How to check the Wattage of a scrapped resistor?

[_q12x_]

- I want to know what these -unknown- resistors Wattage are !?
First of all, they are scrapped. Then, we really dont know in what part of the circuit they had sit, in the gate of the circuit where high power was entering? or in the core of the circuit, where the power didnt matter that much, like a logic circuit for example. We also dont know how much stress they took, they may be brand "new" but --old-- stock, or they took some hard unforgiving heat? Also how much time they got electrical exposure, minutes? days? months? years? That will tire the component as well. Also, what technology was used in building them, using high Wattage resistance or small resistance to wattage? We dont know all of these things.
And this question is more universal than to these ones I have in my hand. I also have a few other candidates, very strange in shape, also regional origins (some made in Romania, some in URSS, some in Poland, even CHINA!!! haha).
I also highly suspect the technology used in the past, like 70's,80's,90's,2000's, differed greatly from today's. For a 1/4W or 1/8W was used a BIG or LONG resistor case, while compared to today SMD's, the same wattage is literally on a tip of a needle.

• I want to test and to --KNOW-- what's the wattage they can run safely !
• How are you doing it, personally? What is the proper technique? What you recommend? I mean to built, not to buy. This should be a fun little mini-project.

 

[Lucien Nunes]

I've done it for you to prove the point: see attached pics.
Resistor A / left PSU: 2.13W.
Resistor B / right PSU: 2.02W
Resistor C (not shown) : 2.08W
All three are at 100 +/-2 °C
From those figures, tell me which resistor is rated at 1W, which at 2W and which at 3W?
See the problem?

 

[_q12x_]

Aaah you are fast. Thank you for playing. Really.
I did the test as well just now.
We will compare notes and the math guys should jump into this data I collected. Also yours.
---
edited some minutes later:
Yes, I confirm your calculations:

From those figures, tell me which resistor is rated at 1W, which at 2W and which at 3W?
See the problem?

We are definitely miss a variable that will give us the Wattage. Is my strong belief. But what is it?

 

[_q12x_]

 

[Lucien Nunes]

We are definitely miss a variable that will give us the Wattage. Is my strong belief. But what is it?

It's what I've been saying all along - the maximum working temperature.

This depends on the type of materials used and the quality and method of construction of that particular resistor. For example, a silicone-coated wirewound will typically withstand 250°C or even 300° with no problems, but a carbon composition might fail rapidly at 120°. And with encased resistors there can be significant thermal resistance from the element to the dissipative surface, so the critical temperature (of the element) might be significantly higher than the temperature of the case.

My three samples all achieved nearly the same dissipation at the same temperature, meaning that they all have nearly the same thermal resistance from case to ambient. That is to be expected, because they are all the same size with similar radiation characteristics. The ambient temp was 18°C so the thermal resistance of all of them is about (100-18)/2 = 41°C/W. This gives the gradient of the line in the derating graph while the max temp gives its X-intercept.

The 1W was resistor C, a ceramic-cased carbon composition made in the 1960s. It's working temperature is limited by the tendency of the slug material to change in characteristics. Above its rated temperature, it will tend to drift badly out of tolerance, both reversibly with temperature change and permanently, and have a shortened life. At 2W it started oozing wax.

The 2W was resistor A, a carbon film type. These are more stable and can work at a higher temperature,. If overrun, the first thing to burn is usually the paint, even while the element stays in tolerance. They are not however as resistant to high pulse dissipations as carbon compositions.

The 3W was resistor B, a metal oxide film type. These have generally better characteristics than carbon-film, lower noise, better tempco and long-term stability, higher working temp. This one would probably withstand 6W or even 9W, 3x its rated dissipation, for a short time (its life span might be reduced) although the coloured bands would burn off. At that power, if mounted on a PCB it would scorch the PCB and the leads would desolder themselves but the resistor would survive. In the 1970s there were wirewound resistors in TVs with a thermal cutout that worked by desoldering a spring-loaded contact if the resistor overheated due to a fault in the line output stage.

