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Power Resistor

Power Resistor

Power Resistor

(OP)
I got a basic question regarding power resistors. I know when specifying a power resistor, the power rating shall be twice the expected dissipated power. For example, I am using 3000 ohm, 10W resistor in a circuit application in which 120VAC will be across it. It is a nice power resistor from ohmite, 20 series. If you do the simple calcs I am only drawing 40mA, which gives me 4.8Watts. now it is a 10W part and it is still getting hot to the touch. I even lowered the AC voltage down to 100VAC , which gives me 3.68W and is still getting hot to the touch. It is not a heatshinkable resistor, it's one of the tubular, ceramic axial leaded resistors.  I ran it for 10minutes across 120VAC, then 30minutes just to make sure the resistor wouldn't burn up. And it did not. Measured it and still read 3000 ohms on the meter. Current was steady also! In the end application the power resistor will be sitting inside a enclosure full of oil. So the oil will keep it cool. I'm just curious on why they get so hot, even at 3.65W (10W rated part), and there is no heatsink required (so the datasheet states).
Thanks
:)

RE: Power Resistor

I once had to investigate the failure of a wire-wound ceramic power resistor (5 watt).

To double-check the relationship of applied power to failure modes, I took a new sample and applied full power. It got very hot and nothing happened. I went to TRIPLE the power rating (15 watts on a 5 watt resistor). Nothing happened except that the painted markings evaporated leaving the test resistor completely blank.

I then wrapped it in high temperature insulation and put it in a tiny box to stop any air cooling at all. I reapplied the triple power. Nothing happened for a while, but then it went open circuit. I opened the box to discover that the soldered connection from the test resistor to the test wire had melted, an inch away from the resistor body, and the test wire had simply fallen off opening the circuit.

After if cooled down, I check the resistor. It hadn't even changed value by any significant amount.

Tough little buggers.

RE: Power Resistor

the bottom line is that 3W is more than enough to get a resistor hot.  One problem that you need to contend with is whether your hot resistor changes value DURING operation.

If you thermal glue some cheapo fins on it, you'll keep the resistor even cooler in the oil bath.  They used to make those sheet metal clip-on fins for small-signal transistors that would significantly improve the heat transfer of the overall structure.

TTFN



RE: Power Resistor

Hear let me toss in:  fuseshut, it has a great deal to do with surface area. Picture an electric stove element. It's sitting there damn hot, glowing red.  Now without changing the power shrink it to a quarter its size.  It would get much, much, hotter probably turning yellow hot!

Now consider your paltry 4.8W.  Have you never touched one of those 7-1/2W night light bulbs?  They're hot!  Now your resistor probably has half the surface area!  So even with half the wattage it's going to be hot! I would expect you could  hold it with you fingers at 1/2W but little more. If this is a ROV app and the oil touches an external wall with a large area you probably won't have any problem. You do need to be careful that the surface temp of the resistor won't reach some temperature that would cause changes in the oil or you could have a real problem.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

(OP)
Itsmoked,
It is in a ROV. There are only three of these buggers inside a aluminum can, 10"ID x 12"L, filled with oil. In our tank the temp (in FLorida) is 80-90F, but the ROV will be stationed on a ship, with sea temps of 50-60F. oil is the best medium for transferring heat from an element(component) to the aluminum, then to the cool water. SHouldn't be a problem.
VE1BLL, wow, these things are tough. Just for kicks, on MOnday I am going to power up the resistor and let it sit inside a large beaker of oil.
THanks for all your input.

RE: Power Resistor

I used to work for Welwyn Components, a well known hybrid and resistor manufacturer in England. The glazed wirewound resistors were capable of running continuously with the glass coating softened by the heat. They weren't designed to run that hot at rated power, but they put up with our abuse in Engineering when we exceeded their rating by a fair margin and very rarely failed. I think the glaze prevented oxidation of the wire even when almost glowing, hence the low failure rate. Like you say, tough little critters!

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Power Resistor

Does all this toughness go with the standard rectangular power resistors? 10Wattish
  The ones with the five sides and then it looks like something white poured in the bottom..

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

Probably yes; it depends on the exact constuction details. There might be substandard imports floating around these days. One can only imagine that a start-up company might solder the internal connections instead of welding.

