the external
surface does.

You really need to consider all of the paths and materials that lie between
the spot where the heat is produced and the spot where it is dissipated to
"ambient". It's all about thermal resistances -- and, in dynamic situations,
the specific heat of materials (thermal capacitances, if you will) comes into
play as well.

-- Dave Twee

2011\10\03@094331 by Mike Harrison

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On Mon, 03 Oct 2011 06:17:40 -0700, you wrote:

>Here's what's confusing me... I tend to think of the wattage rating as  
>the ability (or rate) of the device to dissipate heat (given it's  
>package size, surface area, materials, etc), which is why I imagine  
>higher wattage being no problem as long as the heat is being sucked  
>away.  
True as long as the thermal resistance from the element to the outside world is not high enough to
be an issue.
As mentioned elsewhere, welds may be a weak point.
If you want to look at in detail, you could measure the element temperature via its change in
resistance.

I've seen a grossly overloaded metal-clad aluminium resistor violently fire one of its ends off,
leaving a taser-like trail of (live) wire behind it...!

2011\10\03@094604 by PICdude

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Interesting data.  Do you know what the max specified operating temp  was on these?  And I assume the power to these resistors was higher  than rated right?  This answers a lot.

Cheers,
-Neil



Quoting Antonio Benci <RemoveMEnino.bencispamTakeThisOuTmonash.edu>:

{Quote hidden}

2011\10\03@094702 by Bob Blick

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On Monday, October 03, 2011 6:17 AM, "PICdude"  wrote:
> Here's what's confusing me... I tend to think of the wattage rating as  
> the ability (or rate) of the device to dissipate heat (given it's  
> package size, surface area, materials, etc), which is why I imagine  
> higher wattage being no problem as long as the heat is being sucked  
> away.  For your situation and anyone else who has had power resistors  
> fail, were they within specified max operating temp (but higher than  
> rated wattage)?  Or did the temps go over the specified max operating  
> temp?


They were part of a de-icer, clamped and glued to a metal plate, and
shut off when they reached a little over room temperature. So they never
got very hot(externally).

Bob

-- http://www.fastmail.fm - One of many happy users:
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2011\10\03@094952 by PICdude

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Not shown in that pic is a bunch of kapton tape wrapper around the  resistors and aluminum block.  It helped a lot, but it needs just a  bit more.  If I can find fire-cement or similar in a small quantity I  would try that too, or just build a nichrome-wire head.  But that  cement has been the reason for the resistor-heater design.  Think it's  time I hit the 3D-printer forums.

Cheers,
-Neil.


Quoting EraseMEalan.b.pearcespamstfc.ac.uk:

{Quote hidden}

>

2011\10\03@095747 by PICdude

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Quoting Wouter van Ooijen <RemoveMEwouterEraseMEspamEraseMEvoti.nl>:

>> For your situation and anyone else who has had power resistors
>> fail, were they within specified max operating temp (but higher than
>> rated wattage)?  Or did the temps go over the specified max operating
>
> I think the problem will be that the temperature will *locally* go over
> the maximum temperature.

Yes.  I admittedly haven't done the math on this, but cannot imagine  that difference could be 100 deg C.  The data/numbers others have  posted seem to now tell me that my "gut feel" is wrong though.

Cheers,
-Neil.

2011\10\03@100246 by David VanHorn

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Is it really worth all this sweat?   How hard are they to replace?
How sensitive are you to failure? (cost and downtime)

Try running a pair at the anticipated power level 24/7 and see how
long they last

2011\10\03@100756 by alan.b.pearce

face picon face
> Not shown in that pic is a bunch of kapton tape wrapper around the
> resistors and aluminum block.  It helped a lot, but it needs just a
> bit more.  If I can find fire-cement or similar in a small quantity I
> would try that too, or just build a nichrome-wire head.  But that
> cement has been the reason for the resistor-heater design.  Think it's
> time I hit the 3D-printer forums.

