Monday, 27 August 2018

I2C Voltage Level Translators

I2C (Inter-Integrated Circuit) is a multi-master to multi-slave two-wire serial bus standard that enables serial…

I2C (Inter-Integrated Circuit) is a multi-master to multi-slave two-wire serial bus standard that enables serial communications at a number of bit rates (depending upon supported mode) over several meters of cable. I2C is a relatively old, but still extremely popular standard that arrived on the scene in 1982.
Since then, 5V logic interfaces have given way to much lower voltage standards and I2C bus systems need to accommodate for level shifting to enable intercommunications with different interface voltage devices on the same bus. Communication data rates have also increased from the original 100 kHz clock rate to up to 5MHz in ultra-fast modes, and level shifting must be able to accommodate for these higher data rates.
I2C signaling comprises a single data signal that carries logic levels validated by a clock signal. Both signals are bidirectional, driven by one or more masters or slaves depending on the state of the system. In order to prevent damaging contention caused by outputs driving each other, open drain or open collector outputs with pull up resistors drive the bus.
Figure 1: Open Collector Outputs Prevent Contention
Figure 1: Open Collector Outputs Prevent Contention
An open drain (FET based) or open collector (BJT based) output is one that does not have an active gate pulling up to the positive supply. Turning the output gate off places the output in a high impedance state. The signal wire floats high as the charge stored by the bus capacitance discharges through the pull up resistance as shown in figure 1.
The speed of a transition between low to high on the signal bus is therefore limited by the time it takes for the electric charge stored on the parasitic capacitance on the wire ‘C’ to discharge through the pull-up resistance ‘R’, the 3dB point being defined by equation: f3dB= 1/(2π.R.C)
For example, if the combined capacitive bus loading is 100pF, and combined pull-up resistance on the signals on the bus being 1.5kΩ, the 3dB point is approximately 1MHz. This limits the maximum data rate possible on the bus but has the clear benefit of never having a situation where low impedance outputs drive each other, causing damaging high currents to flow through the gate. The bidirectional nature of I2C signals, the variation of bus voltage levels and the open drain output requirements considerably complicate level translation circuitry.

Logic Levels Compatibility

Different logic levels have different logic threshold voltages that determine the high/low logic transition. Different voltage standards also cause issues due to the presence of diode junctions on input and outputs of semiconductor devices. Signal levels driven higher than the supply voltage force the diode into conduction in a destructive condition called “latch-up”.
I2C has traditionally complied with 5V logic standards, but potentially needs to work with many other lower voltage interface standards. Even when the interface standard thresholds and regions seem within acceptable ranges, reduced noise margins compromise bus performance without proper logic level translation.
Figure 2: Typical Logic Family Voltage Levels
Figure 2: Typical Logic Family Voltage Levels
A bus system must be compatible with the worst-case combination of logic levels relating to the individual devices over the device variation and anticipated temperature range. Make note that each device on the common bus increases capacitance and may further reduce margins. Single-ended buses, especially ones with long wires, are subject to impedances on the ground or supply that cause edge ringing and ground bounce – effects that further reduce level margins.
The use of a bidirectional buffer may not specifically offer different voltage translation, but it can help maintain voltage tolerances where the signal levels are compatible between standards. I2C Repeaters also provide a way to isolate the capacitive load of a bus by splitting it into two, extending cable runs or number of devices supportable in a system. I2C interfaces are very tolerant of long cable runs due to the low data rate, 5V signalling support and the open drain nature. These characteristics lead it to use in situations where devices plugged in and out of the serial bus add complexity of level translation circuitry and exposed connections add the requirement for electrostatic discharge (ESD) protection.

