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Old 2nd March 2009, 12:31 PM   #1
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Default Your views on these resistor values

I recently bought some carbon resistors from ebay. They were advertised as 1 Watt 0.1 OHM and 2 Watt 0.47 ohm. Both were + or - 5%.

I've just received them and measured them with my meter. The 0.1 Ohm resistors measure as 0.3 Ohm and the 0.47 Ohm measure as 0.7ohm.

Even allowing for the + or 1 5% tolerance these measured values seem to be way out of line with what they should read. Are my observations correct or are those measured values what you would expect to see?

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Old 2nd March 2009, 12:44 PM   #2
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If you short your measurement leads, does it read 0.2 ohms?

Jan Didden
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Old 2nd March 2009, 01:00 PM   #3
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Yes if I short the leads it reads 0.2 ohms, but I'm not shorting the leads when measuring the resistors.

At the same time as measuring the above I also measured a batch of 2.2K resistors and 2.7 ohm resistors. Both of these batches measured correctly (as have others) so it's not bad measuring skills.
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Old 2nd March 2009, 01:59 PM   #4
es44 is offline es44  Denmark
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Sure Your not shorting the leads, but they are in series with the DUT. You have to add the resistance of the lead's wire/plugs/junctions when you measure such "small" resistors.

What if you had a 0.01r and tried measuring that the leads would still have ~0.2r.

Best regards
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Old 2nd March 2009, 02:04 PM   #5
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Your meter's leads have a resistance of about 0.2 ohms which are in series (they add up) with the resistor you're measuring. Thus 0.1 resistors appear to be 0.3 and 0.47 ohms show as 0.7 ohms.
You don't notice the difference while measuring 2.2K because the meter can't "see" the extra 0.2 ohms.
Measuring very low or very high resistances with a standard DVM can show noticeable errors, that's why special instruments are sold.
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Old 2nd March 2009, 02:28 PM   #6
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It is true that you learn something new every day

But now I feel silly

Thanks for clearing this up.
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Old 2nd March 2009, 02:59 PM   #7
AndrewT is offline AndrewT  Scotland
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string all your 0r1 resistors in series.
Measure the resistance of the whole string.
Subtract the 0r2 error that your leads have measured out at.
Divide by the number of resistors.
How close to nominal resistance are they now?

Repeat for a string of 0r47 resistors.

While they are in string apply a 100mA CCS source to them.
Each 0r1 resistor will drop about 10.0mV read on a 200.0mV scale DMM.

Compare all the voltage drops. now find matched pairs or quads. label them for future use when tight tolerance is called for.

If you have a 100r 0.1% (four cheap 1% in series parallel would be better since 100mA will run very hot, or better still, 9 in series parallel) resistor you can add this to the string and measure the actual CCS current at the same time and from there calculate the resistance of every resistor to better than 1%.

If you build a 190mA CCS you can do matching to better than 0.5% with care.
Ingenuity and not much money can get you to 0.05%.

Order up some lm317 regs and a decade range of 10r to 100k 0.1% precision resistors, included in your next purchase.
Start buying your 1% metal film 600mW resistors in 100off packs. Rapid sell them for <1.50/100. eventually you'll have the whole E24 range from 10r to 2M2 and E6 range extending out to 1r0 and 10M
regards Andrew T.
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