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Power Resistor

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fuseshut

Electrical
Oct 16, 2005
76
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
:)
 
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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.

 
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



 
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.-
 
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.
 
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!

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

 
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.



 
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.-
 
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 !!!!
 
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...
2ugksy8.gif


Keith Cress
Flamin Systems, Inc.-
 
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.
 
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.

 
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.
 
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.
 
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.-
 
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.
 
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.
 
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