The 5 watt version I was testing was the little cylinder about 1.25" long and 0.25" diameter.

RE: Power Resistor

The answer is that most of these wire wound power resistors are rated to run with a rated surface temperature rise of 250C above ambient. Check out the suppliers small print in the specifications to confirm this.

Now the resistor itself will be fine running with a surface temperature of maybe 300C. It will be made of glass or ceramic or some other extremely temperature tolerant material. But realise that is probably a lot hotter than your soldering iron tip.

But it may just tend to unsolder itself, set fire to the immediate surroundings, toast everything in the vicinity, smoke, create "hot smells" and otherwise not behave itself.  But the resistor itself will work fine.

The trick is to decide how hot you want your resistor to run. To prevent burns to fingers 50C, or 25C rise might be appropriate. That means a 50 watt resistor is immediately limited to 5W dissipation.

Or maybe you don't want your FR4 fibreglass PCB to go black after several months running. 80C surface temperature is the limit. So your 50W resistor can only dissipate maybe 11W.

Hot smells start around 100C surface temperature, so again derate that 50W resistor to 15W.

Running it at half rating may be fine, provided a temperature rise of 125C above ambient is acceptable. Just don't expect to use PVC wire or mount it near anything heat sensitive. Ordinary 60/40 solder is rather questionable too for long term reliability. It will also smell, particularly if it has not been run for a while.

A much better way is to use the gold coloured aluminium clad wirewound resistors, and bolt them to a suitably rated heatsink for any really serious power.

 

RE: Power Resistor

I agree.

I find that if you can't touch them you will have reliability problems in the long run. (board mounted) Due to the board and the solder living in the fast lane.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

Ha-Ha-ha Keith, the voice of experience.

It is called the rule of thumb method.  You stick your thumb on the resistor, if you can smell burning flesh and feel real pain, the the resistor is too hot !!!!

RE: Power Resistor

Yep!

Made that mistake.. Customer wants to know why there is a large charred-black hole in the board.

Ulp.. The res roasted the life out of the FRP4..
Then the solder finally evaporated from the hottest lead..
Then the black carbon took over as part of the resistor.. Then the arc formed and ran for a while...

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

It is a funny thing though. Running a ten watt rated resistor at only one watt. But you are quire right, if it is too hot to keep your finger on it for maybe a a full minute, it will not be reliable in the long term.

If it is not reliable, the sales guys, and the service guys will not be happy. They will call you very bad bad names behind your back, and not buy you lunch.

Much better to de-rate your resistors and become a legend.

RE: Power Resistor

Keith, those block resistors you describe are not nearly as rugged as the tubular types. We were playing around with some resistors one day and we ran a Dale CW5 series (which is a tubular type) so that it was glowing for 1/2 hour plus without it failing or even changing value once it cooled. However, the ones you describe would split in the middle if we attempted this.

Now, not all tubular ones are good either. We had some cheap 5W ones used for a board and they would fail even though they weren't even getting to 100C.

RE: Power Resistor

Thanks for that info Mr. Hutz.. I shall file it away. Now none of my stock have to die in the service of testing. :)

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

I used to use a 2x or even 3x safety figure for power resistors that were in a position to cook things around them. The resistor was in no danger but the radiated heat can do aweful things to stuff placed nearby.

RE: Power Resistor

What I can't figure out about many of the ceramic or 'gold' aluminum body power resistors is why they're not made with a black body material?

At times, I've encountered a power resistor that is just a little warmer than I would like. What I do is take my black sharpie pen and make it black, and it runs cooler. I've verified this with a temperature probe. Same thing with some TO-220 transistors without heatsiniks - darken that shiny metal and they run cooler.

RE: Power Resistor

That's interesting.  Too bad sharpie ink fads rather quickly in my experience.

  I am also trying to square that with the fact that while a white 7-1/2W bulb is hot to the touch a dark color one will simple fry you instantly.  I presumed this was because the darker color filtered out a larger percentage of the visible light requiring the remainder to be lost thru IR radiation that was based solely on the blocked visible light converted to surface temperature.  So.. hmmmm.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

Black is only going to help with infrared heat radiation, and that is only going to be significant if the resistor is "looking into" much cooler infrared surroundings than itself.

