Why don't typical digital multimeters measure inductance?

Question

Even with predominantly digital circuits, I am using inductors much more often than I used to, generally because of all the buck or boost converters (a recent board I was involved in has 12 different voltage rails -- six of them needed just by the TFT LCD).

I've never seen a standard digital multimeter (DMM) with an inductance range. So I ended up buying a separate meter that does LC measurements.

However a lot of DMMs have a capacitance scale. Since capacitors and inductors can be thought of as mirror images of each other with voltage and current flipped, why don't DMMs include an inductance scale also? What's so difficult about measuring inductance that it is left off of DMMs and relegated to specialty meters?

Since inductance meters are usually LC meters (even LCR), do they measure capacitance in a different way than DMMs? Are they more accurate than the capacitance scale of a DMM?

Answer 1

The only reason DMMs can't measure inductances is that it is more difficult to measure inductance than resistance or capacitance: this task requires special circuitry, which is not cheap. Since there are relatively few occasions when inductance measurements are required, standard DMMs do not have this functionality, which allows for lower cost.

Simple DMMs can measure capacitance by just charging the capacitor with a constant current and measuring the rate of voltage build-up. This simple technique provides surprisingly good accuracy and wide dynamic range, therefore it can be implemented in almost any DMM, without significant cost penalties. There are other techniques as well.

Theoretically, one could measure inductance by applying a constant voltage across an inductor and measuring the current build-up; however, in practice this technique is much more complicated to implement, and the accuracy is not that good as for capacitors due to the following reasons:

• Inductors may have relatively high parasitic resistance and capacitance
• Core losses (in cored inductors)
• EMI (incl. stray inductance and capacitance)
• Frequency dependent effects in inductors
• More

There are few techniques for measuring inductances (some of them are described here).

LCRs are special meters designed for inductance measurements and containing the required circuitry. These are costly tools.

Since the hardware for measuring the inductance may also be used for accurate measurement of R and C, LCRs also employ this circuitry in order to improve the accuracy of capacitance and resistance measurements (for example: AC resistance, AC capacitance, ESR etc.). I believe that the difference between measuring inductance and capacitance with LCR is just a matter of different firmware algorithms, though it is just a guess.

Therefore, the general answer to your question is "yes, LCRs are usually more accurate in RC measurements than DMMs, and they can measure a wider range of measurable quantities". However, this is just a rule of thumb - there are many superb DMMs and lousy LCRs out there... Read specs.

Answer 2

Resistors are very pure compared to an inductor in that a typical commonplace resistor has a very small amount of leakage inductance and capacitance. The resistance in 99.9% of the time, dominates the reading.

Capacitors are reasonably pure too when it gets to surface mount devices typically. Self inductance is quite low and ditto leakage resistance and ESR. Again, capacitive reactance across a vast swathe of values dominates a measurement and gives decent results with simple testing methods.

Inductors are a different story. It can be hard to seperate ESR from reactive value at low frequencies unless a dc resistance measurement is also taken. ESR also gets bigger with frequency due to skin and proximity effects too. Added to this is the problem that a wound component has relatively high leakage capacitance and this capacitance can throw a reading off as you approach and rise through and above the self-resonant-frequency making inductors difficult to pin-point in value with relatively simple tests.

Answer 3

True that inductors can be more complicated components than resistors or capacitors. But the reason common DMMs do not have L measurement is probably more because of market forces. I actually once had a cheap DMM with L measurement, but my current collection of DMMs cannot measure L.

You can measure all aspects of a magnetic component like a simple inductor or a multi-winding transformer with complicated devices like in Vasili's link, or buy a simple LCR for just the inductance measurement like this 60 euro thing. According to the online user manual, it applies a sine of 250 Hz to the inductor under investigation, in series with a resistor. The series resistor can be selected with the scale knob. For a more detailed explanation, look e.g. here.

As for OP's second question, I do not believe that inductance meters are "LC" meters. That would suggest that these measure using a resonance circuit. The simplest method to measure L or C is with a series resistor and a low frequency oscillator. A cheap DMM and a cheap LCR will both use this method. The accuracy with DMM or LCR will therefore be similar. However, because inductors have more parasitic effects than capacitors, like resistance, leakage flux, saturation, non-linearity, hysteresis, eddy currents, frequency depending mu, the simple measurement of the inductivity may not be sufficient for you.