I've found a something quite odd with measuring the resistance of a 36uH inductive load (see this earlier question). I can measure the resistance, which is quite low, but then disconnect the probe and for some reason the subsequent resistance measurement of other loads become a bit messed up. I would have to wait a bit for it to settle down. Only then can I measure the other loads and it'll be accurate.

At first I thought it was just me (like usual, when doing experiments), but now I'm more sure.

  • \$\begingroup\$ Could be the dI/dt putting the ohm meter into some kind of voltage overload \$\endgroup\$ – Mike Feb 25 '17 at 14:48
  • \$\begingroup\$ I really doubt it. Modern multi-meters, even the absolute cheapest ones are pretty robust. If you measure a long coil of wire it might take a while for the reading to settle due to inductance or antenna pickup, but it shouldn't cause any lasting change to the meter. \$\endgroup\$ – Evan Feb 25 '17 at 19:38

First consider the relationship between resistance, current and voltage:

V = I x R

In order to find any one of these values the other two need to be known. They can either be controlled or measured. Given the unknown is the resistance (R) we can control the current (I) and measure the voltage (V). Most multimeter operate this way. They control the current (I) and measure the voltage (V) across the unknown resistance (R) and calculate the value of the resistance:

R = V / I

In the real world measuring instruments have an optimal range where they operate well or operate better than in other ranges. It is also best to assume the smaller the current through the unknown resistance the safer the measurement can be carried out. Finally, most modern multimeter are auto ranging meaning they (normally) start with small amounts of current and step up the current over time until the voltage across the resistance falls with in a measurable range for the meter.

Now, consider your case where you are measuring an inductive load. An inductive load by definition will resist any current change. If an auto ranging multimeter is used it can be assumed the current is being adjusted over time to optimize the meter's ability to measure the voltage. However each time the current is change the inductor resists the change. This causes the momentary voltage reading and consequently the reported resistance to take on unexpected values.

(I can not offer an explanation for subsequent resistance measurements of other load. Other than to assume they are also inductors and are behaving similarly to the first inductor you measured.)


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