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Resistor overheated and melted color code, how do I determine type?

  1. Jan 28, 2012 #1

    I am trying to repair this device and have determined that the issue is with this melted resistor... In order to find the correct replacement i need to determine specifically what resistor this is.
  2. jcsd
  3. Jan 28, 2012 #2


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    Measuring the resistance won't answer the question, because if it got hot enough to burn off the paint its value will have changed (and it might now be either open or short circuit, of course).

    If you can't find the circuit diagram, your best plan is to draw out the circuit, figure out how it works, and find the value that way. Presumably you know what the circuit is supposed to do.

    If you post the circuit here after you've drawn it out (including the other component values) somebody here will probably help if you don't know how to analyse it.

    Also, the resistor has obviously failed, but that might be collateral damage, not the root cause of the failure. In that case, just replacing the resistor won't achieve anything, except to incinerate another resistor.
  4. Jan 28, 2012 #3


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    You can get an idea of the resistor value by measuring what its resistance is now.

    Resistors tend to go higher in resistance when they are cooked like this, so the original value of the resistance was likely to be less than it is now.
    So, if it is 30 ohms now, its previous value may have been less than 30 ohms.

    However, resistors rarely fail catastrophically like this by themselves and (as already mentioned) they usually get destroyed by something else failing.
    Prime candidates are semiconductors (diodes, transistors...) or capacitors.

    So, you could look for shorted capacitors and semiconductors.

    If you haven't already got a multimeter, you can get very adequate ones for less than $10.

    Also, the actual fault may be outside the board itself.
    It seems to have input and output wires so the output device may have a fault that is causing this resistor to get overheated.
  5. Jan 29, 2012 #4
    Thank you both for your replies. First, I will be attempting to create a circuit diagram. I honestly know almost nothing about this side of electronics, I normally stick with the programming. I will be googling my way into this project some more, but any links to good beginning resources to help me reverse engineer this board would be greatly appreciated.

    Second, I will be getting my hands on a nice Multimeter tomorrow so I will use that to measure the resistance. As for outside damage, there is some minor burning on what I think is the capacitor (that big red thing?)... first off, is that in fact a capacitor? If so, it has numbers and such on it that I imagine I could google into a replacement part.

    Another theory for outside interference that could have damaged it: This device used a plug that looks like this
    This connection was really loose, and any bumping of this device would cause the device to flicker and make an ominous quiet sparking sound. I got so fed up with this that I ended up cutting the connector head off of the cable, feeding it through the hole in the box where the female connector was and soldering it straight to the power wires. This fixed the problem quite nicely (don't worry I taped off the exposed solders to prevent shorts). However I worry that the amount of time that I let the device be derpy might have caused enough power spikes to fry stuff..

    Does that sound like circumstances that could cause this kind of damage?
    Last edited: Jan 29, 2012
  6. Jan 29, 2012 #5


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    That plug is usually used for mains voltage appliances, which means you probably shouldn't be doing this.
    Do not touch any part of the circuit board if power is applied.

    It is possible for these plugs to make a bad connection, but that would not normally cause a resistor to burn out.

    The capacitor next to the burnt resistor could be faulty, or it may just have got hot from the resistor next to it.

    What is the function of the circuit board with the burnt out resistor on it?

    The large black thing next to the resistor looks like a power transistor or possibly a triac.
    If you test the resistance between the leads of this, then none of them should be less than, say 300 ohms.
    If you find any with resistance less than this, the transistor may be faulty.
  7. Jan 29, 2012 #6
    Although I am not familiar with this aspect of electrical engineering, I am familiar with electronics. I have built several pc's and have done minor soldering repairs and such. I always disconnect the power before running and I use an insulated screwdriver to bridge the capacitor to prevent any danger to me.

    How do I check for a capacitor being faulty?

    This board takes a 110 volt input (red and black wires in first pic) and uses it to power a heating element (wires disconnected in first pic). This element is attached to a temperature sensor(white wires). the device can be set to a certain temperature, and it displays its actual temperature after being set.

    Ok thanks, I will check this out tomorrow and post results. Thanks for the help!

