12 Replies Latest reply on Dec 5, 2013 8:54 PM by kas.lewis

    How do I tell a resistor's wattage?

    hafcanadian

      Forgive my ignorance, but I ultimately found my way here after watching the Ben Heck Show and learning about Element 14. I have little experience with electronics but hope to learn by immersion in discussions, tutorials, basic instructional books, and videos online. As a 65 year-old homeowner, I've been frustrated many times over the years by having to replace entire circuit boards or appliances/devices at steep costs, when I likely could have replaced one tiny and inexpensive faulty component. This year, finally, I've become more confident and capable in my fine-circuit soldering skills, and have resurrected 3 failed devices this Fall, a wireless laser mouse, a TV converter box, and a $600 convection microwave that's aggravated us for two years.

       

      But I've a lot to learn and understand about integrated circuits. Given their increased control over our lives, in the last half of mine at least, such understanding will be crutial to daily functionality, as well as financially, to an enjoyable remaining years. In pursuit of such an understanding, I have a question related to the mentioned microwave repair.

       

      The 6 micro-cracked power relay pin solders, that I overlooked during a dozen previous tear-downs, have been finally noticed and repaired, and the unit operates great once more. But I noticed on the board a resistor that is somewhat discolored. In my clumsy attempt to closely examine it, I very slightly chipped its ceramic coat with needle nosed pliers. It is still functional, but I'd like to replace it anyway. The color banding is red black black gold on a white body, which I've interpreted as a 20 Ohm resistor at 5% tolerance. But when I look into obtaining a replacement, the websites classify by wattage also. My question is how do I determine the watts involved here?

       

      Thanks,

      Joel

        • Re: How do I tell a resistor's wattage?
          dougw

          Hi Joel,

          Your color decoding is correct.

          Only large resistors are marked with their rated power, usually 5 Watts and up.

          You can get a good idea of power rating by the physical size of the resistor since power in a resistor always ends up as heat and must be dissipated. A larger surface area allows more heat to be dissipated. If you look at typical resistors (datasheets) online where the power rating and dimensions are specified, you should be able to match your resistor to one of similar physical size. It would be prudent to substitute one power size larger than the existing resistor, since it is already getting hot enough to become discolored. It is also possible there is another circuit issue that is causing the resistor to overheat. And check that there is sufficient air flow around the resistor - if there is too much dust or lint insulating the resistor or blocking air flow, or clogging the fan (if there is a fan) it can lead to overheating.

          You could try posting a picture of the resistor - just put something in the picture to provide a size reference.

          Doug

            • Re: How do I tell a resistor's wattage?
              hafcanadian

              Since I've reassembled and reinstalled the microwave in its kitchen cabinet over the oven, I wasn't keen on undoing everything for the umpteenth time to get a photo and measurements.  But I did recall taking pix during one of the tear downs, and located them in my album from last March.  The board is about 90mm wide (left to right in the photo, although a short piece of the left side of the board is cut off in this view), so I'm guessing the resistor must be about 7 to 10mm long, if that helps.  Note the heat discoloration on the board under the resistor.  From some of the links you guys contributed, I'd assume the thing must be between .25 and .5 watt;  so from what I've read, if it would fit I should shoot for a .5W 20ohm replacement.  The problem with using the formulas, like Ohm's Law or Watts = V*V/R is I don't know what the voltage is at the resistor;  presumably it isn't 120v at that point in the circuit and it's unlikely a (120 X 120)/20 = 720 watt resistor .

               

              When I can get time again I'll take it apart one last time (hopefully), get the physical measurements of the resistor, and see if our local Radio Shack happens to have one in stock, or order from Newark.  I sure appreciate you fellas providing such quick and great guidance!

               

              Joel

               

              The big white one at upper right.  Perhaps there is some familiar component visible that you can derive perspective from, and advise a pretty confident wattage for the resistor:

              image.jpg

                • Re: How do I tell a resistor's wattage?
                  michaelkellett

                  You should play safe on this. Farnell 1277013 is a suitable size and rated at 2W. If you can do so safely you could measure the voltage drop across the resistor and calculate the power (P = V*V/R) but you might not pick the right conditions for maximum power in the resistor.

                  The 20R value is a problem - much easier to get 22R.

                  The 1277013 is expensive in the UK so you should look for something else but I would go for a  2.5 or 3W resistor and bend the leads so as to keep it raised above the board.


