Tutorials: Electronics fundamentals (ELEFU)

Capacitor C and Inductor L

last updated: 2022-01-30

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Song of this chapter: "The capacitor song" (teacherhaggis on youtube (there's also an inductor song :))

The three elementary linear electronic components are the resistor R, the Capacitor C and Inductor L. Circuits with only linear electronic components are called linear circuits. Their characteristic is that for a sinusoidal input voltage of frequency f, any output of the circuit is also sinusoidal with frequency f. Linear circuits are easier to understand and analyze (they obey the superposition principle) than non-linear circuits.

Important nonlinear electronic components are the diode and the transistor. They will be treated in the next chapter.

The capacitor uses an electric field and the inductor a magnetic field.

Capacitor (condenser) (wiki)


A capacitor is a passive two-terminal component storing electrical energy in an electric field.

Physically, a capacitor is made of two conductors separated by an insulator referred to as a dielectric. This can also be seen in his symbol.

Symbol of a capacitor, a polarized capacitor and a variable capacitor:

symbol capacitorsymbol capacitor polarizedsymbol capacitor variable

For polarized capacitors, the American symbol has a curved plate for the negative side.

"Just do it" C1:

Capacitance C (wiki)

As a resistor has a resistance, the capacitor has a capacitance C (C is used as symbol for the capacitance and also as name for a capacitor). Capacitance is the ability to store energy in the form of an electric field. Capacitance stores charge (electrons).

You know the effect of a capacitor from real world. When you touch the door of a car, you get a slight electric shock. You car is a capacitor. One conductor is the car itself, the tires are the insulator and earth is the second conductor. By friction the car can be charged (acquire a surplus of electrons) to some thousand volt, and when you (a resistance ;)) touch the car a minimal current can flow.

Capacitance exists everywhere, but is mostly very small. The capacitor is a component designed to add specific capacitance to a circuit and can have very high values.

symbol capacitor

A parallel-plate capacitor, consisting of two conductive plates insulated from each other is a good model to analyse the capacitance. One can conclude by reasoning and then verify in experiments that the capacitance is nearly proportional to the surface area of the conductor plates A and inversely proportional to the separation distance between the plates d.

A third factor is the influence of the material between the plates. This influence is reflected by the permittivity ɛ = ɛ0·ɛr of the insulating (dielectric) material. A part of the permittivity is the relative permittivity ɛr, a material property. ɛ0 is the vacuum permittivity an baseline physical constant:

formula vacuum permittivity

The permittivity consists of the vacuum permittivity multiplied with the relative permittivity:

formula permittivity

The capacitance of a parallel plated capacitor can be calculated with :

formula C

The symbol for the capacitance is C (not to confound with the unit of the charge Q, the Coulomb C), the SI derived unit is the farad F.

A capacitor of 1 farad, if charged with 1 coulomb of electrical charge, has a potential difference of 1 volt between its plates.

formula farad

This gives us another formula for the capacitance:

formula C=Q/U

If we want to calculate the charge Q:

formula farad

"Just do it" C2:

Newer multimeter often have the possibility to measure the capacitance.

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Breakdown voltage and temperature (wiki)

The voltage marked on a capacitor is the breakdown voltage of the used dielectric. The breakdown voltage is the maximum voltage that can be applied to an insulator before it conducts. When a current flows through an insulator this is called an electrical breakdown.

Even in (near) vacuum we get a breakdown voltage (vacuum arc). The breakdown voltage of air is about 3 kV/mm ; for Mylar its value is about 430 kV/mm. so the thinner the insulator, the lesser the voltage that can be applied to the capacitor. Normally one uses a capacitor with a marked voltage that is clearly higher than the marked voltage.

Polarized capacitors have a liquid inside and are aging quicker at high temperature. The maximum temperature is marked on the capacitor. For hotter environments it's better to use capacitors with 105° instead of the standard 85°.

Dangers when working with capacitors

A breakdown in a (mostly polarized) capacitor can generate heat that causes the liquid to evaporate very quickly. As the capacitor is sealed, it can turn into an explosive. This is especially true for older capacitors. New capacitors have a special designed top that allows them to vent instead of bursting violently. Defective capacitors have a swollen top.

Capacitors may retain a charge long after power is removed from a circuit!! This charge can cause fatal shocks. When you open a seemingly innocuous camera with flash unit, the flash unit has a capacitor which may contain over 15J of energy and be charged to over 300 volts!! So always discharge capacitors before working on a circuit.

Charging and discharging

Charging a capacitor means accumulating a charge on its conductors. The formula Q = C·U shows that U~Q. The voltage is increasing while charging and we are moving charge (electrons) onto one of the capacitors conductors. This is a current, and we can control with the help of a resistor how quickly the charge moves to the capacitor’s conductor.

Here is the typical circuit to charge and discharge a capacitor:


The capacitor charges through the resistor (switch to power supply (1)). When it the switch connects to ground, the capacitor discharges through the resistor.

"Just do it" C4:

RC time constant (wiki)

The charging and discharging follows an exponential curve. The formulas for charging:

formula charging

and for discharging:

formula discharging

When using different capacitors and resistors we see that the time of charging and discharging changes. By multiplying the value of the resistor with the value of the capacitor we get a time!!