    Also just to verify, should I desolder the black thing to check it, or can I just connect my multimeter to the ends on the circuit board?
    Last edited: Jan 29, 2012
  8. Jan 29, 2012 #7
    I am going to stick my neck out here and suggest another analysis.

    Firstly the resistor, although obvously overheated may well not be destroyed, because it looks like a wire wound power type and the wire appears intact. If so, this will not have changed its resistance on cooling. Obviously the protective ceramic coating has burned away so the components needs replacing.
    This resistor is likely to have quite a low value, difficult to check accurately with a cheap multimeter.

    I can see (not very well) three caps in the picture. they are unlikely to have suffered, in particular the big orange one looks very healthy.
    You can check caps with a multimeter by connecting the ohmeter. As you connect the reading should 'blip' and then gradually rise towards infinity on a good cap. Analog meters with a needle are best for this. You cannot readily measure value this way.

    The power controller?

    I can't see any rectifiers so this is likely a thyristor/triac. Does it have any markings/ letters. Post these. It may well have failed. Multimeter check on these are unreliable.

    And the probable cause of failure?

    Well I'd say that the load element has overloaded the controller circuit for some reason. Perhaps it has gone (partial) short circuit.

    go well
    Last edited: Jan 29, 2012
  9. Jan 29, 2012 #8
    Thats the interesting thing, this device actually works for a bit. When I plug it in, it lets me set the temperature, but it cuts out when it exceeds roughly 100 degrees. As long as I let it cool off, it will work again. My hope was that I can get a mostly accurate measurement when it is cool. Also I the multimeter I am borrowing is not cheap so hopefully it can get an accurate read.

    Are you referring to the black thing next to the burned resistor?

    Is there some way I could test this? If I hook up a multimeter to both leads of the heating element will it be able to tell me if it has a partial short?

    I took some more pictures, focusing on numbers and such.. hope this helps!
  10. Jan 29, 2012 #9

    Looking at the photos, in paricular 6 of 12 I can see the whole circuit and it is more complicated than the first pic showed.

    I can now see a rectifier so that this may well be a DC power circuit, and 'that black thing' may well be a voltage regulator. So the type number is vital, none of your pics show this.

    The flat black thing on the left of pic6 is an integrated circuit. This may be a standard type but many manufacturers of power controllers make their own custom ones which are not then replaceable.

    I can't make out the numbers on either of these.

    Also the rectifier is behing the spade terminal between the large tub capacitor and 'that black thing'

    They also need type numbers?

    Incidentally to trace the circuit, hold the board up to a strong light. You should be able to see the tracks through it and see the connections.
  11. Jan 29, 2012 #10


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    The integrated circuit is a EM78P260N which is apparently an 8 bit micro.

    Not sure I like where this is headed, but maybe you should take it back to whoever made it and get them to fix it?

    If you can set the temperature, then the display must be working, so it is likely that the micro is OK or at least partly OK.

    Incidentally, this would be a good time to go and get your own multimeter. It won't take a good meter to test this circuit, but if you magage to blow up someone else's expensive one, you will have to replace it at some cost and embarassment.

    What about one of these:
    About $12 delivered.
  12. Jan 29, 2012 #11
    I am 90% sure of the following but I write it as if I am 100% sure. You need to confirm with a meter.

    The large capacitor is a series ballast. It has most likely gone bad. It works by acting like a large lossless voltage drop at 60 Hz. The black cube is a three-terminal voltage regulator which still works and is powering the micro. It has a built-in thermal shutdown which is why it quits.

    Im not sure where the power resistor sits but if post the schematic it might confirm the above speculation.
  13. Jan 29, 2012 #12


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    BTW, that is a VERY common plug type. If still possible, you might consider undoing your soldering job and simply picking up a cable at an electronics surplus shop or off one of uncountable dead appliances.
  14. Jan 29, 2012 #13


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    The ballast idea seems good, but it is hard to accept that it could go faulty without taking the regulator and micro with it.

    Anyway, a quick resistance check across the capacitor should give an answer.

    A schematic would be great.