                  MK

                    • Re: How do I tell a resistor's wattage?
                      hafcanadian


                      Thanks Michael.  I will look for that one or similar suggested wattage.

                       

                      Apparently my photo got skewed somewhere along the line;  when I put it up last night it was upright and correct on the site.  But I see now that it has rotated left and it stretched out of kilter, which doesn't help a viewer's attempt to get perspective and judge relative size.  My description in the accompanying text therefore doesn't match the image as shown.  Nevertheless, your analysis is appreciated.  Clicking the image opens an unskewed version, though still rotated.  The large (9mm?) white resistor in question, #R11, and board discoloration stands out regardless.

                       

                      Though the size, according to one document, seems in the 0.5W ball park, a 2 to 3W would dissipate heat better if it can be made to fit cleanly.  I will explore further when I can get the unit apart again and take accurate physical measurements and run my voltmeter leads across its wires while running the microwave.

                       

                      Joel

                       

                      addendum:  I couldn't find the resistor on Newark, but did elsewhere.  The Farnell is U.K.  Vishay Dale is U.S. version. But further searching brought me to Mouser, where I zeroed in on units that look very much like the existing one, and I don't need to order 100 of them;  the shipping for one will likely be far more than the device.  The 2W models are about 15mm.  The 1W ones are 9mm and I think the original's size.  By bending the wires I could probably get a 2W in there, and could stand it off the board more for better cooling since it is wide open above that side of the board.  I'll still check at Radio Shack the next time I'm in to see if they've got one that will suffice.

                      MOS1CT52R200J KOA Speer | Mouser

                • Re: How do I tell a resistor's wattage?
                  bad_boy_y

                  http://www.mikroe.com/old/books/keu/01.htm

                   

                  going to Fig. 1.3: Resistor dimensions on this page this will give you the wattage rating versus the size of resistor.

                  • Re: How do I tell a resistor's wattage?
                    hafcanadian

                    Update:

                    With some input from Radio Shack personnel, I got a 2-pack of 1 watt, 10 ohm, 5% resistors for a buck and a half.  No one locally seems to carry 20 ohm in smaller wattages or exactly like the old one.  By soldering the two together and bending the free tails, the wide-open clearance above the R11 spot on the board should let me solder these two in place of the old white one.   If I get things correctly, the two 10 ohm resistors in series should offer the same functionality as a single 20 ohm one.  What the wattage will be, I dunno, but presume it would be either 1 watt or 2?

                    Joel

                      • Re: How do I tell a resistor's wattage?
                        kas.lewis

                        Hey Joel,

                         

                        Resistors in parallel will get you half the resistance in this case (5 ohms). If you are connecting them in series then yes you would get 20 ohm and the watt rating would not change, it will still only be able to handle 1 watt. The only way hey could handle a "higher wattage" is in parallel but then its not really a higher wattage its just that the current has a "wider" path and therefore each resistor is only passing half the current (in this case).

                         

                        Hope that helps a bit

                        Kas

                        • Re: How do I tell a resistor's wattage?
                          dougw

                          Series connection will work fine - it will provide a well ventilated 2 watt 20 ohm resistor.

                          The old resistor in the picture is probably fine - it is the phenolic PCB that is discoloured. Phenolic PCBs have a temperature limit of about 100 degrees C. Keeping the resistors slightly higher off the PCB will minimize PCB temperature. The resistor can stand over 150 degrees C.

                          Doug

                            • Re: How do I tell a resistor's wattage?
                              kas.lewis

                              Doug

                               

                              Can you explain how the series connection of two 1 watt resistors will allow 2 watts to pass with no issue ? I am not following on this one.

                                • Re: How do I tell a resistor's wattage?
                                  dougw

                                  Hi Kas,

                                  Each resistor can dissipate 1 Watt regardless of how it is connected. It is a property of the resistor.

                                  The voltage across each resistor or the current through it dictates how much power the resistor is actually required to dissipate.

                                  In this case if the voltage supplied by the circuit stays the same when a single smaller value resistor is substituted the power required will increase.

                                  power = V2/R

                                  as R gets smaller P gets larger.

                                  If the resistors are put in series their total resistance stays the same as the original resistor and the total power stays the same which splits the power dissipated between the two resistors.

                                  So each resistor would only be required to dissipate half of what the original resistor had to dissipate. However, the 2 resistors could dissipate 2 Watts between them if the circuit required them to - by supplying a higher voltage or current.

                                  Doug