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  • ImmortalSpirit commented on magkopian's instructable SMD Decade Resistance Box3 years ago
    SMD Decade Resistance Box

    Yes! Here is a *used* precision ohm meter for only $2795.00! https://www.valuetronics.com/detail/Used-keithley...It has 4-1/2 digits! https://www.valuetronics.com/Manuals/KEITHLEY_580..."Resistance measurements can be made from 10μΩ to 200kΩ on seven ranges, with 41⁄2-digit resolutionand accuracy to within 0.04% of reading."

    Yes! Here is a *used* precision ohm meter for only $2795.00! https://www.valuetronics.com/detail/Used-keithley...It has 4-1/2 digits! https://www.valuetronics.com/Manuals/KEITHLEY_580...

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  • ImmortalSpirit commented on magkopian's instructable SMD Decade Resistance Box3 years ago
    SMD Decade Resistance Box

    Say I want 33M ohms. I set the 10M dial to 3 and the 1M dial to 3. With all 1% resistors, setting 33,000,000 will give me anywhere from 32,890,000 to 33,110,000 actual ohms.With that, setting it to 33,003,124 or some such thing is insane; indeed, 33,xxx,xxx, is all silliness. The uppermost “x” (in the 100,000 position) is plus or minus 1, and every lower “x” is off my more than its entire range!Thus with 1% resistors, only two dials can be set on this device, maybe three. All digits below that are completely meaningless.If you had 0.1% resistors, you could use three or four dials.For 10 dials covering 1 ohm to 10M ohms to be meaningful, you’d have to have 0.0000001% resistors.And that is all not to mention that the temperature coefficient of typical resisto...

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    Say I want 33M ohms. I set the 10M dial to 3 and the 1M dial to 3. With all 1% resistors, setting 33,000,000 will give me anywhere from 32,890,000 to 33,110,000 actual ohms.With that, setting it to 33,003,124 or some such thing is insane; indeed, 33,xxx,xxx, is all silliness. The uppermost “x” (in the 100,000 position) is plus or minus 1, and every lower “x” is off my more than its entire range!Thus with 1% resistors, only two dials can be set on this device, maybe three. All digits below that are completely meaningless.If you had 0.1% resistors, you could use three or four dials.For 10 dials covering 1 ohm to 10M ohms to be meaningful, you’d have to have 0.0000001% resistors.And that is all not to mention that the temperature coefficient of typical resistors is about 0.01% per degree C (per wikipedia). Thus, even with hypothetically “perfect” valued resistors you can’t use more than 4 digits (1000 could be 9999) if you only keep the device within 1 degree C for all measurements! Copper wire is 0.4% per degree, better temperature control your leads too!

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