    The last photo shows an F1 on the board, and assuming this is a fuse, I can't see it on the other side of the board. Maybe that is supposed to blow if the capacitor fails, except it may have been bypassed in the rewiring effort.
  15. Jan 29, 2012 #14

    jim hardy

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    can you unsolder that capacitor and ohm it as Vk6 suggested?

    the burnt resistor shows a spiral
    which might be wire element
    or the spiral cut in a metal film
    metal films are often used as a fuse

    might tack in two wires to a candelabra lamp socket and screw in a 10 or 20 watt bulb to replace resistor, but only AFTER you know cap isn't shorted..
    if bulb lights when you hit that magic 100 deg number -
    something is trying to apply power to something else that lets a lot of power through.
    but the measure and control part is working

    cap looks like 4.7uf which would be 564 ohms at 60 cycles, and that'd limit current to about 0.2 amp - not much for a heating element.

    what does this thing control?
    what connects to those metal tabs?

    one step at a time
  16. Jan 29, 2012 #15
    a heating element. as you can see here, this heating element is placed in the glass and metal tube, with the thermal sensor resting immediately behind it, on top of its wires.

    I have desouldered the burnt resistor(Pictures), and in an hour or two I will be able to post the results about what it reads..

    Is it easier to see in the pictures after I have desouldered the resistor? Is that the small red dongle that is behind where the resistor is?

    also I will be unsoldering the cap, but I am going to wait until I get my multimeter before I want to take anything else off the board.

    Just to verify here is an image that I have labeled each part as I understand it

    also as I understand it that way I resouldered my device should have not circumvented anything, I just attached the power straight into the leads... there were not any other wires coming off of the female plug, and this device has been acting funky before I did my resouldering, and was working after it for quite some time as well. It is only just now that it dies at 100 degrees every time.

    Also thanks for the link to the multimeter... next paycheck day I am definitely getting that!
    Last edited: Jan 29, 2012
  17. Jan 29, 2012 #16
    The rectifier is the object to the right of the legend on the board R13 and D4. It has a number beginning 1NXXXX. Filling in the XXXX will identify it. You do not need to desolder it to test it. Just use the ohms range on the multimeter to check the it has high resistance with the leads connected one way across it and low resistance with the leads connected the reverse way round.

    The black object where you have ?rectifier? is a capacitor.

    You have several small 'surface mount components' around the integrated circuit. Do not attempt removal of these. They look like tiny cubes and are labelled RX or CX where X is some number.
  18. Jan 29, 2012 #17
    Regrettably the rectifier is placed in such a way that I can only see the first number (4).. I could feasibly see more if I were to desoulder the pin that connects to the heating element, however I am going to test it first, if it is working I would rather minimize any changes to the board.

    There are two more ?rectifiers? (d1 and d2) on the board behind the 3 terminal voltage regulator, similarly difficult to read. Should I test these as well.

    here is an image listing each marking on the board, and here is an updated pic of the items with labels

    I found that there is in fact no fuse on this board, nor was there. The board came with the power wire soldered straight to where the fuse should go. Should I/could I put a fuse in to prevent this in the future, and what type?
  19. Jan 29, 2012 #18

    jim hardy

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    in the burnt resistor picture, the round black capacitor (1000 uf 25 volts) has split his plastic skin.

    Is he bulged?
    see this link

    if you can measure from one end to say halfway up that burnt resistor you might guess-timate what it was. i'm guessing it's about ten ohms and intended for a fuse.
  20. Jan 29, 2012 #19


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    You should test the rectifiers. You have to use the "diode" function on a multimeter to do this though.

    On the red one I pictured above, it is just above the "ON" marking of the on-off switch.

    This gives a higher test voltage than the "OHM" ranges.

    The big capacitor can initially be measured in-circuit. If it measures very low resistance (like less than 10 ohms) then it has failed.
    These capacitors do not usually get slightly dented. They fail dramatically and stay failed.

    It appears to be adequately rated, so it probably hasn't failed. However you have to check.

    You may be able to measure some higher resistance across the capacitor and this may be due to other components in the circuit.

    Can you try to read all the writing on the "3 wire regulator". I don't think we know for sure that is what it is.

    You could consider a fuse later if this ever gets fixed.
  21. Jan 29, 2012 #20
    the writing is as such
    Code (Text):
    How do I determine the correct one?
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