Discrete MOSFET Method of Level Translation

Bidirectional level shifting must conduct in both directions. The simplest method uses MOSFETs as depicted in figure 3, and described in more detail in application note AN10441. Although a MOSFET level shifter is acceptable for 100kHz to 400kHz communications standards, it does not have the performance required for faster modes of operation due to the previously mentioned frequency limitations imposed by RC time constants. In cases where higher bandwidth modes are used, special purpose level translator devices are required.
Figure 3: MOSFET Based Level Translation
Figure 3: MOSFET Based Level Translation
I2C interfaces have positive going edges limited by this effect. The MOSFET circuit introduces additional parasitic capacitances (for example the Miller effect) and switching levels that further distort the characteristics of both edges during direction changes. This can cause problems if the logic interface is marginal as they have a transition region around the threshold voltage where the output is indeterminate.
Signals that reside inside this range cause what is termed ‘meta-stable states’ that cause signal glitching. Schmitt trigger inputs are special interfaces with voltage hysteresis caused by positive feedback from the input gate output. This increases noise margin and reduces the potential for metastability due to slowly changing signals.
Also, noisy, slow or non-monotonic signals, can exist on I2C serial buses are also corrected using a debounce algorithm. A debounce algorithm introduces a dwell time where the input signal is monitored over a time period for a continuous signal level before changing, rejecting glitch transitions that fall within this period. This is a technique used with FPGAs and ‘bit-banging’ software implementations on microcontrollers.

Isolated Methods

A system as described in figure 4 is a non-isolated system with direct electrical connection between the two devices, irrespective of the voltage level translation. As I2C interfaces are unbalanced and can have long cable interconnects, these potential differences can lead to spurious noise currents on ground signals. Non-isolated grounds can lead to dangerous fault conditions when dealing with high voltage circuits. I2C interfaces used in applications that cross isolation boundaries have to meet safety standards and require galvanic isolation.
Figure 4: Non-Isolated Level Translation
Figure 4: Non-Isolated Level Translation
The common method of electrically isolating I2C buses also incorporates level translation as the isolation boundary and enables any voltage support on either side of the isolation boundary. Conventional isolation techniques include photovoltaic isolation via opto-couplers or devices that modulate serial bus signal information across a capacitive bridge or inductive transformer.
All these tend to share the common characteristic of splitting/recombining the bidirectional signals into two unidirectional paths. Some applications extend the length of cable supported and increase noise immunity by converting the signals to a differential standard (like RS485). They may also utilize higher signalling voltages.
Figure 5: Multi-drop Isolated I2C with Independent Voltage Levels
Figure 5: Multi-drop Isolated I2C with Independent Voltage Levels
Opto-electrical isolation of the I2C-bus, like the circuit described in figure 5, is capable of operating at speeds in excess of 1Mbps. NXP’s application note AN10364 is a comprehensive description of various topologies for interfacing between I2C devices with different local ground potentials and voltages. Similar topologies are found in applications that require primary to secondary side communications in isolated AC mains power circuits, medical equipment and Power of Ethernet (PoE).

Bidirectional Voltage Level Translators

Bidirectional voltage translators can work on an I2C bus. It is important to ensure the device is the correct bus orientation – matching the bus voltage level to the correct translator side. Most translators will specify one side to be greater than a minimum voltage (typically around 1V) than the other side. Both sides will still require pull-ups for correct I2C operation as described in application note AN11127.
Figure 6: Bidirectional Level Translator in I2C Circuit
Figure 6: Bidirectional Level Translator in I2C Circuit
Such devices are excellent for supporting I2C connection from 5V and 3V3 slave peripherals to low voltage I/O banks of FPGAs or microcontrollers. Dual open drain bidirectional voltage level translator devices such as the NVT2001 simplify multi-voltage bus architectures due to the low component count, high flexibility of solution, integrated ESD protection and the tiny physical packages.
They also serve to provide isolation between slower and faster elements on an I2C bus, enabling them to interoperate in the same design. Controlling the enable pin to disconnect slower buses during fast mode communications provides for coexistence. These devices feature less than 1.5ns propagation delay (not including bus loading) and 33MHz clock performance in an open drain system. High frequency support enable solutions for ultra-fast modes of operation to incorporate level shifting and raises the possibility of even higher speed I2C interface standards in future.

Combinational Logic

Combinational Logic Circuits

Unlike Sequential Logic Circuits whose outputs are dependant on both their present inputs and their previous output state giving them some form of Memory.

