This article is about measuring resistance, capacitance and inductance. The measurements described here talks about measuring components that not wired into any circuit.
Resistors, Inductors and Capacitors are the most commonly used passive components in almost every electronics circuit. Out of these three the value of resistors and capacitors are commonly marked on top of it either as resistor colour code or as numeric marking. Also the resistance and capacitance can also be measured using normal Multimeter. But most of the inductors, especially the ferrite cored and air cored ones for some reason does not seem to have any sort of marking on them. This becomes quite annoying when you have to select the right value of inductor for your circuit design or have salvaged one from an old electronic PCB and wanted to know the value of it.
Resistance with a multimeter
Resistance is the measure of difficulty electrons have in flowing through a particular object. Resistance can be measured with an analog or digital multimeter or ohmmeter. Resistance is can be usually usually most easily measured using an instrument such as digital multimeter.
Resistance measurement does not involve measuring the circuit’s resistance value itself. Instead, resistance is calculated by measuring the current and voltage applied to the circuit. Resistance can be calculated by measuring the current and voltage using Ohm’s Law.
Many multimeters typically apply a low current to the circuit under measurement, the circuit (resistance) exhibits a voltage a voltage drop related to resistance value. In most cases, when measuring resistance with a digital multimeter, you’ll use the two-terminal measurement method. This method applies a constant current and measures the resistance value using the instrument’s voltmeter. There are advanced meters that use a four-terminal resistance measurement for more accurate low-resistance measurement.
Many modern “better” multimeters have a measurement range to measure capacitance. 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: A multimeter determines capacitance by charging a capacitor with a known current, measuring the resulting voltage, then calculating the capacitance. Those multimeter capacitor measurements are simple to use, but can have limited accuracy. Capacitors measured by means of a multimeter in capacitance mode may be expected to read low by as much as 10%. This accuracy is sufficient for many applications such as the starting circuit for an electric motor or for power supply filtration. Greater accuracy is available by performing a dynamic test.
A capacitance meter is a piece of electronic test equipment used to measure capacitance, mainly of discrete capacitors. Depending on the sophistication of the meter, it may display the capacitance only, or it may also measure a number of other parameters such as leakage, equivalent series resistance (ESR), and inductance. For more details on capacitor measurements check out IFIXIT Introduction to Capacitors.
You can to check bad Capacitors in circuit boards with ESR meter and multimeter that can measure capacitance. For more details on this check out 3 Ways to Check Capacitors in Circuit with Meters & Testers video. If you are interested in capacitor ESR measurements, check my posts on ESR meters and DIY ESR meter.
Inductance is usually measured in units called millihenrys or microhenrys. It is commonly measured by using a frequency generator and an oscilloscope or an LCR multimeter. 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.
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. It can be also hard to separate ESR from reactive value at low frequencies unless a dc resistance measurement is also taken. The DC resistance itself is easy to measure separately with a multimeter resistance measurement range.
RLC meter instruments
LCRs are special meters designed for inductance measurements and containing the required circuitry. These are costly tools.
A basic LCR meter is very similar to a multimeter. With the RLC measurement electrical components can be measured in detail.
determine resistance (R), inductance (L) and capacitance (C).
To measure the inductance of a device, intrinsic inductance of a circuit or more widespread distributed inductance, an LCR meter is the instrument of choice. It subjects the device under test (suitably discharged and isolated from any ambient circuitry that could energize it or create irrelevant parallel impedance) to an ac voltage of known frequency, typically one volt RMS at one kilohertz.
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. The cheaper ones are accurate enough, but don’t distinguish resistance from inductance when measuring coils, so you need to know both to get an accurate reading. Like all things, you get what you pay for.
There are also more advanced meters that simultaneously measures the voltage across and current through the device. From the ratio of these amounts it algebraically calculates the impedance. The measurement results from programming (stimulus) an AC voltage/current with a given frequency and the measurement of the voltage curve and the current profile.
Subsequently, advanced meters measure the phase angle between the applied voltage and resulting current. The simultaneous determination of current (I) and voltage (U) in short intervals over a period of several intervals allows for the determination of the phase angle between U and I.
They use this information to display the equivalent capacitance, inductance, and resistance of the device in question. The meter operates under the assumption that the capacitance and inductance it detects exist in either a parallel or series configuration.
Multi component testers
There are many cheap tester circuit boards, DIY kits and ready made instruments marketed like $20 LCR ESR Transistor checker. Those instruments can measure transistors, resistors, capacitors and inductors. You can but those from online market, and their capacitance meter usually also ESR meter built in.
For capacitance, inductance and resistance measurements I have successfully used component testers like this:
They are good for many things, but have limitations also – do not always give accurate inductance reading with some coils that use large iron or ferrite core. The reason is that those testers typically measure the inductance with a short measurement pulse that can behave somewhat differently with magnetic core than continuous waveform sent by specific RLC meters.
Audio measurement instruments
There are audio measurement tools that are designed to measure different audio system components like speaker elements and passive components. For example DATS V3 can measure the exact values of passive components like resistors, capacitors and inductors.
If you are a DIY person, it is also possible to measure RLC parameters using a sound card. You will need some inexpensive DIY hardware and suitable software. Check for example 2-Pound RLC meter article and Sound Card LCR Meter Mini-App with ESR, DF, and Q web pages.
Vector network analyzer
Vector Network Analyzers are used to test component specifications and verify design simulations to make sure systems and their components work properly together. they are typically used to verify radio frequency circuit component and wiring impedance matching. R&D engineers and manufacturing test engineers commonly use VNAs at various stages of product development. Vector Network Analyzer (VNA) helps measure both phase and magnitude related measurements. They are primarily designed to measure system impedance matching, but many VNAs can also measure resistance, capacitance and impedance values over certain range.
A Vector Network Analyzer contains both a source, used to generate a known stimulus signal, and a set of receivers, used to determine changes to this stimulus caused by the device-under-test or DUT. They have used to to be very expensive special instruments only found on advanced RF and high speed electronics circuit design laboratories.
But over last few years, cheap VNAs have come to the market. One of the most commonly used hobbyist budget VNA is called NanoVNA, which is available on many version starting at well below 100 US dollars. You can use such a Vector Network Analyzer (VNA) to measure and understand RLC parasitics in radio / wireless design and construction. it can be used to measure resistors, capacitors, and inductors and their non-ideal behaviors at radio and even microwave frequencies. I have tested first NanoVNA and S-A-A-2 NanoVNA V2. I found both to be useful for measuring RLC values in RF circuits (meaning quite low inductance and capacitance values).
Here are links to a few videos how to do RLC measurements with NanoVNA:
Keep in mind that NanoVNA uses RF frequencies to measure the RLC components. The shunt method is most accurate within a range of impedances or capacitance. The impedance seen by VNA is a function of frequency, thus as the frequency of measurement moves further away from the ideal the measurement will be less and less accurate. This is one reason (not the only reason) the observed inductance changes as you go up in frequency. Stray capacitance in the test fixture, between the coils of the inductor, as well as proximity effect and skin effect… all of these things are frequency dependant and will play a role in determining the behaviour of your inductor at different frequencies while being measured with the nanoVNA. If one wishes to simply obtain a “marked value” measurement of a given inductor, a relatively crude test fixture can work.
For accurate result it is best to perform the characterization the component at the frequency the component is designed to be used, user proper test jug and do good calibration of VNA. It all depends on what your goals are.