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  1. #1

    Default How to test resistors wattage rating

    Already did my homework and tried finding threads on this subject. No Luck.

    In the middle of a recap, and about to start another.
    Have some flameproof resistors that are solid
    colored and only have ohm ratings. They check out
    for ohms, but can not determine what their stock
    wattage ratings are. Schematics do not state
    their wattage rating either.

    What is a quick, painless, easy way to determine
    their wattage rating with only a DMM or other?

    Thanks

  2. #2

    Default Re: How to test resistors wattage rating

    I had a prof years ago who loved to find the wattage rating by blowing them up

  3. #3
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    Default Re: How to test resistors wattage rating

    Determine, based on the schematic or with your DMM, what voltage is being presented to the resistor. The measured value and ohms law will take care of the rest. ;-)

    Voltage(E) = Current(I) x Resistance(R)

    Power(P) = Current(I) x Voltage(E) in a purely resistive circuit. vee-ola. Add 25% to the wattage rating or round up for good measure if you aim to replace it. Too much capacity never hurt.

  4. #4
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    Default Re: How to test resistors wattage rating

    Mark, I know that you know what you're doing, but please could you explain to the doofus over here in the corner (that would be me) how knowing the resistance and voltage have to do with finding the power, when the current is still a variable?

    I'm sure that it's simple but I'm sure missing what the answer might be.

    Thanks,

    Mark

    Edit: ooops you already answered. Never mind....
    You go your way, I'll go mine. I don't care if we get there on time.

    ~Pink Floyd

  5. #5
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by Mark65 View Post
    Mark, I know that you know what you're doing, but please could you explain to the doofus over here in the corner (that would be me) how knowing the resistance and voltage have to do with finding the power, when the current is still a variable?

    I'm sure that it's simple but I'm sure missing what the answer might be.
    .
    MattK had it right: I had a prof years ago who loved to find the wattage rating by blowing them up

  6. #6
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by billfitzmaurice View Post
    MattK had it right: I had a prof years ago who loved to find the wattage rating by blowing them up
    There might be a temptation here for someone to think people are leg pullin'. They're not. There is no test tool to determine this non destructively.

    Joe.
    New to speaker design? Click here.

  7. #7
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by joekraska View Post
    There might be a temptation here for someone to think people are leg pullin'. They're not. There is no test tool to determine this non destructively.

    Joe.
    If the OP hadn't said that they were in circuit (schematic comment), I'd have to agree. I may still agree, purely because I like to blow stuff up. By testing though, he'll at least know what his minimum rating is. To find maximum... well, this isn't a flaming thread yet. Hehe.. you can make one smell pretty bad just before failure.. without killing it... ok I'm digressing now.

  8. #8
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by Mark65 View Post
    Mark, I know that you know what you're doing, but please could you explain to the doofus over here in the corner (that would be me) how knowing the resistance and voltage have to do with finding the power, when the current is still a variable?

    I'm sure that it's simple but I'm sure missing what the answer might be.

    Thanks,

    Mark

    Edit: ooops you already answered. Never mind....
    You can derive another set of equations for ohm's law, Mark.

    Since; V = I*R, and P = VI, substitute I*R for V...
    P = I*I*R or I^2*R, and the same kind of substitution also yields...
    P = V^2/R

    You can then find values with 2 variables instead of 3. It's also a way to choose fuse values.
    Later,
    Wolf
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by billfitzmaurice View Post
    MattK had it right: I had a prof years ago who loved to find the wattage rating by blowing them up
    That's like how they rate bridges. Build the bridge and drive heavier and heavier trucks across. When the bridge fails, they know its limit and build the permanent one.

    Seriously, though, if this is a tweeter network, get a Mills 12W and call it a day. The tweeter will go first.
    nothing can stop me now

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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by Wolf View Post
    You can derive another set of equations for ohm's law...
    Sure.. I didn't wanna break out the power wheel on him though especially if he doesn't know ohms law yet; I'm just curious what the OP's into that he needs to know. There's a stray cat outside that I'm willing to sacrifice for curiosity's sake; keeps getting in my garbage.

  11. #11
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    Default Re: How to test resistors wattage rating

    Here's an idea. Apply a known voltage, and then shoot this (or equivalent) at it

    http://www.mcmelectronics.com/produc...T-205-/DIT-205

    noting when the temperature starts to rise precipitously. I think based on the resistor type you can set an upper temperature limit. Although I don't know for a fact, I'd call it a day when the resistor temperature rises above 50C, or perhaps 100C.
    Action speaks louder than words but not nearly as often. -- Mark Twain

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    Default Re: How to test resistors wattage rating

    If Wolf's saying what I think he's saying... is the circuit still working? If so, determine the voltage at the resistor. That will determine the current that it carries, which isn't the same as its maximum rating, but this should get you into your ball park. Mind you, the last time I had an electronics class was in 9th grade, but so it goes: I don't think anyone here is attempting to tell you how to determine a resistors rated maximum current...

