Announcement

Collapse
No announcement yet.

How to test resistors wattage rating

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • 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
    Re: How to test resistors wattage rating

    I had a prof years ago who loved to find the wattage rating by blowing them up
    It is estimated that one percent of the general population are psychopaths - New Criminologist: Understanding Psychopaths

    Comment


    • #3
      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.
      ~Mark

      Stuff I've builded

      Comment


      • #4
        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

        Comment


        • #5
          Re: How to test resistors wattage rating

          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
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

          Comment


          • #6
            Re: How to test resistors wattage rating

            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.

            Comment


            • #7
              Re: How to test resistors wattage rating

              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.
              ~Mark

              Stuff I've builded

              Comment


              • #8
                Re: How to test resistors wattage rating

                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
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                *InDIYana event website*

                Photobucket pages:
                http://photobucket.com/Wolf-Speakers_and_more

                My blog/writeups/thoughts here at PE:
                http://techtalk.parts-express.com/blog.php?u=4102

                Comment


                • #9
                  Re: How to test resistors wattage rating

                  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

                  Comment


                  • #10
                    Re: How to test resistors wattage rating

                    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.
                    ~Mark

                    Stuff I've builded

                    Comment


                    • #11
                      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

                      Comment


                      • #12
                        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.

                        Comment


                        • #13
                          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.

                          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, 05:00 PM.

                          Comment


                          • #14
                            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

                            Comment


                            • #15
                              Re: How to test resistors wattage rating

                              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.

                              Comment

                              Working...
                              X