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How a 25,000-Count Auto-Ranging Multimeter Measures Resistance

Writer's picture: Paul TaylorPaul Taylor

Updated: Jan 31

Did you know your digital multimeter "shifts gears" like a car?


An automobile internal combustion should operate within a certain RPM range to provide best performance and yet operate within safe limits. Therefore, a transmission has multiple gears as shown in the graph below.




Although you can be anywhere on the long thin lines, all the magic happens along the dark path.


Automatic transmissions are like auto-ranging multimeters measuring resistance, they pick the best range for you. But manuals still exist, because…


Some like to do the shifting themselves!


To measure resistance, a multimeter must provide a sufficient voltage, but not too much. Here’s the meter we will use to demonstrate.



Notice how similar this graph below is to the one for the sports car!


I left it in auto-ranging to collect the data. The meter is trying to provide the best precision it can by its range selection.



If you calculate the current from the data above, a new graph can be made. This reveals that the multimeter is providing a test current, and then determining the voltage drop that results. This graph looks like the gear ratio steps a transmission makes.



For auto-ranging, it needs a little time of trial and error to pick the best current value this can be annoying. One of the reasons you might want to use manual ranging is to prevent the meter from jumping across these steps, for the sake of consistent accuracy and less time due to no auto-ranging. Similar to a car, we don't always want the car to downshift unless really needed.


The power dissipated by the component under test is very small. Unlike an analog meter, a digital multimeter is not likely to damage smaller components or overheat low current fuses.



Just imagine, based on extrapolation of the first part of the curve, what would happen if the current wasn't reduced along the way. Too much power would be dissipated in large ohm resistors! Plus, the test voltage would be quite high. The meter provides around 1V at most.


So now we know: the ANENG SZ20 is a 5-speed. Hope you enjoyed this little diversion!



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