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Tool Talk Discussion Board |
Re: Re: Re: analog multimeter
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Posted by Bob M on March 15, 2004 at 18:04:11 from (66.67.10.249):
In Reply to: Re: Re: analog multimeter posted by Clint@MD on March 15, 2004 at 14:54:48:
Clint - Generators are notorious for creating electrical noise due to the rapid switching going on at the commutator. Also the vibrating voltage regulator contacts, and to some extent ignition point action create noise and cause all manner of weird AC and chopped DC voltages to ride along on the "power" DC. I've had cheap DVM's read as high as 20 - 30 volts when placed across a 6 volt battery under charge. (An analog meter will read 7.2 volts at the same spot.) This is especially so on equipment such as old tractors where no design consideration is given to RFI surpression. Want proof: Find someone with an oscilloscope and have 'em slap it across the battery while the engine is running. It'll show a virtual electrical storm going on when the motor is running. Or simply tune an AM radio between stations then bring it within 15 ft or so of a running tractor - you can hear the generator hash and ignition popping. Now recognize not all DVM's have this problem - it's generally a case of you get what you pay for. I have a cheapie ($15) Radio Shack pocket DVM that is worthless for this kind of work. However I also have Fluke 77 and 87 meters ($125 and $225) which do fine on the tractors.
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