Sanity check for a cheap DVM

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bob prohaska 8 years ago

I'd like to check an old Radio Shack DVM for accuracy around 5 volts DC. On a brand-new cr2032 coin cell it measures 3.25 volts open circuit, about 3.15 volts with a 3 kohm load. That's rather high compared to

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which shows ~2.95 V with a

15 kohm load.

I've noticed that brand-new alkaline D cells seem to measure around

1.65 volts and credited it to improved battery chemistry. Now I'm not so sure and a reading that is 200 mV high might explain some trouble I'm seeing.

Thanks for reading, and any thoughts.

bob prohaska

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Phil Hobbs 8 years ago

For a buck and a quarter at Digikey you can get a 5V reference with 0.2% accuracy in a convenient through-hole TO92 package:

If you're okay with surface mount stuff, 78 cents will get you a 4.096V

+- 0.1% reference,

For about $5 you can get 5V +- 0.01%:

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com

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sdeyoreo 8 years ago

Make an electrochemical cell. Look up the resulting voltage using , say silver and copper, then check your dmm. We make our on humidity references using different salt pastes.

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Dimitrij Klingbeil 8 years ago

For $28 you can get a reference in this class, fully assembled into a battery operated module, trimmed and calibrated.

See

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They also have other versions for voltages from 1 to 10 V as well as a DMM check module ("DMMCHECK Plus"), which also includes precision resistors and a current source to cover the other basic functions.

These modules seem to have some popularity among the members of the "volt-nuts" mailing list.

Regards Dimitrij

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Phil Hobbs 8 years ago

Cool, thanks.

I generally use an ensemble of 5-1/2 and 6-1/2 digit meters plus some HP

61xx precision power supplies. Last week I got a 6114A (0-20V @ 2A, 20-40V @ 1A) supply that tests out within 0.01% everywhere. $79 plus shipping--eBay rocks.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com

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Phil Allison 8 years ago

** 3.25V open cct is normal IME, that data sheet is miseading.

** Also normal, been like that for decades.

** Your cheap DVM is fine.

.... Phil

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bob prohaska 8 years ago

That's a relief, sort of. The trouble in question is then more complicated than a bum wallwart.

Thank you very much!

bob prohaska

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Sanity check for a cheap DVM

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