This resistor has me puzzled

Discussion in 'General Electronics Chat' started by ComputeRick, May 7, 2013.

  1. ComputeRick

    Thread Starter New Member

    May 4, 2013
    3
    0
    I've been working on an LCD power inverter, seems like a lot has been damaged. Replaced most of the capacitors, going through testing resistors but I am missing something or I can't read them yet.

    I am still learning, but I have done due diligence and can't make sense of these 2.

    A higher res picture is available at this link.

    The top left is easy, the top right and bottom left are the confusing ones. The top right is Red, Brown, Silver, Gold, gap then White. I understand that the white is not important to me, but the gap shows that I'm reading it the right direction. Red, Black, Silver, Gold = .3Ω ±5%:confused:. I haven't been anything like a 1w .3Ω resistor.
    Testing it, I get 9Ω on my multimeter.

    The bottom left is similar: Yellow, Purple, Silver, Gold. Again, by the charts, .47Ω ±5%:confused: I get 9Ω on this also with my meter.

    I appreciate any insights that you can offer. I just doubt the color charts and the silver strip is messing me up. But I believe both of these resistors are damaged.
     
  2. Rbeckett

    Member

    Sep 3, 2010
    205
    32
    Rick,
    Are you reading across the resistor in the circuit? You must disconect at least one end to isolate the component or you will sead the resistance in that part of the circuit which may contain other resitors and components. Just makin sure we are all starting from the same place.

    Bob
     
  3. absf

    Senior Member

    Dec 29, 2010
    1,490
    371
    Could it be something is wrong with your meter. Did you try to read a new 0.47Ω resistor and see what the value is displayed on your meter.

    In my experience reading a resistor with such a low value in-circuit, would normally yield a resistance reading lower than that displayed on the resistor. But still the most accurate way is to measure it externally like what Bob said.

    Allen
     
  4. ComputeRick

    Thread Starter New Member

    May 4, 2013
    3
    0
    I understand about testing it independent of the circuit, I will certainly do that in the morning. The question remains, I've never seen (not that I've seen a lot) a 1w .47Ω resistor. That could be accurate? I was reading the resistor codes correctly? How would I find a replacement for something like that? More than the test results, which I will verify, was I correct about the resistance ratings of both of those? The silver band threw me off.
     
  5. Ramussons

    Active Member

    May 3, 2013
    557
    92
    .47Ω resistors of 1W (and more) is a standard one used in the Emitters of Power BJT's for DC stability. Paralleling 2 pieces if 1 Ω resistors to get .5Ω (as good as .47Ω) is one way.

    Incidentally, how is the other resistor .3 Ω ? looks to me as .2 Ω

    Ramesh
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Firstly to correct a wrong impression.

    If you measure resistance across a resistor in circuit you will always get a low reading, not a high one, since the rest of the circuit is inparallel with the resistor.

    So 9 ohms is likely the parallel resistance of the rest of the circuit and the component in question has gone open circuit.

    Secondly check here for fusible resistor colour codes here.

    http://www.google.co.uk/search?scli...0j7j4j1.12.0...0.0...1c.1.12.serp._Q9vjZCk47Y
     
  7. Rbeckett

    Member

    Sep 3, 2010
    205
    32
    The resistors do not appear burned or otherwise damaged, but anything is possible. Definitely isolate the resistors and you will know pretty quick. I would verify that your meter is correct also. Try several of the resistors in you parts bins to see if they are actually reading in tolerance too. That will confirm the accuracy of your meter and solidify your findings when you measure the resistor isolated from the rest of the circuit, I check the calibration and accuracy of my meter pretty frequently because it does rely on the batter you have installed in your meter and could be giving you a false or inaccurate reading too. Isn't troubleshooting a blast. Don't give up, just keep following the circuit till you find whatever is out of range and that will probably be it. Don't forget to try and determine why the part failed too. Otherwise you might have to replace it again if some other component down the line is causing too much current or too much amperage for the resistor.

    Bob
     
  8. ComputeRick

    Thread Starter New Member

    May 4, 2013
    3
    0
    Ok, so I'm not some noobie that can't figure out the color codes, Thank you. Even if I don't have a resistor like that around to replace it with. Second, I will probably make a little test board, with some components just to be sure that my tester is reading accurately. Thank you very much.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Yes. Those are metal oxide power resistors and there definitely are values below an Ohm available (used them many times).
     
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