The outputs of Combinational Logic Circuits are only determined by the logical function of their current input state, logic “0” or logic “1”, at any given instant in time.
The result is that combinational logic circuits have no feedback, and any changes to the signals being applied to their inputs will immediately have an effect at the output. In other words, in a Combinational Logic Circuit, the output is dependant at all times on the combination of its inputs. So if one of its inputs condition changes state, from 0-1 or 1-0, so too will the resulting output as by default combinational logic circuits have “no memory”, “timing” or “feedback loops” within their design.

Combinational Logic

combinational logic circuits
 
Combinational Logic Circuits are made up from basic logic NAND, NOR or NOT gates that are “combined” or connected together to produce more complicated switching circuits. These logic gates are the building blocks of combinational logic circuits. An example of a combinational circuit is a decoder, which converts the binary code data present at its input into a number of different output lines, one at a time producing an equivalent decimal code at its output.
Combinational logic circuits can be very simple or very complicated and any combinational circuit can be implemented with only NAND and NOR gates as these are classed as “universal” gates.
The three main ways of specifying the function of a combinational logic circuit are:
  • 1. Boolean Algebra – This forms the algebraic expression showing the operation of the logic circuit for each input variable either True or False that results in a logic “1” output.
  • 2. Truth Table – A truth table defines the function of a logic gate by providing a concise list that shows all the output states in tabular form for each possible combination of input variable that the gate could encounter.
  • 3. Logic Diagram – This is a graphical representation of a logic circuit that shows the wiring and connections of each individual logic gate, represented by a specific graphical symbol, that implements the logic circuit.
and all three of these logic circuit representations are shown below.
combinational logic
 
As combinational logic circuits are made up from individual logic gates only, they can also be considered as “decision making circuits” and combinational logic is about combining logic gates together to process two or more signals in order to produce at least one output signal according to the logical function of each logic gate. Common combinational circuits made up from individual logic gates that carry out a desired application includeMultiplexersDe-multiplexersEncodersDecodersFull and Half Adders etc.

Classification of Combinational Logic

combination logic circuit
 
One of the most common uses of combinational logic is in Multiplexer and De-multiplexer type circuits. Here, multiple inputs or outputs are connected to a common signal line and logic gates are used to decode an address to select a single data input or output switch.
A multiplexer consist of two separate components, a logic decoder and some solid state switches, but before we can discuss multiplexers, decoders and de-multiplexers in more detail we first need to understand how these devices use these “solid state switches” in their design.

Solid State Switches

Standard TTL logic devices made up from Transistors can only pass signal currents in one direction only making them “uni-directional” devices and poor imitations of conventional electro-mechanical switches or relays. However, some CMOS switching devices made up from FET’s act as near perfect “bi-directional” switches making them ideal for use as solid state switches.
Solid state switches come in a variety of different types and ratings, and there are many different applications for using solid state switches. They can basically be sub-divided into 3 different main groups for switching applications and in this combinational logic section we will only look at the Analogue type of switch but the principal is the same for all types including digital.

Solid State Switch Applications

  • Analogue Switches – Used in Data Switching and Communications, Video and Audio Signal Switching, Instrumentation and Process Control Circuits …etc.
  • Digital Switches – High Speed Data Transmission, Switching and Signal Routing, Ethernet, LAN’s, USB and Serial Transmissions …etc.
  • Power Switches – Power Supplies and General “Standby Power” Switching Applications, Switching of Larger Voltages and Currents …etc.

Analogue Bilateral Switches

Analogue or “Analog” switches are those types that are used to switch data or signal currents when they are in their “ON” state and block them when they are in their “OFF” state. The rapid switching between the “ON” and the “OFF” state is usually controlled by a digital signal applied to the control gate of the switch. An ideal analogue switch has zero resistance when “ON” (or closed), and infinite resistance when “OFF” (or open) and switches with RON values of less than  are commonly available.

Solid State Analogue Switch

analogue switch
 
By connecting an N-channel MOSFET in parallel with a P-channel MOSFET allows signals to pass in either direction making it a Bi-directional switch and as to whether the N-channel or the P-channel device carries more signal current will depend upon the ratio between the input to the output voltage. The two MOSFET’s are switched “ON” or “OFF” by two internal non-inverting and inverting amplifiers.

