EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Agony on May 12, 2013, 07:51:27 pm
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so i "salvaged" a led ring/strip and looked at the resistors.
There are some 105-s, that look like 0.06W range, really tiny.
measured i get 720-730k ohms.
105 is rated at 1M ohm.
is that huge tolerance normal? thats around 27% less.
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Maybe you fried them during removal?
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didnt remove them, in circuit testing. .. too small to even bother removing.
https://www.circuitlab.com/circuit/fhp5xv/led-ring/ (https://www.circuitlab.com/circuit/fhp5xv/led-ring/)
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They probably are 1M but other components in the circuit are skewing the measurement.
Or maybe your DMM is a cheapish brand and has a flat battery (cheap meters read wrong when getting flat)
Most likely the former.
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No, but if you look at your schematic, you've got resistances in parallel, and if you're trying to measure those 105 resistors in circuit, assuming all of those resistors are actually 1M and they are right on the mark for tolerance, you should read about 750K. Factor in a bit of tolerance, and 720K-730K isn't that far out.
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the R3,R4,R5,R6 - they all were marked as "105" - 105 should be 1M.
But when i did some measuring - had a unregulated DC supply (17.6V under load) - the current was very close - 7mA.
Thats what simulation gets me with 720k ohm resistors. With 1m ones the simulation suggested a 11mA - way mroe then it actually measured.
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Measure the resistors out of circuit and tell us what they read. ::)
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the R3,R4,R5,R6 - they all were marked as "105" - 105 should be 1M.
But when i did some measuring - had a unregulated DC supply (17.6V under load) - the current was very close - 7mA.
Thats what simulation gets me with 720k ohm resistors. With 1m ones the simulation suggested a 11mA - way mroe then it actually measured.
You're not listening and/or reading...
You can't measure the value of those resistors "in-circuit". There's too much other stuff going on.
And an unregulated 17.6v. Who knows what kind of wave garbage is coming out of that thing...
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You should look up how a voltmeter works. Also understanding that taking a measurement at two points is between two "nodes", so anything connected between those two nodes is measured. It might give you some clarity as to why you're getting the result you are.
The multimeter is putting out a small test voltage, the current from this is travelling through all the resistors in the circuit. What you are measuring is the combination of all the resistors at once in that configuration. You pretty much have to remove most components from a circuit if you want to test their values.