finding harvested resistors power rating - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: November 26, 2019, 09:05:21 am
content: I harvested a bunch of resistors and I am now facing a question... what are the power rating for these resistors?Is there a test circuit capable of measuring how much power a resistor can handle? Therefor "measuring" it is power rating?

author: wraper
author_rank: Unknown
timestamp: November 26, 2019, 09:27:11 am
content: Not possible. Same size resistors will heat to same temperature with same power applied. And power rating depends on their maximum temperature rating which you don't know.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2019, 11:34:06 am
content: Sort of possible. What do they look like, for starters?Tim

author: rsjsouza
author_rank: Super Contributor
timestamp: November 26, 2019, 12:01:05 pm
content: Apart from through hole carbon/metal film resistors that usually have well determined sizes between1/16,1/8,1/4,1/2, 1 and 2 Watts, it becomes quite difficult if you are talking about SMD. Similar scenario trying to find the voltage rating of SMD ceramic capacitors, for example.A good reference that can help you is at:http://www.resistorguide.com/resistor-sizes-and-packages/

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2019, 06:29:29 pm
content: If they are common leaded film resistors, then they power derate to zero at about a 150C surface temperature. So applying enough power to raise them to that temperature will give a good estimate of their power capability.The same can be done for other resistor types if you can find a representative power derating curve for them.

author: gkmaia
author_rank: Frequent Contributor
timestamp: November 26, 2019, 06:55:49 pm
content: Here are some of them.This size table seems somewhat accurate when compared to other resistors I bought previously. BUt not 100%...

author: AndyC_772
author_rank: Super Contributor
timestamp: November 26, 2019, 07:12:47 pm
content: You'll really need to have an idea of what constitutes a "rating", to you, in your application.The rating of a component is one of those things that's often misunderstood or taken out of context. If, say, a resistor is rated 10W, and you apply 10.1W, is it suddenly going to burst into flames? Of course not.Will its resistance go out of spec? Maybe, but only if your test conditions are identical to those used for picking a value and calling it the "rating".Maybe you don't actually care if its resistance varies by 1.2% instead of 1%, and it'll continue to work perfectly well for the rest of your natural life under those conditions. Maybe in your application there's a heat sink or fan which means it'll run cooler, and can therefore handle more power before something bad happens - and it is, of course, completely up to you to determine what that "something" is.Or maybe the manufacturer's rating represents the power level at which it doesn'tquitecatch fire under lab conditions, in which case you might want to choose a different figure anyway.If you want to truly understand what a resistor "can handle", you first need to define "handle", and what constitutes a failure to "handle" that amount of power.You could choose an arbitrary surface temperature under some equally arbitrary test conditions, or perhaps you could pick a change in resistance from its nominal value at 20C.If you're concerned about long term reliability, you might instead pick a criterion which relates to thermal stress cracks forming over repeated heating and cooling cycles, which would be much harder to test but no less valid, and probably something you really would have to obtain from the manufacturer as they'd be much better equipped to carry out that kind of testing than you would.Whatever you choose, be sure to pick a way of determining the rating that actually applies to how you're going to use them, and what failure mode concerns you.And then subtract 30%, just to be on the safe side

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2019, 09:46:41 pm
content: Pictured are 1/4 and 1/2W axial resistors.Tim

author: gcewing
author_rank: Regular Contributor
timestamp: November 27, 2019, 02:06:37 am
content: My rule of thumb for resistors is that if it's getting too hot to put my thumb on, I'm passing too much current through it.This is probably a long way on the conservative side, but I don't like to run things near their thermal limits.

author: james_s
author_rank: Super Contributor
timestamp: November 27, 2019, 02:43:03 am
content: None of those are power resistors so in most cases the rating doesn't really matter. Just assume 1/4W in most cases, the more chunky ones are usually 1/2W. As someone else said, if it's too hot to touch then it's too hot, unless it's a ceramic or metal power resistor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2019, 05:51:17 am
content: Well, resistors of that style can operate at 150°C or so quite comfortably. Noteworthy that the finger method needn't be very accurate, not just because of the meat sensors and computer behind it (which, if adequately trained^Hcalibrated on various materials and temperatures, can be pretty accurate), but because the meaty finger has a lot more thermal mass than a tiny resistor, so you're more likely to feel a pulse of heat, and not its actual quiescent temperature.One "feature" of those kinds, the enamel is usually some kind of epoxy compound that smells funny as it heats up. This may not apply to old (salvaged) resistors, but it's true for new parts at least. When I'm breadboarding with them and notice the smell, that's a good hint I've wired something wrong, or used too low a rating.Tim