Contact Types

Just like mechanical switches, analogue switches come in a variety of forms or contact types, depending on the number of “poles” and “throws” they offer. Thus, terms such as “SPST” (single-pole single throw) and “SPDT” (single-pole double-throw) also apply to solid state analogue switches with “make-before-break” and “break-before-make” configurations available.

Analogue Switch Types

analogue switch types
 
Individual analogue switches can be grouped together into standard IC packages to form devices with multiple switching configurations of SPST (single-pole single-throw) and SPDT (single-pole double-throw) as well as multi channel multiplexers.
The most common and simplest analogue switch in a single IC package is the 74HC4066which has 4 independent bi-directional “ON/OFF” Switches within a single package but the most widely used variants of the CMOS analogue switch are those described as “Multi-way Bilateral Switches” otherwise known as the “Multiplexer” and “De-multiplexer” IC´s and these are discussed in the next tutorial.

Combinational Logic Summary

Then to summarise, Combinational Logic Circuits consist of inputs, two or more basic logic gates and outputs. The logic gates are combined in such a way that the output state depends entirely on the input states. Combinational logic circuits have “no memory”, “timing” or “feedback loops”, there operation is instantaneous. A combinational logic circuit performs an operation assigned logically by a Boolean expression or truth table.
Examples of common combinational logic circuits include: half adders, full adders, multiplexers, demultiplexers, encoders and decoders all of which we will look at in the next few tutorials.

Capacitor


Introduction to Capacitors

Capacitors are simple passive device that can store an electrical charge on their plates when connected to a voltage source

The capacitor is a component which has the ability or “capacity” to store energy in the form of an electrical charge producing a potential difference (Static Voltage) across its plates, much like a small rechargeable battery.
There are many different kinds of capacitors available from very small capacitor beads used in resonance circuits to large power factor correction capacitors, but they all do the same thing, they store charge.
In its basic form, a capacitor consists of two or more parallel conductive (metal) plates which are not connected or touching each other, but are electrically separated either by air or by some form of a good insulating material such as waxed paper, mica, ceramic, plastic or some form of a liquid gel as used in electrolytic capacitors. The insulating layer between a capacitors plates is commonly called the Dielectric.
introduction to capacitors
A Typical Capacitor
Due to this insulating layer, DC current can not flow through the capacitor as it blocks it allowing instead a voltage to be present across the plates in the form of an electrical charge.
The conductive metal plates of a capacitor can be either square, circular or rectangular, or they can be of a cylindrical or spherical shape with the general shape, size and construction of a parallel plate capacitor depending on its application and voltage rating.
When used in a direct current or DC circuit, a capacitor charges up to its supply voltage but blocks the flow of current through it because the dielectric of a capacitor is non-conductive and basically an insulator. However, when a capacitor is connected to an alternating current or AC circuit, the flow of the current appears to pass straight through the capacitor with little or no resistance.
There are two types of electrical charge, positive charge in the form of Protons and negative charge in the form of Electrons. When a DC voltage is placed across a capacitor, the positive (+ve) charge quickly accumulates on one plate while a corresponding and opposite negative (-ve) charge accumulates on the other plate. For every particle of +ve charge that arrives at one plate a charge of the same sign will depart from the -ve plate.
Then the plates remain charge neutral and a potential difference due to this charge is established between the two plates. Once the capacitor reaches its steady state condition an electrical current is unable to flow through the capacitor itself and around the circuit due to the insulating properties of the dielectric used to separate the plates.
The flow of electrons onto the plates is known as the capacitors Charging Current which continues to flow until the voltage across both plates (and hence the capacitor) is equal to the applied voltage Vc. At this point the capacitor is said to be “fully charged” with electrons.
The strength or rate of this charging current is at its maximum value when the plates are fully discharged (initial condition) and slowly reduces in value to zero as the plates charge up to a potential difference across the capacitors plates equal to the source voltage.
The amount of potential difference present across the capacitor depends upon how much charge was deposited onto the plates by the work being done by the source voltage and also by how much capacitance the capacitor has and this is illustrated below.
introduction to capacitors the symbol
The parallel plate capacitor is the simplest form of capacitor. It can be constructed using two metal or metallised foil plates at a distance parallel to each other, with its capacitance value in Farads, being fixed by the surface area of the conductive plates and the distance of separation between them. Altering any two of these values alters the the value of its capacitance and this forms the basis of operation of the variable capacitors.
Also, because capacitors store the energy of the electrons in the form of an electrical charge on the plates the larger the plates and/or smaller their separation the greater will be the charge that the capacitor holds for any given voltage across its plates. In other words, larger plates, smaller distance, more capacitance.
By applying a voltage to a capacitor and measuring the charge on the plates, the ratio of the charge Q to the voltage V will give the capacitance value of the capacitor and is therefore given as: C = Q/V this equation can also be re-arranged to give the more familiar formula for the quantity of charge on the plates as: Q = C x V
Although we have said that the charge is stored on the plates of a capacitor, it is more correct to say that the energy within the charge is stored in an “electrostatic field” between the two plates. When an electric current flows into the capacitor, charging it up, the electrostatic field becomes more stronger as it stores more energy.
Likewise, as the current flows out of the capacitor, discharging it, the potential difference between the two plates decreases and the electrostatic field decreases as the energy moves out of the plates.
The property of a capacitor to store charge on its plates in the form of an electrostatic field is called the Capacitance of the capacitor. Not only that, but capacitance is also the property of a capacitor which resists the change of voltage across it.

