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.

 

[pc1966]

Those are wire-wound resistors and probably 2.5W under ideal conditions. But that means they will be running hot, and I mean well over 100C body temperature, so the real-world limit is sometimes the thermal endurance of the parts nearby.

However, for the exact answer you would need to track down the manufacturer and measure the body dimensions to see which series of parts they are from. An example of similar parts is here:

https://docs.rs-online.com/635c/A700000007299740.pdf

The 2.5W version is 14.5mm long body, but others are also made looking very similar.

 

[rapparee]

Calculate 4 digit SMD resistor: 05R1

kiloohm.info

This website will give you the resistance

You can work the rest out from there

 

[pc1966]

Just to say if you wanted to measure the power rating you would have to know the maximum running temperature, or to impose a lower value based on where/how it is mounted, as well as the ambient temperature.

Then run some known power, measure how much hotter it gets, and from that you can estimate the power needed to get to, say, 125C body temperature.

 

[_q12x_]

Those are wire-wound resistors and probably 2.5W under ideal conditions.

Idont want to guess. That Ive been doing all along by myself. I want !to know! what Wattage they really are !
- You dont get the real picture yet. Let me clarify it for you a bit more.
First of all, Very good idea, with comparing the size and shape of the unknown resistor with the sizes of other commercial resistors !
But... hahahahaha, here is what you dont get it yet:

from my other forum:
...you can probably get a good estimate of a resistor's power rating by applying power to it and measuring it's temperature...

Yes, exactly on my line of thought. I have to find a way to apply power to the resistor now. My immediate thought is to put it in series with a 240VAC bulb. And ---something--- in between them that can be dimmed to 0 or max current flow. Or voltage dimming.
But I stay and think, maybe there is something not so drastic, that works at 5VDC perhaps, and not directly at 240VAC. And some guy here: "No, there is not!" hahaha. 5VDC with 2 Amps probably will be suffice, that equals 10W. Or even lower because I need to test components up to...5W I'd say. Im not really sure what I have but I guess a lot are in the range of a couple of hundreds mA. The large majority of them that I want to test. And not only resistors but other components as well, some transistors, diodes, just to check if they are what they say they are (for the known components values).
I wait your ideas.
Thank you.
Aham...#1 Yesterday I built that circuit I mentioned from : #6 ; (not all of it, only a module); (you can see it in the picture, in the top left corner, having a 10R 10W cement white long brick resistor on it). This circuit didnt worked for resistors. Only for very low resistors like 10R and smaller. That LM317 current driver circuit is good for leds and transistors. Not resistors.
Aham...#2 Just to see the diversity that I have, and to make you forget comparing with the known sizes and shapes of existing commercial ones :
The very small ones, down in the -center- bottom of the image, that have long and sexi legs, are the known value, size and shape, they are the commercial ones. They are 1/4W (250mW). I leave them there for comparison. I also leave my finger there for reference, and a rule with both cm and inch scale. The rule is 6 inch long for my american friends (or 15cm for the rest of the world).

Aham...#3 To be clear, the large majority of my power resistors I have, are marked. But I have some BLACK sheep there, unmarked, unusual shape and size, different brand if you will, different construction time and original country. So I cant compare them with the rest. These black sheep I have my eye on them for a very long time, decades even. And these are the big power resistors. The small ones are not so diverse but I have some romanian brands, a lot of them actually, that also they are unmarked. I assume they are 1/4W and I treat them as such for a very long time. But they are 2 times bigger than the 1/4 ones I have in the picture. I assume the way they build them was ancient technology compared with these new commercial ones.
Aham...#4
The context: ...how many I have... not THAT many, right? But boy they are from everywhere there. And some are brand new !
You see those on the left side? near my hand? Those are the small ones, romanian brand. is my best guess really. They may be polish or russian for what I know. But I know I have a lot of them. And like I said, they may be 1/4W. like the small blue one near the ruler. I suspect they are 1/4 because ancient technology used back then in the 40's/50's/60/70/80/90's when they were made.

 

[Lucien Nunes]

All resistors of the same size and shape, produce the same surface temperature rise when dissipating the same power. So for resistors of the same general construction, the power rating is determined only by the temperature their materials can withstand, without being damaged or prematurely aged or going out of tolerance.

There's no point heating them up electrically and waving thermometers at them, because all resistors of the same size will reach the same temperature at the same power. The thermometer won't tell you the quality and chemical composition of the materials.

If you want to know the rating exactly, officially, then only the original manufacturers data will tell you, because only they knew what their materials could withstand over time. You can't find this out by testing them at the workbench, unless you want to spend a year testing batches of 100 identical resistors under many different sets of conditions, as the manufacturer probably did. Without that data, comparison with similar resistors is the only realistic method. The size and shape, and general construction and choice of materials must be similar. To recognise the construction, you need to study resistor technology a little.

