How to check the Wattage of a scrapped resistor?

Thread Starter

q12x

Joined Sep 25, 2015
1,448
- 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.
IMG_20220930_230900.jpgThank you.
 

WBahn

Joined Mar 31, 2012
27,475
To answer the general question, there's no definitive way to know what the specs are on some resistor found in a pile of rubbish. When you know nothing about a component or it's history, the safest thing to do is to not use it. The second safest thing to do is only use it in places where nothing that you don't know is going to be important.

Having said that, you can probably get a good estimate of a resistor's power rating by applying power to it and measuring it's temperature. You might get a baseline and take some known resistors and run them at, say, 3/4 of their rated free-air power and measure what temperature they settle at. If these are in the same ballpark, it's probably reasonable to use that as a criterion. Then take your resistor of unknown rating and increase the power until it reaches that same temperature. To give yourself some margin, use that power as the limit for that resistor.
 

Thread Starter

q12x

Joined Sep 25, 2015
1,448
And a quick circuit that I can make and test ?
Also what devices should I use? I have 3 different ways to measure temperature. One of them is very precise and close to a professional way of doing it. I also have DMM, OSCiloscope, and why not Leds.
I was looking back in one of my previous similar post : #6 ; I believe that may be a good circuit to try, right? or any other circuit?
 

MisterBill2

Joined Jan 23, 2018
13,176
The simple way to be reasonably close is described in post #4. But go to a good distributor website and examine their catalog for similar products. If you can measure the size of the resistor and compare that with the sizes in the catalog you should come close.

But in general it is best to not use a resistor close to it's maximum ratings. The exception being in the design of cheap junk consumer electronics products, where reliability is not a concern.
 

Thread Starter

q12x

Joined Sep 25, 2015
1,448
Very good idea, with comparing the size and shape of the unknown resistor with the sizes of other commercial resistors !
...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).
IMG_20221002_171033.jpg
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.
20221002_180921.jpg
 
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dl324

Joined Mar 30, 2015
15,120
For resistors in this form factor, I'd go by dimensions. Since you're saying these are "regional", you can only hope that they conformed to manufacturing standards. For example, Chinese resistors tend to use skinny leads (that may be ferrous).

This is from Yaego. Note that miniature resistors are rated for twice the power dissipation of standard resistors. Including specs for carbon film, metal film, and wirewound so you can see that the lengths are the same for all categories; so, with experience, you can eyeball them to determine wattage.

Note that power rating is derated above 40-70C and conservative designs derate at room temperature (we never operate at 100% of the power rating).

Carbon FIlm:
1664724234414.png
1664724852446.png

Metal Film:
1664724390432.png
1664724821608.png

Wirewound:
1664724353926.png
1664724764439.png
 

WBahn

Joined Mar 31, 2012
27,475
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).
With a resistor, you don't get to pick both the voltage and the current. You pick one and the resistor picks the other. Lets say that you have two resistors that are both 5 W, but one is 10 Ω and the other is 1000 Ω resistor. The first one will require 7 V and consume 700 mA (for 4.9 W) while the second will require 70 V but only consume 70 mA (yielding the same 4.9 W).
 

BobTPH

Joined Jun 5, 2013
5,801
I calculate 1.6V at 3.1A for the 0.51 and 16V at 0.31A for the 51 to get 5W.

I would guess that they are 2W myself.

A lab supply should be enough to test them. I would start at 1W.
 

sagor

Joined Mar 10, 2019
756
If the resistors get too hot to keep your finger on them (just a bit too hot), I would estimate you are close to 1/2-3/4 the maximum rating.
If they are hot, but you can keep a finger on them for a couple of seconds, you are probably 1/4 to 1/3 maximum power rating.
Just my "estimate" for resistors in the 2W to 5W categories. I'd call it rule of thumb, but I use a finger instead ;)
Just a guideline, don't expect accurate results....
Another method is the "volume" of the resistor. Measure length and width (and thickness in some cases) and compare to volume of known wattage resistors of the same composition (carbon vs carbon or wirewound vs wirewound)
 

ThePanMan

Joined Mar 13, 2020
496
If you know what voltage they operated on you can use Ohm's Law. Say it was a 24V system: 24V^2 ÷ 51Ω = 11.29W. Those don't look close to anything that can handle that kind of amperage. Of course there's other factors like the load itself. But not knowing that you'll have to make some other assumptions.

12V^2 ÷ 51Ω = 2.82W. Max said it might be a 3W resistor. Again, not knowing the load resistance (or reactance) we're only guessing.
5V^2 ÷ 51Ω = 490mW. It's way larger than that. So my guess would be closer to the 3 watt wire wound as @MaxHeadRoom said.

But remember, these numbers are worst case scenario's. If your load were to short would the resistor survive?

Do you KNOW the operating voltage before they came to be in the palm of your hand?
 