So there you have it - you need at least two parameters to determine the rating (at a given ambient temperature):

• Thermal resistance to ambient (which we find out with the heating test but all similar case sizes will have similar thermal resistances.
• Maximum working temperature (which comes from manufacturer data or comparison with other resistors of similar construction)

 

[_q12x_]

Thank you again for playing. Nice explanations !
I did some advances as well. Very unorthodox ones. With some good and consistent result --over the same category-- of the component - for example only over all 1/8 0.125W ones. But when I expected the same result over the larger ones, the 0.250W, it didnt work. But I got somewhat close. I could touch it but not grab it. Aaah... so close. Here is the data: link Tell me what you think if you get through it. Thanks. My best friend, mister @Lucien Nunes !!!

 

[_q12x_]

 

[_q12x_]

Test finished !
Final Conclusion: the most important variable is the material used in making the resistor itself.
That material is dictating the temperature the resistor will rise to and its maximum wattage.
Unfortunately it is not a SINGLE TYPE of material used on all the --resistor watage types-- as I originally presumed. (I didnt think too hard)
Because the material varies, also the temperatures is varying from model to model and I can not obtain the Wattage if the temperature is not constant.
This only works if the temperature is --constant-- for every value type of resistor (10R,100R,1K,10K,100K,etc) and also for every watt type of resistor (1W, 10W, etc)
DataFiles

 

[Lucien Nunes]

I did mention this in post #6 - including a list of the common materials!

The material only affects the temperature rise slightly, because of different emissivities affecting the radiation per unit surface area, and the conductivity affecting the uniformity of temperature over the surface and amount of heat dissipated by the leads. The size and shape are much more important. The materials do however dictate the maximum permissible temperature which is critical.

 

[_q12x_]

This is a great handicap of electronics, not being able to calculate the Real Maximum Wattage of a given component ! Whatever it is. I feel sorry for this side of electronics.
I planned this experiment for a long time and I did some of it before, but now I push it to a complete stage even if it is a negative result, doesnt matter, I tried. Hopefully someone else will get ideas and inspiration from my exercise and who knows maybe in the future there will be a way to read the real maximum wattage of -any- given component. That will be cool.
Until then... I am doing what everyone else suggested so far, guessing and using a good sense in pointing what it may be - if is completly unknown and if possible, comparing to something that exists. Also asking others, always is a good idea, haha.
Thanks a lot for the support and chat !

 

[Lucien Nunes]

I build equipment that has to keep working under harsh conditions. Not space-probe harsh but there isn't space-probe budget either. If someone takes a unit to the desert and it overheats, it could cost tens of thousands of pounds of lost time while the backup is wheeled out. So thermal design is an important factor. In industrial plant you might solve the problem with lots of fans, but my equipment has to be near silent. You might think of using large heatsinks but it has to be portable. You might think of using heat-pumps but it has to run on limited battery power. There has to be intelligent balance between working temps, maximum permitted ambient temps, service life, MTBF etc.

If you have enjoyed thinking about heat dissipation in resistors, now look at the effects of heat on electrolytic capacitors. Get some data sheets and read up on the chemical and physical properties of electrolytics. Imagine you are building a unit with 1000 electrolytic capacitors of which 100 have significant ripple. They will be mixed up with 50 power semiconductors and 25 power resistors of different sizes and dissipations all packed into a unit that is going to be operating in the equatorial sun and must have an MTBF long enough that you don't expect any of 100 units to fail in their 5-year working lives. Suddenly you might find that the power limit on a resistor is not what the resistor can stand, but what it can be allowed to dissipate without overheating the capacitor next to it. What then? Move them apart? (wiring inductance problem). Heat-sink the resistor to the case? (assembly / disassembly problem) Overspec the capacitor? (size problem)... Now you have multi-dimensional thermal puzzle and the Arrhenius equation is your nemesis.

This is why I like my work.

 

[pc1966]

Even for ground-based equipment ESA has (or at least used to have) guidelines for de-rating* parts to achieve real-world reliability that is sufficient for serious applications.

[*] De-rating as in applying a fraction such as 0.8 of the rated power/voltage/current along with a lower max temperature such as 100C for Si devices instead of the manufacturer's 125C or whatever.