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
:)
Thanks
:)





RE: Power Resistor
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
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
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
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
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Sometimes I only open my mouth to swap feet...
RE: Power Resistor
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
The 5 watt version I was testing was the little cylinder about 1.25" long and 0.25" diameter.
RE: Power Resistor
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 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
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
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
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
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
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Power Resistor
RE: Power Resistor
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
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
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
RE: Power Resistor
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
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
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
RE: Power Resistor
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
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
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
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.