I doubt that just going to the white cement ones will change much for you, AFAICT they are the same in the channel in the cement as fed into the inside of the aluminium block and cemented in using similar cement. The Al block will have a lower thermal resistance from element to outside surface. The problem you have is that most of the surface of the resistor is not in contact with the part you are attempting to heat, so 75% (roughly) of the resistor surface area is wasting heat for you. It will take somewhat more than a few layers of kapton tape to sort it, although that may give enough thermal resistance to show what is really needed.
-- Scanned by iCritical.

2011\10\03@103011 by PICdude

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Exactly why I *was* okay with them blowing up, but originally asked to  see if there was any way to reduce potential damage.  Now  though, I  am concerned with the one report of a resistor violently blowing up.   The automated workcell was built for multiple purposes and we thought  we could make a 3D printer out of it too with the addition of an  extruder head.  This is just for the fun of it and I really don't need  it for anything useful, sensitive to downtime = almost zero.

Cheers,
-Neil.



Quoting David VanHorn <RemoveMEmicrobrixspam_OUTspamKILLspamgmail.com>:

> Is it really worth all this sweat?   How hard are they to replace?
> How sensitive are you to failure? (cost and downtime)
>
> Try running a pair at the anticipated power level 24/7 and see how
> long they last.
>

2011\10\03@103914 by M.L.

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On Mon, Oct 3, 2011 at 9:49 AM, PICdude <RemoveMEpicdude3TakeThisOuTspamspamnarwani.org> wrote:
> Not shown in that pic is a bunch of kapton tape wrapper around the
> resistors and aluminum block.  It helped a lot, but it needs just a
> bit more.  If I can find fire-cement or similar in a small quantity I
> would try that too, or just build a nichrome-wire head.  But that
> cement has been the reason for the resistor-heater design.  Think it's
> time I hit the 3D-printer forums.
>
> Cheers,
> -Neil.
>
>

I'm not sure if this answers what you're asking, but refractory cement
can be had in small (5-pound) tubs at home improvement stores.

--
Martin K.

2011\10\03@111422 by Paul Hutchinson

picon face
> -----Original Message-----
> From: EraseMEpiclist-bouncesspamspamspamBeGonemit.edu On Behalf Of PICdude
> Sent: Sunday, October 02, 2011 9:31 AM
>
> How do these things fail?  This is not a volume or commercial product,
> but a one-off 3D printer head.  Resistors are these ones from Yageo...
> search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=8
> 10F5R0E-ND

When thinking about using a power resistor as a heater don't forget the
power derating for temperature. The datasheet linked from the DigiKey page
has the derating specs a bit out of place (just above electrical instead of
below power). The spec is linear 100% @ 25C to 0% @ 275C, so @ 150C the
power handling is only 50% (5 Watts for the 10 Watt version).

Paul Hutch

{Quote hidden}

> -Neil.

2011\10\03@112637 by PICdude

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The local home depot and lowe's here claimed not to have anything of  this sort, though with this new (to-me) term "refractory cement" seems  to be quite available at several sources.  Thanks.





Quoting "M.L." <RemoveMEmKILLspamspamlkeng.net>:
>
> I'm not sure if this answers what you're asking, but refractory cement
> can be had in small (5-pound) tubs at home improvement stores.
>
> --
> Martin K.

2011\10\03@142832 by M.L.

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On Mon, Oct 3, 2011 at 11:26 AM, PICdude <picdude3STOPspamspamspam_OUTnarwani.org> wrote:
> The local home depot and lowe's here claimed not to have anything of
> this sort, though with this new (to-me) term "refractory cement" seems
> to be quite available at several sources.  Thanks.
>

I used several buckets when I built an aluminum melting furnace. It's
good to at least a few thousand degrees!
Mix it with perlite if you're looking for a general high temp.
insulating material.

--
Martin K.

