last updated: 15/12/18
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.
In the next chapter we will meet two important nonlinear electronic components, the diode and the transistor.
The capacitor uses an electric field and the inductor a magnetic field.
Source: Graphic Eric Schrader under Creative Commons Attribution-ShareAlike 2.0 Generic license(CC BY-SA 2.0).
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:
For polarized capacitors, the American symbol has a curved plate for the negative side.
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 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.
A parallel-plate capacitor, consisting of two conductive plates insulated from each other (see symbol) is a good model to analyze capacitance. One can verify 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 a 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:
ɛ0 = 8.854187817·10−12 F/m.
The capacitance of a parallel plated capacitor can be calculated with :
C = ɛ·A/d = ɛ0·ɛr·A/d
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.
1F = 1C/1V = 1A·1s/1V = As/V.
This gives us another formula for the capacitance:
C = Q/U
If we want to calculate the charge Q:
Q = C·U
Newer multi-meter often have the possibility to measure the capacitance.
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 mica 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°.
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 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 VCC). When it the switch connects to ground, the capacitor discharges through the resistor.
The charging and discharging follows an exponential curve. The formulas are for charging:
u(t) = U(1-e(-t/τ))
and for discharging:
u(t) = U(e(-t/τ))
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:
F·Ω = (As/V)·Ω = (s/Ω)·Ω = s
τ(tau). As shown the unit is seconds.
τ = R·C
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 2τ charges a capacitor to 85 %, and
5τ to 100 %.
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 witch 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 !!
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.
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.
The circuit we used to charge and discharge a capacitor (RC in series) is called an RC low-pass filter.
Let's see why.
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:
We distinguish active filters (containing amplifying devices) and passive filters. 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).
A passive low-pass filter is a combination of capacitance, inductance or resistance. The goal 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
The cutoff frequency of a low-pass filter is the frequency at which the power output has fallen to one half the pass-band power. This is the case when the voltage drops to
0.707 (70.7 %) (when the voltage drops to 1/√2 = 0.707 on a resistance, Ohm's law says that also the current drops to 0.707:
P' = 0.707U·0.707I = 0.5P). This frequency is also called the
3 dB point (see half-power point).
To calculate the cutoff frequency of our RC low-pass we can use the RC time constant,
τ = RC = 1/(2πfc)
fc = 1/(2πτ) = 1/(2πRC)
Example for an RC low-pass filter with C = 2.2 µF and R = 22 Ω:
fc = 1/(2π·2.2·10-6F·22Ω) = 3.29kHz.
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 100nF 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).
Next we will see a new component, that is similar but almost the opposite of a capacitor.
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: