From the Ground Up
Eric Edwards GW8LJJ explains the basics of capacitance, in the first of two parts.
Eric Edwards GW8LJJ explains the basics of capacitance, in the first of two parts.
Capacitors along with resistors are the most used components in an electronic circuit. A capacitor is a two-terminal electrical device used to store electrical energy in the form of an electric field between the two plates, Fig. 1. The unit of capacitance is the Farad (F) and is named after the English physicist Michael Faraday (1791 – 1867). The Farad measures how much electric charge is accumulated on the capacitor. One Farad is the capacitance of a capacitor that has a charge of one coulomb (C) when applied a voltage drop of one volt. The coulomb is the amount of electric charge (Q) by a constant electric current of one ampere flowing for one second. The higher the capacity, the higher is the amount of (DC) electricity a capacitor can hold.
Capacitors are used in circuits for many different purposes and are common components of filters, oscillators, power supplies, amplifiers and other electrical and electronic circuits. They come in various shapes and sizes, depending on their capacity, working voltage, type of insulation, temperature coefficient and other factors and their capacitance values can be fixed or variable. The photo, Fig. 2, shows a selection of capacitors, while the symbols for capacitors are shown in Fig. 3.
Capacitor or Condenser
We use the term capacitor when referring to capacitance but condenser was the name that many old timers (and I am one) used when describing the component. The name came from the year 1746 when the capacitor (condenser) was invented by a Dutch scientist, Van Mussenbrock. There were others and as with all inventions and discoveries there is always more than one person experimenting and developing alongside and from the works of others. The original condenser was a Leyden (Leiden) Jar, which stores a high-voltage electric charge (from an external source) between electrical conductors (foil) attached on the walls of the inside and outside of a glass jar. A terminal in the jar lid makes contact with the inner foil. Electricity, which was ‘condensed’ in the jar, was capable of storing charge in a very small space and consequently was named condenser. There are microphones that are still called condenser types but generally the term capacitor is used for the actual component. Many of you will have replaced the points and condenser in your petrol car!
The Capacitor at DC
A capacitor is similar to a battery, but whereas a battery generates energy, a capacitor is a much simpler device that can’t produce new electrons but stores them. Unlike a battery, it does not use a chemical reaction and can only hold a very small charge. Inside the capacitor the terminals are connected with two metal plates separated by a dielectric material (such as paper (waxed), mica, ceramic, air, gas and other insulating materials). These separate the plates and allow them to hold opposite electrical charges maintaining an electrical field. Capacitors can be useful for storing charge and quickly discharging into a load. A large (several Farads) capacitor also works as a small rechargeable battery and is used as a temporary ‘battery’ for maintaining the data in EEPROMs. A very large capacitor can only light up an LED for a few seconds because of the current (milliamps) required by the LED, but can supply a backup voltage for some considerable time as the EEPROM draws very little current (nanoamps).
Charging a Capacitor
The term ‘charging’ simply means the transfer of electrons from a battery or other power source to the plates of a capacitor. The positive terminal of the battery is connected via its connecting leads (for through-hole capacitors) to one plate of the capacitor. If a polarising capacitor is used, such as an electrolytic type, the positive of the battery must be connected to the positive connection of the capacitor. The negative terminal of the battery is connected to the other plate (connection) of the capacitor, Fig. 4.
Most capacitors are of the non-polarising types but usually larger capacitor values (1μF, 10μF, 100μF, 1000μF and so on) are polarised and are either electrolytic or tantalum types. It is very important to observe the polarity of these as they can get hot and even explode if incorrectly terminated, especially the tantalum types. The voltage rating of all capacitors must also be observed otherwise that could also be destructive. When using capacitors, the safe voltage rating is about 50% above the voltage applied to the capacitor. If a capacitor is used to decouple (place across) a 12V DC supply, the voltage rating should be 18V but a higher voltage working such as 22V will also be suitable. This applies for non-polarised as well polarised capacitors.
How it Works
Whenever voltage is applied across capacitor plates (also known as charging of a capacitor), current starts to flow and continues to do so until the voltage across both the plates becomes equal to the voltage of the source (applied voltage). The two capacitor plates are separated by a dielectric material, which is an insulator, so no current can pass through it. The dielectric is also used to increase the capacitance of the capacitor. The thickness or amount of the dielectric plays an important part in the capacitance along with the size of the capacitor plates. Capacitance increases with the size of the plates, the gap between them and the nature of the dielectric.
When a DC voltage is applied across the plates of a capacitor, the current is maximum at first but as the voltage across the capacitor plates rises to the same level as the applied voltage, the current decreases, to zero once the voltages equalise. Thus, voltage across a capacitor lags current.