author: rsjsouza
author_rank: Super Contributor
timestamp: November 27, 2019, 03:35:20 pm
content: Quote from: T3sl4co1l on November 27, 2019, 05:51:17 amYes, anything near 60°C is already quite unbearable to touch. That is why I tend to have at least one cheap DMM with temperature around both my work and my home lab.Quote from: T3sl4co1l on November 27, 2019, 05:51:17 amThat or when you realize the plastic hole of the breadboard itself became a bit wider...(this can also happen with power transistors)

author: james_s
author_rank: Super Contributor
timestamp: November 27, 2019, 06:13:21 pm
content: 60C is about that point where I can touch it but I don't want to hold my finger on it for any length of time. I realize that resistors can safely operated at higher temperatures than that but unless they're power resistors I don't really like running them that hot. Even if the resistor survives, a lot of times the PCB will darken and the solder joints are prone to failing.

author: wraper
author_rank: Unknown
timestamp: November 29, 2019, 09:38:26 am
content: Quote from: rsjsouza on November 27, 2019, 03:35:20 pmAnd it's basically useless for measuring resistor temperature unless you will attach thermocouple with thermally conductive compound. And even then nearly useless for small resistors because thermocouple itself will sink a lot of heat. I feel like facepalming every time when someone touches heatsink or some part with junction on thermocouple end and thinks he is measuring it's temperature.

author: rsjsouza
author_rank: Super Contributor
timestamp: November 29, 2019, 10:10:51 am
content: Quote from: wraper on November 29, 2019, 09:38:26 amImprecise? Yes. Useless? Hardly.It surely beats the finger test.

author: wraper
author_rank: Unknown
timestamp: November 29, 2019, 12:32:15 pm
content: Quote from: rsjsouza on November 29, 2019, 10:10:51 amIf you measure something like half of actual temperature, I call it useless.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 29, 2019, 02:30:57 pm
content: Yes, for measuring anything with such a small thermal mass, touching it with just about anything will skew the results so much as to make them nearly worthless.You really need a non-contact thermometer, they're cheap enough these days (<£50) if you don't care much about brand names.My personal preference is to not run anything at more than 75% of its rated values, but personal preference isn't scientific method.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 29, 2019, 02:48:25 pm
content: As someone else commented;Assume the largest one is 1/2w, the mid sized ones 1/4w, and the tiniest one 1/8w, and call it a day.In my much younger days I also harvested a lot of parts. Only used it for hobby stuff, though.

author: wraper
author_rank: Unknown
timestamp: November 29, 2019, 03:29:36 pm
content: Quote from: ThickPhilM on November 29, 2019, 02:30:57 pmWon't work on resistors unless it's something with size of at least as 5W ceramic resistor. You need thermal camera for smaller parts. Preferable with adjustable focus for working at close range. IR thermometer gun won't do the job. They capture too wide spot, and laser pointer is shifted from center of actual measurement spot quite a lot.

author: AVGresponding
author_rank: Super Contributor
timestamp: November 29, 2019, 05:23:21 pm
content: They do work, you just have to be mindful of the limitations, and adjust your measuring technique accordingly.Full on thermal camera time is for the birds, and those with cash to burn. Even used (and useful) gear is well beyond the means of most hobbyists.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 29, 2019, 07:08:45 pm
content: Point-and-shoot thermometers have a fairly large spot size, and a focal distance. The most common usage error I see, is not understanding that the sensor line-of-sight isparallel tothe laser sight -- offset by a fixed distance. You have to aim above the target to measure it. This is easy to demonstrate, but many people don't think to test their instruments but take them at face value instead, and will never discover this.Tim

author: rsjsouza
author_rank: Super Contributor
timestamp: November 29, 2019, 07:49:55 pm
content: Quote from: wraper on November 29, 2019, 12:32:15 pmQuote from: rsjsouza on November 29, 2019, 10:10:51 amTake a chill pill, dude. The OP's resistors have significant mass when compared to a sensor. SMD resistors, sure.

author: wraper
author_rank: Unknown
timestamp: November 29, 2019, 10:13:55 pm
content: Quote from: ThickPhilM on November 29, 2019, 05:23:21 pmYou can try as hard as you can but you will get much lower reading than actual resistor temperature (given that resistor is hotter than background). You will mostly measure background, not actual resistor.

author: magic
author_rank: Super Contributor
timestamp: November 29, 2019, 11:30:26 pm
content: Measuring small things with cheap IR guns is bonkers, the measurement area is easily 1cm across or more even up close.I haven't verified but I suspect that those things don't even focus at all and the sensor is placed closer than one focal length behind the lens for an infinitely diverging "field of view". Anyone knows?