2011\10\03@161158 by Barry Gershenfeld

picon face
Heat loss as pointed out earlier is worth thinking about. (1) The resistors
are designed to cool themselves, so mill the "heatsink fins" off. (2) If the
block could be milled for a slot you could get more contact between the
resistor and the block. (3) It almost looks like you could bolt another two
resistors on; even if smaller, since every bit helps.

The other thought I had that wasn't covered already is that if you have a
cheap plentiful supply of resistors, you could subject them to large
overdrives and observe their failure modes.  (May be some good YouTube
material here :)  Then do successively lower power tests and watch how the
destruction time increases.  Maybe you can then predict how long they would
last at your desired power level

2011\10\04@005936 by Sean Breheny

face picon face
I *think* that such power derating curves are usually intended for a
certain set of conditions - normally the resistor in its typical
mounting, surrounded by open air, so that the temperature mentioned in
the derating curve is the ambient air temperature, not the resistor
surface temperature. Otherwise, that would imply that the resistor
needs active cooling in order to use it at its normal rated power.
Some components, like MOSFETs, are typically rated this way (i.e.,
they give you some ridiculous max power dissipation for a *case* temp
of 25C, which would only be achievable if you had it attached to a
cold plate with liquid nitrogen running through it). However, I'm
under the impression that resistors are not rated that way, especially
those which are not designed to be heat-sunk. That is to say, if you
really are sure that the ambient air will never be more than 25C, then
a 10W resistor *can* actually dissipate 10W and live for its full
normal rated lifetime.

Someone please correct me if I'm wrong!

Sean


On Mon, Oct 3, 2011 at 11:14 AM, Paul Hutchinson
<spamBeGonepaullhutchinsonSTOPspamspamEraseMEyahoo.com> wrote:
> When thinking about using a power resistor as a heater don't forget the
> power derating for temperature. The datasheet linked from the DigiKey page
> has the derating specs a bit out of place (just above electrical instead of
> below power). The spec is linear 100% @ 25C to 0% @ 275C, so @ 150C the
> power handling is only 50% (5 Watts for the 10 Watt version).
>
> Paul Hutch

2011\10\08@130415 by Paul Hutchinson

picon face
Yes you are correct and I should have written "ambient temperature" instead
of just "temperature" in my first sentence.

What I was responding to was the "How do these things fail?" opening that
was quoted in my post. I was trying to point to a possible explanation for
why the resistor would fail even when the nominal power rating was never
exceeded. Let me, hopefully, clarify my point.

When you use a resistor as a heating element you will either be
unintentionally or, in the case of an enclosed space heater, intentionally
increasing the ambient temperature. An increase in ambient temperature
causes a decrease in power handling capacity for most if not all resistors.
I've seen a number of cases where this effect is why a power resistor used
as a heater causes the resistor to fail when used near its nominal power
rating. The particular 10 Watt resistor that was pointed out has a derating
factor of 0.4% per degree rise in ambient temperature. So that resistor
would only have a 9.5 Watt capability with a mere 12.5 degree increase above
the rated ambient temperature. If you can ensure that the air entering this
resistors heat sink never exceeds the 25 degree ambient specification then
it should not fail with 10 Watts applied.

Looking at what I just wrote, I'm thinking I may not have clarified the
point well. I suck at writing quickly and I'm not going to spend the time it
takes for me to write well.

Paul Hutch

> {Original Message removed}

2011\10\09@063416 by RussellMc

face picon face
> Looking at what I just wrote, I'm thinking I may not have clarified the
> point well. I suck at writing quickly and I'm not going to spend the time it
> takes for me to write well.


Paul - came across perfectly clearly. You said:

Resistors are rated at full power only at say 25C and as ambient
temperature rises their power handling capability is derated.

So, if a resistor is used to add energy to air but the incoming air
that it heats is always at 25C then it should operate safely at full
power.

But, if a resistor is surrounded by the air that it heats then it's
ambient temperature will rise as the air heats, and the manufacturer's
derating formulae or tables must be consulted.


   Russell

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