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How to find the (original) value of a burnt resistor? Let's suppose a condition in which initially I don't know the value of its resistance. Now because of its low watt, it burnt. So is there any method to find out the resistance?

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  • \$\begingroup\$ The circuit diagram? Find the datasheet of the used components around it, hope for an application note in one of them that resembles the diagram? Make an educated guess and hope for the best? \$\endgroup\$
    – jippie
    Commented Jun 22, 2013 at 13:27
  • \$\begingroup\$ @jippie am just asking whether it is possible or not?No any circuit no any other datasheet?How an educated guess? \$\endgroup\$
    – Atom
    Commented Jun 22, 2013 at 16:10
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    \$\begingroup\$ Get experience in electronics, check datasheets and application notes, develop a feeling for how a circuit works, reverse engineer the circuit diagram that you have and try to understand how it works. Get a pencil and a piece of paper to do some calculations on. Nobody will be able to give you a cookbook style solution for it. \$\endgroup\$
    – jippie
    Commented Jun 22, 2013 at 17:48
  • \$\begingroup\$ @JYelton: Your edit makes a major change in meaning to the question. The OP originally asked how to find the value of a burnt resistor, which I answered. You have now changed it to aks how to find what the original value of a burnt out resistor is. This level of change is inappropriate without clarification from the OP. He has written only two comments so far, and none of them indicate such a drastic change in meaning. At this point there is so much confusion this mess needs to be closed unless the OP can be more explicit. \$\endgroup\$ Commented Jun 24, 2013 at 12:29
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    \$\begingroup\$ Well, we know that the original didn't put up much of a resistance. :) \$\endgroup\$
    – Kaz
    Commented Nov 13, 2013 at 0:06

6 Answers 6

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A burnt out resistor can only be read one of two ways. The first is the color code on the resistor, if it is through hole, or the number code if it is a smd resistor with a a number code. If that is not available, the only other way is a circuit diagram, or a reference design around the IC it is supporting. Most manufacturers tend to stick with the manufacturer's reference design, rarely deviating unless they have a specific reason to, so if the reference design specifies a specific resistor, you have a very good chance that is it.

Aside from that, no, if the resistor code is unreadable, and the reference design is not applicable, you cannot know the original resistor value once the resistor goes bad. There is no point in measuring a burnt out resistor, as if it actually measured the right ohm-age, it wouldn't be ""burnt out"".

The other option is context. Sometimes, based on the circuit, you can simply assume what the resistor is. If it is a transistor base resistor, it might simple be a saturation value resistor. If it is a led resistor, it's value might allow for 20mA through the led. This is very contextual, and it depends on the IC, so it pretty much falls under the "manufacturer reference circuit" standard.

Engr Kahn's answer below also seems to have a reasonable chance of working. As resistance in series is cumulative, if you can measure half the resistor (like you would the wiper of a pot) and then the other half, aside from the burnt section, you can get an approximate value then adjust upwards to standard values. And depending on the circuit, the exact value is not always necessary. A 1.2k resistor might work well enough in place of a 1k resistor.

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  • \$\begingroup\$ If you can just trace one of its pins to an IC you can find a datasheet for, there is a good chance there will be enough information in the data sheet to find the role of the resistor. \$\endgroup\$ Commented Mar 31, 2016 at 2:07
  • \$\begingroup\$ @john yes... that's what I said in the very first paragraph. \$\endgroup\$
    – Passerby
    Commented Mar 31, 2016 at 3:38
  • \$\begingroup\$ Doh. brain fart. I was trying to state a concrete example of context and somehow fazed on the fact you already mentioned it in the paragraph i just read. sorry bout that. \$\endgroup\$ Commented Mar 31, 2016 at 3:44
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Once a resistor has been abused, you don't know what its value might be without explicitly measuring it. Disconnect one end of the resistor and measure the resistance with a ohmmeter. That's the only way to know.

However, you can't really trust a abused resistor, even if you know its value under one set of conditions at one point in time. It could be much noisier than normal. It's value could change unpredictably with temperature and small mechanical stresses, phase of the moon, or what you had for dinner last night. A burnt resistor is trash.

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  • \$\begingroup\$ Somebody told me to calculate its like this way. \$\endgroup\$
    – Atom
    Commented Jun 24, 2013 at 12:55
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Three things you can try:

  • See if you can read the colour code (probably not)
  • Try a multimeter, but this won't work if the resistor is broken
  • See if you can get the (sub)circuit of the device / some chip around the resistor and find the resistor there

Otherwise, I don't think you can find it out.

Note that you shouldn't use a damaged resistor (or any damaged device or component): it might give unexpected results and the properties may change over time. You should only use these methods to find a good replacement for the damaged transistor.

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  • \$\begingroup\$ Color code won't mean much here. The resistor has been used outside of its absolute maximum values, and nothing is guaranteed \$\endgroup\$ Commented Jun 22, 2013 at 13:38
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    \$\begingroup\$ @ScottSeidman I think he wants to know the original value (for replacement e.g.), otherwise a multimeter is the obvious solution. \$\endgroup\$
    – user17592
    Commented Jun 22, 2013 at 13:38
  • \$\begingroup\$ @camil staps I randomly choose a resistance that's why i said "suppose that" in my question \$\endgroup\$
    – Atom
    Commented Jun 22, 2013 at 13:41
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    \$\begingroup\$ Probably has a single brown or black ring ;o) @ScottSeidman \$\endgroup\$
    – jippie
    Commented Jun 22, 2013 at 13:41
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    \$\begingroup\$ @SHASWAT I'm giving solutions not only for you, but also for anyone coming here from search engines with a similar question. Not all things I mentioned will be possible in all cases indeed. \$\endgroup\$
    – user17592
    Commented Jun 22, 2013 at 13:42
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You can find the Values of burnt resistors by these three handy methods.

How to find The value of Burnt Resistor ( By three handy Methods )

Method 1
1. Scarp the outer coating.
2. Clean the Burnt Section of the resistor
3. Measure resistance from one end of the resistor to the damaged section Again measure the resistance from damaged section to the other end of the resistor.
4. Add these two value f resistances
5. This is the approximate value of Burn resistor
6. Just add a small value of resistance for damaged section .i.e., suppose the value of burnt resistor was 1k Ω, but you got 970 Ω. So just add 30 Ω, and you will have 1k Ω.

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  • \$\begingroup\$ Agree with scraping and measuring whats left over, done this several times. \$\endgroup\$
    – user105282
    Commented Mar 30, 2016 at 22:10
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If you scrape off the coating, you'll find divots that go around the resistor. l strip or divot is equal to 1 ohm, 2 is 10 ohms, 3 is 100 ohms, 4 is 1000 ohms and so on... it will get you in the right ball park, the rest is up to you.

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Yeah. When it comes to digital logics - use 10K. Works? Fine. Not? hook up 5K. No? Go for 1K. Still no? use 200 Ohm. Keep trying until it works. If you burn it again, go for middle value :D

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    \$\begingroup\$ This is poor advice. That technique will likely cause damage to other components. Even if it does work you have no idea how close to the edge of reliable operation you are. -1, I'm afraid. \$\endgroup\$
    – Transistor
    Commented Jan 25 at 22:52
  • \$\begingroup\$ It is as good as the question is. One shouldn't swap JUST a resistor out of the blue. Burnt resistor is a consequence, not a cause! How one would burn say 100K resistor? Or 1M resistor?? \$\endgroup\$
    – Sasha
    Commented Jan 26 at 22:07

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