The Capacitance of a Capacitor

Capacitance is the electrical property of a capacitor and is the measure of a capacitors ability to store an electrical charge onto its two plates with the unit of capacitance being the Farad (abbreviated to F) named after the British physicist Michael Faraday.
Capacitance is defined as being that a capacitor has the capacitance of One Farad when a charge of One Coulomb is stored on the plates by a voltage of One volt. Note that capacitance, C is always positive in value and has no negative units. However, the Farad is a very large unit of measurement to use on its own so sub-multiples of the Farad are generally used such as micro-farads, nano-farads and pico-farads, for example.

Standard Units of Capacitance

  • Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10-6 F
  • Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10-9 F
  • Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10-12 F
Then using the information above we can construct a simple table to help us convert between pico-Farad (pF), to nano-Farad (nF), to micro-Farad (μF) and to Farads (F) as shown.
Pico-Farad (pF)Nano-Farad (nF)Micro-Farad (μF)Farads (F)
1,0001.00.001 
10,00010.00.01 
1,000,0001,0001.0 
 10,00010.0 
 100,000100 
 1,000,0001,0000.001
  10,0000.01
  100,0000.1
  1,000,0001.0

Capacitance of a Parallel Plate Capacitor

The capacitance of a parallel plate capacitor is proportional to the area, A in metres2 of the smallest of the two plates and inversely proportional to the distance or separation, d(i.e. the dielectric thickness) given in metres between these two conductive plates.
The generalised equation for the capacitance of a parallel plate capacitor is given as: C = ε(A/d) where ε represents the absolute permittivity of the dielectric material being used. The permittivity of a vacuum, εo also known as the “permittivity of free space” has the value of the constant 8.84 x 10-12 Farads per metre.
To make the maths a little easier, this dielectric constant of free space, εo, which can be written as: 1/(4π x 9×109), may also have the units of picofarads (pF) per metre as the constant giving: 8.84 for the value of free space. Note though that the resulting capacitance value will be in picofarads and not in farads.
Generally, the conductive plates of a capacitor are separated by some kind of insulating material or gel rather than a perfect vacuum. When calculating the capacitance of a capacitor, we can consider the permittivity of air, and especially of dry air, as being the same value as a vacuum as they are very close.
capacitor capacitance

Capacitance Example No1

A capacitor is constructed from two conductive metal plates 30cm x 50cm which are spaced 6mm apart from each other, and uses dry air as its only dielectric material. Calculate the capacitance of the capacitor.
the capacitance of a capacitor
Then the value of the capacitor consisting of two plates separated by air is calculated as 221pF or 0.221nF

