resistor question

Discussion in 'General Electronics Chat' started by ZWhooshdoo, Sep 10, 2011.

  1. ZWhooshdoo

    Thread Starter New Member

    Jul 16, 2011
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    I have a 1/4 watt resistor rated at 1k ohms in a circuit and it is showing 12 volts at one end and only .4 volts at the other end. Since that is such a drastic difference in voltage levels, I am wondering if that is normal or could the resistor somehow have too much resistance? Is there a way to tell roughly what the difference in voltage should be across a resistor based on its rating? Thanks very much for any help.
     
  2. justtrying

    Active Member

    Mar 9, 2011
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    if you figure out the voltage drop across the resistor, there does not seem to be any problems in terms of its power rating (that is always the main concern). So the question is what kind of current is flowing through the resistor, the voltage drop will obey Ohm's Law. Also if you are uncertain whether the resistor is good or not, you can always check it with DVM.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    No there is not unless it is some extreme voltage drop. The one you have is within it's wattage specifications. If you are unsure you can measure it in-circuit which will most likely give you the wrong value, but it shouldn't be BIGGER than 1k. To be absolutely sure, desolder one side and measure again.
     
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    I think ZWhooshdoo's problem is how he was measuring the resistor. I think he had the common probe on (-) and put the positive probe on the (+) side of the resistor (of course, giving him the full 12 volts), but then moved the positive probe to the other side of the resistor to get .4 volts. Then again, if that were the case, .4 volts seems awfully high...I would expect it to be closer to 0.
    Der Strom
     
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
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    There is single voltage across the resistor, apparently you didn't measure that.
    I'd think, however that if you measured the voltage at one end with respect to ground and got 12V and the other end also with respect to ground and got .4V then you know the voltage across the resistor, 12V - .4V or 11.6V. Which should be the same as if you place the probes of the DVM on both ends of the resistor. The current then is 11.6V/1K or 11.6mA. The fact that there was a different voltage on both ends of the resistor proves that the resistor is not bad. How close to its stated value is another story.
     
    Last edited: Sep 11, 2011
  6. upand_at_them

    Active Member

    May 15, 2010
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    How many times are you going to ask this question? You've already asked it on Electro-Tech and gotten several answers.
     
  7. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Lot's of people post their questions on both forums. There is absolutely nothing wrong with that. The more answers the better, if you ask me.
    Der Strom
     
  8. upand_at_them

    Active Member

    May 15, 2010
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    It depends.

    This isn't a service, per se. It's not like getting a price on a fridge...You go one place, get a price, go to another and get a price...

    If your neighbor came over and asked, "Hey, Joe...What do you think about such-and-such?" And then, before you could answer, he goes to another neighbor and asks the same question. It certainly looks like he's inconsiderate and rude. Why waste your time answering him if he just wants to treat you like an appliance salesman?

    Long-time members tend not to do this. Newbies do it a lot, because they're not interested in being part of the community they just want in and out.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    I applaud zwhooshdoo's thoroughness in gathering the data from multiple sources.

    Many of the members from both forums can benefit from the question.

    hgmjr
     
  10. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This in any case impossible to answer out of context, beyond saying that a 1kΩ 1/4 W resistor could reasonably have any voltage up to about 15.8V across it, and still be within its rating. P = V^{2}/R, so V=√(PR)

    We can't know what the values of the voltages at each end of the resistor should be without information about the rest of the circuit.
     
  11. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    The difference between this example and what Zwooshdoo is doing is that he's not cutting anyone off--he's not leaving the people he asked in person. I'd agree with you if it was a question like "what is 4 times 4?" where there is only one correct answer. Getting multiple suggestions and opinions on questions like Zwooshdoo's resistor one actually shows good initiative and efficiency in getting answers. There is ABSOLUTELY NOTHING wrong with asking on multiple forums--it's no different from asking on one forum and getting multiple answers. You're just asking on two, and you get more answers.
    As for only newbies asking on two forums, that is not true. I have seen it happen a lot with long-time members here (members with thousands of posts, I won't mention names :p ). I have even done it on several occasions myself, especially with problems that could have many solutions. I recently asked a question about my car (on both forums!) and got a large number of answers that allowed me to narrow the problem down to one or two specific parts. If I had only asked on one forum, I probably would not have gotten enough information.

    Der Strom
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    @ upand_at_them The forums exist to help people. I happen to be a member of the two your talking about. When your new to a subject or even the concept of forums, you may ask questions several places, until you get an idea of the quality of answer or the tone of the forum. Nothing wrong with that. You must be a member of both, or how would you know the OP is? Should you be not allowed to ask questions on both?
     
  13. iONic

    AAC Fanatic!

    Nov 16, 2007
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    In my opinion, if they are separate forums then you have every right to ask on both. If someone complains that you are posing question on both then maybe the two forums should be merged! And if you can't ask the same question on both forums, then why are some members of both forums?
     
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