Multimeters (My first post to sci.electronics.basics)
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Posted by P J King on May 23, 2005, 11:46 am
Please log in for more thread options Hello: I couldn't seem to find a valid FAQ to this newsgruop (unless the Electronics Search FAQ was it, but it doesn't appear to say much), so I apologize if this topic has been "beaten to death" in previous posts. I have about 15 multimeters at the school I teach at. There are 3 different brands (the names escape me at the moment), all of them are digital. I set up two small lamps in series, and test the resistance of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought to be 600 ohms. But when I test the connection across the two lamps, I get 560 ohms. I thought this was a fault of the multimeter, but since I have 15 of them, and 3 different brands, each one I used gave nearly the same reading. BTW, I made sure that each time, the individual loads were measured with the same multimeter as used to measure the total resistance. How is it that two loads hooked up in series give a reading that is *less* than the sum of the individual loads? This makes absolutely no sense. Calibration can't be the problem, since the same multimeter is used for all 3 measurements. The error seems to be independent of the brand of multimeter. PJK | |||||||||||||
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Posted by Chris on May 23, 2005, 10:03 am
Please log in for more thread options P J King wrote: > thought this was a fault of the multimeter, but since I have 15 of
> them, and 3 different brands, each one I used gave nearly the same > reading. BTW, I made sure that each time, the individual loads were > measured with the same multimeter as used to measure the total > resistance. > > How is it that two loads hooked up in series give a reading that is > *less* than the sum of the individual loads? This makes absolutely no > sense. Calibration can't be the problem, since the same multimeter is > used for all 3 measurements. The error seems to be independent of the > brand of multimeter. > > PJK Welcome to s.e.b., P.J. - the water's fine here among the groundlings and basic electrical/electronics questions are (usually) answered without snark. I'm assuming you have two very small incandescent bulbs. Incandescent lamps have a non-linear resistance characteristic. That means their resistance increases along with their temperature. In other words, the measured resistance will decrease as current (power) used to measure the resistor decreases. Resistance is measured by putting current through a resistor, and measuring voltage across it. Resistance is inferred through Ohm's Law (R = V / I). The higher resistance of the two bulbs in series may decrease the measurement current from the meter. If that's the case, each bulb has less power applied across it, and so it's a little cooler. This means lower voltage across each bulb, therefore lower total voltage and lower total inferred resistance. That may be why they don't "add up". Don't depend on accurate resistance measurements of bulb filaments unless you take accurate filament temperature into account. And you should always keep in mind the effect of the instrument on the measurement. A friend of mine used to call our shop claw hammer his "Heisenberg Uncertainty Principle Tube Tester" many years ago. Since he hated wasting time swapping out marginal or possibly bad tubes with other marginal or possibly bad tubes, he always tested them this way last (inside a box in the shop garbage can). He said the advantage of this test was, once you were done, you always had a definitive and satisfying answer to whether the tube was good or not. And the instrument definitely affected the measurement. ;-p Good luck Chris | |||||||||||||
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Posted by P J King on May 25, 2005, 8:58 am
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Thanks for all your responses, they were a great help! PJK >
>Welcome to s.e.b., P.J. - the water's fine here among the groundlings >and basic electrical/electronics questions are (usually) answered >without snark. > >I'm assuming you have two very small incandescent bulbs. Incandescent >lamps have a non-linear resistance characteristic. That means their >resistance increases along with their temperature. In other words, the >measured resistance will decrease as current (power) used to measure >the resistor decreases. > >Resistance is measured by putting current through a resistor, and >measuring voltage across it. Resistance is inferred through Ohm's Law >(R = V / I). > >The higher resistance of the two bulbs in series may decrease the >measurement current from the meter. If that's the case, each bulb has >less power applied across it, and so it's a little cooler. This means >lower voltage across each bulb, therefore lower total voltage and lower >total inferred resistance. That may be why they don't "add up". > >Don't depend on accurate resistance measurements of bulb filaments >unless you take accurate filament temperature into account. And you >should always keep in mind the effect of the instrument on the >measurement. > >A friend of mine used to call our shop claw hammer his "Heisenberg >Uncertainty Principle Tube Tester" many years ago. Since he hated >wasting time swapping out marginal or possibly bad tubes with other >marginal or possibly bad tubes, he always tested them this way last >(inside a box in the shop garbage can). He said the advantage of this >test was, once you were done, you always had a definitive and >satisfying answer to whether the tube was good or not. And the >instrument definitely affected the measurement. ;-p > >Good luck >Chris | |||||||||||||
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Posted by John Popelish on May 23, 2005, 1:10 pm
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P J King wrote: > Hello:
> > I couldn't seem to find a valid FAQ to this newsgruop (unless the > Electronics Search FAQ was it, but it doesn't appear to say much), so > I apologize if this topic has been "beaten to death" in previous > posts. > > I have about 15 multimeters at the school I teach at. There are 3 > different brands (the names escape me at the moment), all of them are > digital. I set up two small lamps in series, and test the resistance > of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought > to be 600 ohms. > > But when I test the connection across the two lamps, I get 560 ohms. I > thought this was a fault of the multimeter, but since I have 15 of > them, and 3 different brands, each one I used gave nearly the same > reading. BTW, I made sure that each time, the individual loads were > measured with the same multimeter as used to measure the total > resistance. > > How is it that two loads hooked up in series give a reading that is > *less* than the sum of the individual loads? This makes absolutely no > sense. Calibration can't be the problem, since the same multimeter is > used for all 3 measurements. The error seems to be independent of the > brand of multimeter. Try the same test with a pair of 220 or 330 ohm resistors. I suspect that the meters pass enough current through the filaments to change their temperature, raising their resistance above the room temperature value. Testing two in series lowers that current so they heat less, and each have lower resistance. Resistors should not have this high temperature coefficient and should not show the effect. | |||||||||||||
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Posted by me on May 24, 2005, 3:34 am
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>Hello:
> >I couldn't seem to find a valid FAQ to this newsgruop (unless the >Electronics Search FAQ was it, but it doesn't appear to say much), so >I apologize if this topic has been "beaten to death" in previous >posts. > >I have about 15 multimeters at the school I teach at. There are 3 >different brands (the names escape me at the moment), all of them are >digital. I set up two small lamps in series, and test the resistance >of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought >to be 600 ohms. > >But when I test the connection across the two lamps, I get 560 ohms. I >thought this was a fault of the multimeter, but since I have 15 of >them, and 3 different brands, each one I used gave nearly the same >reading. BTW, I made sure that each time, the individual loads were >measured with the same multimeter as used to measure the total >resistance. > >How is it that two loads hooked up in series give a reading that is >*less* than the sum of the individual loads? This makes absolutely no >sense. Calibration can't be the problem, since the same multimeter is >used for all 3 measurements. The error seems to be independent of the >brand of multimeter. > >PJK > What is it that you teach?... ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- | |||||||||||||
>
> I couldn't seem to find a valid FAQ to this newsgruop (unless the
> Electronics Search FAQ was it, but it doesn't appear to say much), so
> I apologize if this topic has been "beaten to death" in previous
> posts.
>
> I have about 15 multimeters at the school I teach at. There are 3
> different brands (the names escape me at the moment), all of them are
> digital. I set up two small lamps in series, and test the resistance
> of the lamps. Lamp 1 is 260 Ohms; lamp 2 is 340 Ohms. The total ought
> to be 600 ohms.
>
> But when I test the connection across the two lamps, I get 560 ohms.