Thread Starter

q12x

Joined Sep 25, 2015
1,448
Ok then. Practically, What a --load-- can be for a resistor ? I imagine a motor as a load. It's the only thing I can imagine.
What you may suggest? On ranges of resistors of course. So from 0.1R to 10R, then from 10R to 100R and so on. And probably for each range it will require a different kind of load? Also we need a way of varying the power down or up from a minimum to a maximum. I imagine that will be done with a power transistor. Theoretically, if I would make a coil of some kind, simulating the motor coil, that should be enough --load--, but Im not THAT good. I know people that are that good... hehehe.
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.
1664749512923.png
And 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.
Screenshot_2.jpg
Hmmm? What do you think?
 
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Thread Starter

q12x

Joined Sep 25, 2015
1,448
Or even a SMD resistor, a 1206 to be specific. Which is 0.125 (1/8) W. I just look it up right now.
I also discovered the 1210 smd is 1/4W. I didnt know that. All the smd's that I bought and I have full stock and full complete range values are the 1206. I also have scrapped smd's as well, also 0805,0603,0402 and even smaller, much-much smaller, literally as wide as my tip of my sawing needle small !!! , very unconventional and non-commercial and I have no possible way to solder them back. But I could measure them. These scrapped smd's are Not in complete range values like the ones that I buy. So I prefer the 1206 for the momment.
1664765627249.png
I believe my circuit that I just posted will use less power dissipation over the transistor using a 1/8 smd resistor.
And I stay and thought even more, and I believe with NO Motor in the circuit, it will be more 'direct' and unlimited. But the transistor on the other side must be able to take the heat. Correct? At this point I want your impressions, ideas. With or without a Load as a Motor? And what transistor I should use? Also for what range of resistors?
Here is a more perfected circuit I am thinking:
Screenshot_2.jpg
 
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Thread Starter

q12x

Joined Sep 25, 2015
1,448
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:
Screenshot_1.jpg
Also, doen't matter the 'range' of resistors used with this type of circuit.
 
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MisterBill2

Joined Jan 23, 2018
13,176
Why is it so very critical for the TS to determine the exact power handling capabilities of rescued resistors? Given that they can easily be examined and compared to similar products with a fair degree of certainty, and also given that wise designers allow a fair margin of safety between working values and rated values, it does not make much sense.
Or iis it that the intent is not to use these resistors and other parts, but to market them as "new parts" and gain a lot of profit? While many used components are able to be utilized in assemblies and function quite well, there is a reduced level of reliability because of all the additional manipulation and potential mechanical stressing.
Or is the TS a hobby user who chooses to run components at their "absolute maximum" limits???
 

Thread Starter

q12x

Joined Sep 25, 2015
1,448
Not quite @MisterBill2. Im not after "exact power". Im after some sense of what the power can be. Either is in the ballpark of 3W? either in the 5W? Who knows? I certainly did a lot of guessing over the years. But why not actually knowing the stuff that I collected with so much sweat and risk.Heh. Im not a c... who one is saying in front and other does in the back... I am what I say I am and people are knowing me already. Is easy to assume, but some assumptions must be kept to yourself, as a sign of respect. I also have a electronic shop in my town that is very old and very small, and there lives a small capitalistic billionaire from how many resistors he sold, hahahaha. Oh boy. The imagination people have these days. I collected all these components quite randomly and by luck, not at all by an agenda. And they piled up over the years. Do you want some? For free, dont worry. I can send you some. But not many. Haha.
I get your point very well, to use some sort of good sense on the size and shape of them and trust my guessing, being very close to what they might be. Like I said it before, I want to know it for sure. If possible of course. If not, I will get back on the guessing game like I did it so many years until now.
What Im upset is that you didnt come with a viable circuit so far. I had to guess it what it may be by myself. I believe I come with a workable solution after all. I didnt did any actual experiment yet. But I will and I will update you. Staaaay tuuuned ! Haha
 
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MisterBill2

Joined Jan 23, 2018
13,176
I never guess with resistor power ratings. After the resistance value is decided then it is not hard to know the current, because the current was used to determine the resistance required. so the anticipated power is known. From that number I can selectth next higher resistor wattage. And if it seems that there may be transients, or mechanical challenges, one step higher yet because more wattage comes with larger size, and bigger is often stronger.
 

dl324

Joined Mar 30, 2015
15,120
If the resistors get too hot to keep your finger on them (just a bit too hot), I would estimate you are close to 1/2-3/4 the maximum rating.
If they are hot, but you can keep a finger on them for a couple of seconds, you are probably 1/4 to 1/3 maximum power rating.
Just my "estimate" for resistors in the 2W to 5W categories. I'd call it rule of thumb, but I use a finger instead ;)
Just a guideline, don't expect accurate results....
This is too subjective to be useful. I knew cooks who could grab a baked potato from an oven set at a moderate temperature (~350F) with their bare hands; after squeezing them to check doneness...

I used a 6 ohm 50W resistor to load test a variable supply. I couldn't bear to touch it when it was dissipating about 18W. It was so hot that I could feel the heat radiating from it.
 
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