Conduction and convection are much more effective ways to carry heat away from a resistor than radiation, especially at realistic resistor surface temperatures.

RE: Power Resistor

Thats true Warpspeed. Black can absorb heat just as well, depending upon the surrounding temperatures. Usually, power resistors are the hottest items in a chassis. But if they were in a box of big power devices, or in a tube chassis, that may not be true.

RE: Power Resistor

Comkid, you're right, the low emissivity of gold makes it a poor radiator but aren't those gold resistors meant to be bolted to heatsinks. The white ceramic ones have a high emissivity, typically 0.9 so they radiate well.  

It's not always true that black is a better radiator either as surface texture and properties are more important than colour.  It varies with the waveband of interest but at lower temperatures (<100C) convection is probably the dominant heat remover and surface coatings can actually insulate.  I have used the black pen trick too but don't know what it's emissivity is.

RE: Power Resistor

You're right. The gold ones are meant to conduct heat to what you've bolted them to. But you know how it is - you're trying something real quick, so you take that 25 Watt Dale and hang-it-loose in a 6 watt breadboard application because it's in the parts cabinet. If it's hotter than you want, make it black. The gold does look pretty, but I've seen some in surplus military equipment that were actually produced as black.

If you could see in IR, you could tell if "Black" in visual light was still "Black" in IR. (Jordie of TNG could tell.) Anyway, Sharpies seem to work good. Now, if I spray painted it black, I would be concerned that the paint itself with it's thickness was a thermal insulation barrier.

RE: Power Resistor

This is quite beyond the depth of this topic but I was in the position to take some actual numbers and they might be of interest to others.  

A piece of steel at 330C radiated 33mW/cm2/sr in 9-12u band and 18mW/cm2/sr in the 3.5-4.5u band.  

Metal with laquer paint at 40C radiated 3.73mW/cm2/sr in 9-12u band and 0.13mW/cm2/sr in the 3.5-4.5u band.

Totally useless info.....

RE: Power Resistor

Aside from warning of a potential problem, the finger test isn't a good one for resistors, even if you can read the manufacturer logo on your finger.  50 Celsius will feel hot on a finger, especially after a minute.  Meat cooks at 70 Celsius.  But these are nothing compared to the operating temperature of a power resistor.  Get an IR thermometer and make real measurements.  Burning hot to your finger may not be a problem at all to your product.

RE: Power Resistor

50 Celsius is not actually that hot. The thermostatic mixing valve on my single bath tap is set to 45C, and the whole family by pouplar vote decare it perfect. Here in Oz, the maximum staturory hot water supply temperature for schools and kindergartens is 50C so the kiddies cannot scald themselves.

If immersion in 50C hot water is not going to hurt a five year old, your  gnarled toughened much abused engineers thumb sure isn't going to burst into flames at 50C.

I bet the tea/coffee you drink is hotter than 50C.  

RE: Power Resistor

European standards are set for 48ºC surface temperature for a 10 min contact exposure and 43ºC for an 8 hr exposure.

The implication is that you can indeed get burned at 50ºC, it's just that most people will get the message before any damage occurs.

TTFN



RE: Power Resistor

It all gets back to the rule of thumb: Every 10 degrees doubles the chemical reaction rate.

  That is: The solder oxidizes faster, the board ages faster and the resistor ages faster. Too fast? Depends of course.  

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Power Resistor

Just to show that our body isn't a reliable temperature measuring instrument.  I measured my "hot but drinkable" tea temperature at 54 Celsius with an IR gun.  Sipping very small quantities to feel the heat that very quickly dissipates into the mouth, exciting the tastebuds to make them transmit better the senses.  Within two seconds the IR gun in my mouth showed 41 Celsius.

I also remember making system longevity tests in a 55 Celsius chamber. Keeping my hands in contact with the stainless steel casings was out of question.

Too hot is relative.  The CPU chip on my main product has an operating case temperature of 55 Celsius.  Perfectly within specs; customers are even happy to know that the temp is only 55 Celsius.  I would not keep a finger on it for long even if the presence of the finger drains heat and makes the temperature go lower.

Hey Keith, would people living in northern countries live
longer than people living in the south?  Stats might prove it right.  smile

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