Then you will be able to recognise different types of resistance element such as:
Carbon composition
Carbon film
Metal / metal oxide film
Power wire wound
Precision wirewound

And different types of encapsulation such as:
Moulded case
Ceramic outer tube
Solid ceramic core
Silicone coated
Vitreous bodied
Etc.

I can recognise eight different types of construction in your picture above, and have a pretty accurate guess at the power ratings. That comes from 40 years experience handling resistors made 1900 to 2022, from 0.05W to 20kW, day after day. But I cannot tell you the official power rating with 100% certainty, only the manufacturers' data can do that.

Final note: the power rating is for a single set of ambient temperature and airflow conditions only. A 10W resistor might only be usable at 5W in a high ambient temperature with limited airflow, or 3W if nearby components are sensitive to heat, etc. A precision resistor might be able to dissipate 1W but only accurate at 0.1W or less. So don't get too attached to the idea of a single figure describing the power, just as you cannot describe a car engine's performance by a single figure of kW crankshaft output.

 

[freddo]

Well I don't think anyone can beat the post above, so instead I'll post a photo of large power resistor, rated power handling in the kW's. I don't think I've posted this here before.

 

[_q12x_]

to mister @pc1966 ,we are on the same line of thought, right?
Let's do a 'power tester' for some already known values and shapes. Those blue resistors that are 1/4 or 250mW.

Play with what we know and figure out from there, if successful, how to go to other unknown types. Or... use the SMD version of a resistor.
Here is how I imagine the actual circuit should look like:
Only for a specific range of resistors. In this case from 0.1R to 10R.

 

[_q12x_]

I couldn't sleep. The more
some forum errors when editing - also can not remove this post - no options to remove it. WOW
hahaha

 

[_q12x_]

I couldn't sleep. The more I think the more I realize its much more simpler.
I strongly believe the absolute necessary components to measure a resistor Wattage is down to 3 esential elements in the circuit:
1 - a variable voltage supply; 2- the test resistor itself; 3- a measuring tool of some kind.
Actually there are 2 or 3 measuring tools we need: the Ampmeter, Voltmeter and Thermometer. So 5 components for the final and complete circuit.
Here is a more refined idea that I imagined:

Also, doen't matter the 'range' of resistors used with this type of circuit.

 

[pc1966]

I couldn't sleep. The more I think the more I realize its much more simpler.
I strongly believe the absolute necessary components to measure a resistor Wattage is down to 3 esential elements in the circuit:
1 - a variable voltage supply; 2- the test resistor itself; 3- a measuring tool of some kind.
Actually there are 2 or 3 measuring tools we need: the Ampmeter, Voltmeter and Thermometer. So 5 components for the final and complete circuit.
Here is a more refined idea that I imagined:
View attachment 102109
Also, doen't matter the 'range' of resistors used with this type of circuit.

That will give you the wattage for 100C operation which is usually OK, but as @Lucien Nunes points out the actual wattage to meet and given specification might be rather different.

Also for some resistors, especially smaller SMD types, the "safe and reliable operation" voltage limit can be fairly low and it might be your limiting factor instead of temperature.

 

[Lucien Nunes]

Here's a typical data sheet for a particular series of power resistors.
VTM power resistor data

Note the power derating graph. The maximum working temperature is 275°C and the ratings are based on this. If you were to test based on 100° maximum you would get a much lower rating at the typically quote ambient options of 40° or 70°, which is why you need to identify the construction to make an informed guess at the permissible temperature. E.g. with linear derating, (275-70)/(100-70) = 6.8 i.e. the real 70°C power rating would be nearly 7 x higher than the one you 'measured.'

 

[_q12x_]

I didn't do any measurement yet. I was trying to figure out what the circuit may look like. To get an estimate of a component power. I'm not after exact values. But if I get it exact, Ill be equally happy. I put an arbitrary 100°C but that will be changed when the actual experiment will start.

 

[Lucien Nunes]

But you can't find out the maximum permitted temperature from the experiment, other than by when the part starts smoking, breaks in half or goes out of tolerance. Just like car manufacturers test prototypes with crash-test dummies, resistor manufacturers have to cook a lot of resistors to find out what power they would withstand. That is why you either need the data sheet or experience of resistors, or a direct comparison to one of similar construction.

 

[_q12x_]

mister @Lucien Nunes , I understand everything you said so far and you explained clear and logical enough. Thank you for that. My intention here is to get an idea and the hope is to get close enough to the original value. Not by any means to be perfect on the spot. If it will fail, it will fail, but I give it a try. I'm as curious as you what the real experiment will give me. You are more skeptical than curious but lets say you are curious. Haha.

 

[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 !!!