Voltage Burden

“The measurement system should not affect the measurement.”

This statement is accepted throughout the test and evaluation community as a given whether it is stated in the requirements or not.  We get into the habit of taking the measurement without considering the instrument.  We expect that a voltmeter provides a very high impedance, so it does not load the circuit being measured.  We expect that the current meter has a very low impedance, so it does not restrict current flow.  And naturally, we do not read the manual for using a simple multimeter – after all it is obvious how it works.

As an example, let’s examine the Fluke 87 True RMS multimeter for current measurements (see sketch below).  The current function specifications show that the 60 mA and 400 mA ranges have a typical 1.8 Ω resistance (1.8 mV/mA).  If the load was ~1 kΩ the expected current would be ~3 mA so we would select the 60 mA range.  The meter voltage drop would be ~5.4 mV leaving 2.995 V for the load.  Now let’s change the load to 8 Ω which should produce a current ~375 mA, so the 400 mA range would be appropriate.  Now the meter voltage drop would be ~675 mV leaving 2.33 V for the load.  This causes the applied voltage to be off by near 20% and may affect the operation of the circuit.  The circuit current would be measured at ~306 mA instead of the calculated 375 mA.  Making the same measurement with the 6A (resistance of ~30 mΩ (0.03 V/A) range would produce a meter voltage drop of ~11.3 mV leaving 2.99 V for the load. This issue is not unique to the Fluke meters. Their manual does a good job of explaining the situation.  Do you know the voltage burden for your meter?  Do you have a correction process?  Are your technicians and engineers aware of the burden and effects?

As an example, let’s examine the Fluke 87 True RMS multimeter for current measurements (see sketch below).  The current function specifications show that the 60 mA and 400 mA ranges have a typical 1.8 Ω resistance (1.8 mV/mA).  If the load was ~1 kΩ the expected current would be ~3 mA so we would select the 60 mA range.  The meter voltage drop would be ~5.4 mV leaving 2.995 V for the load.  Now let’s change the load to 8 Ω which should produce a current ~375 mA, so the 400 mA range would be appropriate.  Now the meter voltage drop would be ~675 mV leaving 2.33 V for the load.  This causes the applied voltage to be off by near 20% and may affect the operation of the circuit.  The circuit current would be measured at ~306 mA instead of the calculated 375 mA.  Making the same measurement with the 6A (resistance of ~30 mΩ (0.03 V/A) range would produce a meter voltage drop of ~11.3 mV leaving 2.99 V for the load. This issue is not unique to the Fluke meters. Their manual does a good job of explaining the situation.  Do you know the voltage burden for your meter?  Do you have a correction process?  Are your technicians and engineers aware of the burden and effects?

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