The Capacitor at AC
Immediately an AC (Alternating Current) waveform is applied to the plates of a capacitor, maximum current will be flowing, and minimum voltage will be across the capacitor as when applying a DC voltage. Let’s consider an AC waveform consisting of the alternate reversal of a DC supply (battery). The battery and capacitor with a reversal switch is shown at Fig. 5. The battery is connected to the common contacts on a DPDT (Double Pole Double Throw) switch. The capacitor leads are connected to the contacts that are made
when the switch is ‘thrown’ (placed) in the up position, which puts the battery positive on the left-hand plate of the capacitor with the negative from the battery on the right-hand plate of the capacitor. When the switch is placed in the down position it reverses the polarities of the battery on the capacitor. The capacitor first charges up with the electrons flowing from the negative terminal of the battery to the right-hand plate and stops when the voltage across the capacitor is equal to the battery voltage. If the switch is reversed the electrons now travel from the negative terminal of the battery to the left-hand plate of the capacitor. When the power supply is removed, it will show the voltage across the capacitor plates. The capacitor is now charged and will remain until discharged. However, there is no perfect insulator and the capacitor will discharge over time because of leakage in the dielectric (insulator).
See the Current Peak
The effect can also be seen with an ammeter placed in series with the battery lead and the capacitor plate. As soon as the battery voltage is applied, the pointer on the ammeter peaks then goes back to zero showing that current (very fast) was sent from the battery negative lead to the capacitor plate. Using two AVOs, Fig. 6, or any analogue multimeter or ammeter set to DC100mA (0.1A) and the battery (DC power supply) to 12V (or 13.8V) placed in series with both capacitor leads, when the power supply is switched on or battery connected, the pointers on both of the meters will indicate a sharp increase in current then return to zero.
The power supply positive lead is connected to the one AVO DC+ terminal and the positive lead on the electrolytic capacitor is connected to the negative DC− terminal of the same AVO. The power supply negative lead is connected to the DC− terminal on the second AVO and the DC+ terminal connected to the negative lead on the capacitor. This indicates that although current does not pass through the capacitor, it flows through anything in its path. In this case the two AVOs. But once the capacitor is charged no more current flows and the capacitor stays at the voltage it has reached. The AVOs shows the surge of current and if a scope were connected across the capacitor (the earth lead of the scope connected to the negative terminal of the battery) with DC selected on the scope and the probe connected to the positive side of the capacitor, with the power supply removed the scope will display the voltage held in the capacitor.
Sinewave
If we apply a sinewave (such as our UK mains, which is at 50Hz, or frequencies within the audio spectrum), the current will reduce as the capacitor charges up while the voltage on the capacitor plates will increase. When the voltage is at maximum, the current will have reached minimum. This is effectively a pair of sinewaves (one voltage, one current), 90° out of phase alternating minimum and maximum. The current first as it charges the capacitor then it is followed by the voltage as the current flow to the plate of the capacitor reduces.
Push Pull
Or perhaps that should ‘push push’. The voltage increases quickly and the electric (force) field strength in the dielectric of the capacitor is changing quickly. As the field gets stronger, it pushes more electrons out of the positive plate (due to increasing electric force on them created by the field). A capacitor is an open circuit and current does not flow through a capacitor but to or from one plate or the other. This causes an electric field to build in the dielectric, which affects the free electrons on the other plate via electric force. The capacitor plates are of conductive metal, so lots of free electrons exist in them. The voltage difference between plates, generated by the sinewave source, will push free electrons from the negative side of the source onto the plate it is connected to. This builds an electric field within the dielectric of the capacitor such that electrons are pushed by the
electric force out of the opposite plate. The circuit carries them back to the positive leg of the source supply.
The force field acts like a flexible barrier, Fig. 7. The direction of the lines of force shown in the dielectric in the diagram shows electrons being repelled from one plate making it positive. This results in storage of charge. The electric field distorts the molecular structure so that the dielectric is no longer neutral but is stressed by the electric field force. This results in a charge in the dielectric. When the phase of the sinewave changes, the lines of force are reversed, the dielectric is once again distorted and a charge is held but in the opposite polarity. This alternation of the field force and distortion of the dielectric pushes out the electrons from the opposite plates and gives the impression that current flows through the capacitor.
Elastic Barrier
To reiterate, the electrons on the negative capacitor plate try to cross to the other plate connected to the battery positive terminal and fill in the gaps left by the depletion of electrons (they were attracted to the positive terminal of the battery) and as they try to get across, a force field is created that is like an elastic barrier as it bulges out to reach the other plate. They cannot get there because of the insulation but in doing so they ‘push’ out more electrons from the positive plate. As more and more charge is pushed into the negative plate, the field grows stronger and more electrons are pushed off the other plate. However, since the rate of change of voltage is slowing as we reach maximum voltage (at 90°), our field strength is still increasing, but more slowly all the time. For that reason, fewer and fewer electrons are pushed off the positive plates so the current flow is getting smaller. At the point of maximum voltage, the rate of voltage change is zero, so there are zero more electrons being pushed off that positive plate. At that point the voltage begins to fall, and the field weakens. This allows some of the pushedout electrons from the positive plate to come back into it. As the voltage rate of change accelerates and the voltage falls back toward zero, the rate at which electrons return to the positive plate accelerates (current rises). When the voltage is at zero, it’s changing at its maximum rate, so you have maximum current flow in the circuit (electrons are coming back to the plate as fast as they ever will for this circuit). The other half of the waveform (negative lobe of the voltage sinusoid) is the same, but switches the plates from negative to positive.
(To be continued in part 2)