Let's look at the units:

formula tau unit

This time is called RC time constant, τ (tau). As shown the unit is seconds.

formula tau

The RC time constant is the time required to charge a capacitor, through a resistor, from 0 V to 63 % of the value of an applied DC voltage, or to discharge the capacitor through the same resistor to 37 % of its initial charge voltage. This can be calculated with the above formulas.

The time of charges a capacitor to 85%, and to 100%.

To charge or discharge a capacitor we need the time of or 5·R·C

Charging and discharging diagram


Measuring current with an oscilloscope

Whats about the current? The only way to see a current is to measure the voltage with an oscilloscope on a resistor in the circuit. Fortunately we have already a resistor in our circuit, so we don't need to add a shunt. The ground of the 2 oscilloscope channels are connected together! Also most grounds of oscilloscopes are connected to the housing and to earth of the power cord. The same is true for the ground of the generator. So it is here not possible to use the oscilloscope like a voltmeter in parallel to the resistor, because we will have a short circuit which can be pretty dangerous when measuring in circuits working with mains voltage (230 V).

!! If you use an oscilloscope all grounds have to be connected together !!

"Just do it" C5:

measuring current with an oscilloscope

Combining capacitors

As seen above, the bigger the surface area of the conductors, the higher the capacitance. If we connect capacitors in parallel, the conductors are combined and we are getting a larger effective surface area. The capacitance gets bigger by connecting capacitors in parallel. With the formula of the capacitance we can show, that the total capacitance is equal to the sum of the individual capacitance's.

When we connect capacitors in series, we are effectively combining the thicknesses of their insulators. A bigger thickness results in a lower capacitance!With the formula of the capacitance we can show, that the total capacitance is the reciprocal of the sum of the reciprocals of the individual capacitance's.

In a parallel circuit of capacitors:

In a parallel circuit of capacitors the total capacitance is equal to the sum of the individual capacitance's.

formula C parallel

In a series circuit of capacitors:

In a series circuit of capacitors capacitance is the reciprocal of the sum of the reciprocals of the individual capacitance's.

formula C series

"Just do it" C6:


Low-pass filter (LPF) (wiki)

The circuit we used to charge and discharge a capacitor (RC in series) is called an RC low-pass filter.

low pass

Let's see why.

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Ideal filters

Filters are named according to the frequency range of signals that passes through them. The ideal amplitude response characteristics of the four basic filter types is shown in the following image:

ideal filters

We distinguish active filters (containing amplifying devices) and passive filters. A passive filter is a combination of two or more of the three basic linear elements resistors, capacitors and inductors. The RC-filter is a passive filter with two passive components. The output signal has a smaller amplitude than its corresponding input signal, so the passive RC-filters attenuate the signal (gain less than one).

Cutoff frequency (wiki)

The goal of our low-pass filter is to get a high attenuation above a specified frequency and little or no attenuation below that frequency. The specified frequency at which the transition occurs is called the cutoff frequency fc.

The cutoff frequency of a low-pass filter is the frequency at which the output power has fallen to one half the input power. This is the case when the output voltage drops to 0.707 (1/√2, 70.7 %) of the input voltage!

When the voltage drops to 70.7 % of it's value on a resistance, Ohm's law says that the current drops also to 70.7 % of its value. This explains the drop to 50 % for the power:

formula P cutoff

This cutoff frequency is also called the 3 dB point (see half-power point).

To calculate the cutoff frequency of our RC low-pass we meet again the RC time constant τ:

tau cutoff

And so we get the formula for the cutoff frequency:

f cutoff

Example for an RC low-pass filter with C = 2.2 µF and R = 22 Ω:

f cutoff example


For the frequency in relation to the cutoff frequency a logarithmic scale is used.

Order of a filter

As seen the filter is characterized by its cutoff frequency (attenuation of the input power by half or 3 dB), but also by the rate of frequency rolloff. The order of a filter determines the amount of additional attenuation for frequencies higher than the cutoff frequency.

lin filter_response 1-3 order

The filter frequency response is generally represented using a Bode plot. In this plot the gain (Uout/Uin) is represented in decibel giving us a diagram with two logarithmic scales.

log filter_response

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The capacitor has the ability to pass an AC voltage but to block DC voltage. This is very useful, especially in the world of analogue and audio electronics. Often 100 nF capacitors are used on the power lines in a circuit to bypass (shunting) high frequency noise to ground (bypass capacitors). Large capacitors on power lines near regulators are used to smooth slower changes, and to deliver high currents for a short time. Capacitors in combination with resistors are often the basis for timing and oscillator circuits (changing time constant). Capacitors in combination with resistors and inductors are used as filters (see above).

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Next we will see a new component, that is very similar but almost the mirror of a capacitor.

Inductor (wiki)

These chapter on inductors is short. Why?

An inductor, (coil) is a passive two-terminal component storing electrical energy in an magnetic field when electric current flows through it.

Physically, an inductor is made of an insulated wire wound into a coil around a core.


Symbol of an inductor, an inductor with magnetic core and a variable inductor with magnetic core:

symbol inductorsymbol inductor w. coresymbol inductor core variable

Capacitance is the ability to store energy in the form of an electric field. The inductance L of an inductor is the ability to store energy in the form of a magnetic field.

The symbol for the inductance is L, the SI derived unit is the henri H.

The basic functionality of an inductor is equivalent to that of a capacitor if you swap current and voltage.

"Just do it" L1: