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Old 06-25-2017, 08:29 AM
Mich48041 Mich48041 is offline
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,739

Digital voltmeters can be misinterpreted. Supposing that a digital meter is used to measure the voltage at the downstream side of a blown fuse and the meter reads 1.75 volts. Is it really 1.75 volts? If a load is connected that requires 1.75 volts, would it work? No, it would not. So is the meter lying? No, it is displaying exactly what it sees. If a digital meter has, say, 5 megohms of resistance and it is connected in series with a blown fuse with a resistance of 30 megohms, an infinitesimal amount of current flows from the source through the blown fuse and through the meter back to the source. If the source is a 12.75 volt battery, then 11 volts are dropped across the blown fuse and 1.75 volts are dropped across the digital meter. The meter reads 1.75 volts.
If a 5 watt, 12 volt, indicator lamp is connected as a load, now the meter will read zero because the infinitesimal current is flowing through the lamp instead of the meter. I like to test a circuit when it is under load to get more meaningful meter readings. By the way, the definition of infinitesimal is the distance that an aircraft carrier sinks in the water when a fly lands on its deck. :-)
Fluke Meter pdf talks about input impedance.
Joe Gores
RV-12 Flying
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