    Joe.
    New to speaker design? Click here.

  13. #13

    Default Re: How to test resistors wattage rating

    The quickest and most painless way would be to go by the size of the resistor. Standard sizes. When in doubt, simply use a bigger size.

    Note that the nominal power rating of a resistor is not the same as the power that it can safely dissipate in practical use. Air circulation and proximity to a circuit board, ambient temperature, and other factors can reduce acceptable dissipation significantly. Rated power dissipation may be given for an ambient temperature of 25 C in free air. Inside an equipment case at 60 C, rated dissipation will be significantly less.

    Quote Originally Posted by HuskerNation View Post
    Already did my homework and tried finding threads on this subject. No Luck.

    In the middle of a recap, and about to start another.
    Have some flameproof resistors that are solid
    colored and only have ohm ratings. They check out
    for ohms, but can not determine what their stock
    wattage ratings are. Schematics do not state
    their wattage rating either.

    What is a quick, painless, easy way to determine
    their wattage rating with only a DMM or other?

    Thanks
    Last edited by ; 01-05-2010 at 05:00 PM.

  14. #14
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    Default Re: How to test resistors wattage rating

    When the resistor reaches its thermal limits, won't resistance climb sharply? I've never tried it with a wirewound or sandcast, but is a resistor run to its thermal limits forever damaged?
    nothing can stop me now

  15. #15

    Default Re: How to test resistors wattage rating

    Quote Originally Posted by Dirk View Post
    When the resistor reaches its thermal limits, won't resistance climb sharply? I've never tried it with a wirewound or sandcast, but is a resistor run to its thermal limits forever damaged?
    Yes, but at risk of damaging the resistor. The temps required for a large increase in resistance will very likely damage it. If it doesn't fail open circuit, it will probably be OK once cool provided the resistor case hasn't been deformed which could allow moisture to get in.

    Let's just say if it lets out smoke, the case has been damaged.

    I usually look at the size. I've got enough hands on experience with circuits that I can ID most by size alone....and the bigger ones usually state their wattage.

    Tell us how big they are and we might be able to tell you the approximate wattage.

  16. #16
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by hardwareguy View Post
    Yes, but at risk of damaging the resistor. The temps required for a large increase in resistance will very likely damage it. If it doesn't fail open circuit, it will probably be OK once cool provided the resistor case hasn't been deformed which could allow moisture to get in.

    Let's just say if it lets out smoke, the case has been damaged.

    I usually look at the size. I've got enough hands on experience with circuits that I can ID most by size alone....and the bigger ones usually state their wattage.

    Tell us how big they are and we might be able to tell you the approximate wattage.
    Yeah, after you've worked with them enough you can tell by the package size...but this doesn't help you, HuskerNation. I would suggest taking it out of the circuit and going to your local Radio Shack. Compare it to resistors there, and then go one package size higher, just to be safe.

  17. #17
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    Default Re: How to test resistors wattage rating

    BTW- Don't trust the guy at Radio Shack to know anything. They'll talk like they do, but they don't. At least that's the case in all the stores down here.

  18. #18
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    Default Re: How to test resistors wattage rating

    They still sell resistors at Radio Shack??????

    Take a picture of the resistor(s) in question with something next to it for size reference. I bet I can tell you fairly accurately what the wattage rating is from my 30+ years of working with electronics.
    Craig

    "I used to drive WAY too fast to worry about cholesterol. Now I worry about my blood pressure."

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  19. #19
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    Default Re: How to test resistors wattage rating

    Quote Originally Posted by joekraska View Post
    There might be a temptation here for someone to think people are leg pullin'. They're not. There is no test tool to determine this non destructively.

    Joe.
    Joe, of course that's not true.

    You can determine the power rating of a resistor by gradually increasing the current until the device under test is too hot to touch, but not yet smoking.

  20. #20
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    Default Re: How to test resistors wattage rating

    This is an approximate method, Pete, and not an exact one. Although I can see how it will get you there. And it isn't a tool, although I suppose one could put a digital thermometer on the resistor during this test and decide on a specific temperature point based on the resistor's materials. Perhaps this method of approximation was what the OP was looking for, however I had the sense that OP was looking for a channel on their multimeter that would determine the answer definitively: no such channel.

    Joe.
    New to speaker design? Click here.

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