When run at its rated power?
I'm referring to 1/4 - 10 watt resistors in TVs and PC power supplies, probably in 30-50 Celcius surrounding air.
I thought that resistors were supposed to be chosen for twice the actual power, but I've seen many 0.5W to 1.0W resistors run at almost exactly their rated power continuously.
"larry moe 'n curly" wrote in news:e18113f3- snipped-for-privacy@h40g2000prf.googlegroups.com:
How long is a piece of string? I think you need to rethink what you're asking. More to endurance than duration.
-- That\'s because it costs more to use a larger resistor than is necessary. Let\'s say that you can get 1/2 watt resistors for a penny in quantities of a million, and one watters for 1.5 cents. If the 1/2 watters will do the job like they\'re supposed to and you buy a million one watters instead, you\'ve just thrown away $5000.00.
Designing for longevity is a thing of the past. Making a buck is. Most problems in electronics is caused by under rated components (not enough power or voltage headroom). Caps are a big culprit.
The life at rated wattage will depend on many factors. The legs of the resistor will conduct the heat away. How much copper they hook to will control how well this works. The brand of the resistor also matters. One from Uncle Chin's Resistor Company will last much longer than those from the Shaw Dee Resistor Factory because Uncle Chin uses better ceramic.
If you use that urethane conformal coating that the US navy seems to like so much, it will turn dark and look very ugly long before the resistor fails. The ceramic materials will do their best when clean.
Depends on what it's made of and how well it's cooled. We use 0603 surface-mount resistors at half a watt, because we heat sink them well. Half-watt carbon resistors will die at half a watt in a confined space.
Enameled wirewounds are very tough, up until the enamel melts.
Some resistors will die from temperture cycling stress.
But in general it's good to derate 0.5 maybe.
John
What additional information should I specify, other than rated power, actual power, and ambient temperature? I'm not dropping high voltages.
"larry moe 'n curly" wrote in news:ef1a610c- snipped-for-privacy@s21g2000prm.googlegroups.com:
Materials. What I was getting at is that resistors aren't always nicely behaved metal/ceramic thingers, and carbonisation of an epoxy or urethane or other coating might affect a high-value resistance enough to consider it failed even though the resistive element is fine. Basically it can get complicated, so you need to think of the specific conditions to eliminate guesswork.
Another way of looking at it is, if it's a ceramic/metal type and isn't glowing with heat, or melting solder if mounted that way, then it's likely to outlast everything else in the circuit and is therefore the least of concerns...
Resistors can last for well over 100 years...
..only if you are stupid enough to not increase the sale price of the widget by 50 cents per one watt resistor used.
-- Then your widget will cost more than an identical widget designed to be lean, and your competition will eat your lunch.
And steal your dessert! ;-)
-- The movie \'Deliverance\' isn\'t a documentary!
oh, i forgot to mention that the old wet electrolytics were known to last up to 50 years...
Robert Baer wrote in news:dsGdnej0YOnJEkzXnZ2dnUVZ snipped-for-privacy@posted.localnet:
Common knowlege to me. :) It's kind of what amazes me about electrolytics. So much variation in lifetime, I'm surprised that there are so many short-lived ones, I'd have thought that good answers were so long out of patent that it would be a no-brainer to choose ways to make them last. Maybe it had to do with them not pushing for high capacity though, I mean, for a given size, the capacity was a lot lower than now.
At this point, either the thread will morph into days and perhaps hundreds of posts on the subject, or everyone will decide it's been done to death before and there won't be any.
There is another dimension to consider: duty cycle. I've been told (but don't have first hand experience) that old fashioned carbon composition resistors are much better at pulse loads that newer carbon film resistors.
-- These are my opinions, not necessarily my employer\'s. I hate spam.
snipped-for-privacy@ip-64-139-1-69.sjc.megapath.net (Hal Murray) wrote in news:DO2dnZ1gIacDNU_XnZ2dnUVZ snipped-for-privacy@megapath.net:
They're also a lot better at high voltage. I'm just mentioning this because when I used some for a HeNe supply current limit resistor, they really impressed me. Most other things I tried eventually burned. Basically, a solid mass is better than a film for HV, it seems.
Except for the horrible voltage coefficient. Carbon comps' resistances decrease by about a quarter at their rated voltage. That's okay for a HeNe, since the tube's load line is pretty steep.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Phil Hobbs wrote in news:0badnQb6Bo8xok7XnZ2dnUVZ snipped-for-privacy@supernews.com:
Well that's cool cos that's about all I ever use them for. :) But they were the perfect answer in this singular case.
Phil Hobbs wrote in news:0badnQb6Bo8xok7XnZ2dnUVZ snipped-for-privacy@supernews.com:
Veering swiftly offtopic... this interests me because people often rave about the sound of valve amps. I don't mean audiophool ravings, I mean those who like the effect especially in overdriven amps for bass guitar and other instruments where the sound is appropriate, and very good. It's normally attributed to the harmonic distortion and saturation effects in the valves, but now I'm wondering how much of it might be due to that voltage coefficient in carbon resistors. Maybe designers deliberately tried to reduce the effect by careful biasing, or maybe there is a strong effect that is overlooked, I really have no idea...
Comments welcome, so long as they come from either experience or interest in this subject.
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