The Dielectric of a Capacitor

As well as the overall size of the conductive plates and their distance or spacing apart from each other, another factor which affects the overall capacitance of the device is the type of dielectric material being used. In other words the “Permittivity” (ε) of the dielectric.
The conductive plates of a capacitor are generally made of a metal foil or a metal film allowing for the flow of electrons and charge, but the dielectric material used is always an insulator. The various insulating materials used as the dielectric in a capacitor differ in their ability to block or pass an electrical charge.
This dielectric material can be made from a number of insulating materials or combinations of these materials with the most common types used being: air, paper, polyester, polypropylene, Mylar, ceramic, glass, oil, or a variety of other materials.
The factor by which the dielectric material, or insulator, increases the capacitance of the capacitor compared to air is known as the Dielectric Constantk and a dielectric material with a high dielectric constant is a better insulator than a dielectric material with a lower dielectric constant. Dielectric constant is a dimensionless quantity since it is relative to free space.
The actual permittivity or “complex permittivity” of the dielectric material between the plates is then the product of the permittivity of free space (εo) and the relative permittivity (εr) of the material being used as the dielectric and is given as:

Complex Permittivity

capacitor permittivity
In other words, if we take the permittivity of free space, εo as our base level and make it equal to one, when the vacuum of free space is replaced by some other type of insulating material, their permittivity of its dielectric is referenced to the base dielectric of free space giving a multiplication factor known as “relative permittivity”, εr. So the value of the complex permittivity, ε will always be equal to the relative permittivity times one.
Typical units of dielectric permittivity, ε or dielectric constant for common materials are: Pure Vacuum = 1.0000, Air = 1.0006, Paper = 2.5 to 3.5, Glass = 3 to 10, Mica = 5 to 7, Wood = 3 to 8 and Metal Oxide Powders = 6 to 20 etc. This then gives us a final equation for the capacitance of a capacitor as:
capacitance of a capacitor
One method used to increase the overall capacitance of a capacitor while keeping its size small is to “interleave” more plates together within a single capacitor body. Instead of just one set of parallel plates, a capacitor can have many individual plates connected together thereby increasing the surface area, A of the plates.
For a standard parallel plate capacitor as shown above, the capacitor has two plates, labelled A and B. Therefore as the number of capacitor plates is two, we can say that n = 2, where “n” represents the number of plates.
Then our equation above for a single parallel plate capacitor should really be:
actual capacitance of a capacitor
However, the capacitor may have two parallel plates but only one side of each plate is in contact with the dielectric in the middle as the other side of each plate forms the outside of the capacitor. If we take the two halves of the plates and join them together we effectively only have “one” whole plate in contact with the dielectric.
As for a single parallel plate capacitor, n – 1 = 2 – 1 which equals 1 as C = (εor x 1 x A)/d is exactly the same as saying: C = (εor*A)/d which is the standard equation above.
Now suppose we have a capacitor made up of 9 interleaved plates, then n = 9 as shown.

Multi-plate Capacitor

capacitor construction
Now we have five plates connected to one lead (A) and four plates to the other lead (B). Then BOTH sides of the four plates connected to lead B are in contact with the dielectric, whereas only one side of each of the outer plates connected to A is in contact with the dielectric. Then as above, the useful surface area of each set of plates is only eight and its capacitance is therefore given as:
eight plate capacitor
Modern capacitors can be classified according to the characteristics and properties of their insulating dielectric:
  • Low Loss, High Stability such as Mica, Low-K Ceramic, Polystyrene.
  • Medium Loss, Medium Stability such as Paper, Plastic Film, High-K Ceramic.
  • Polarized Capacitors such as Electrolytic’s, Tantalum’s.

Voltage Rating of a Capacitor

All capacitors have a maximum voltage rating and when selecting a capacitor consideration must be given to the amount of voltage to be applied across the capacitor. The maximum amount of voltage that can be applied to the capacitor without damage to its dielectric material is generally given in the data sheets as: WV, (working voltage) or as WV DC, (DC working voltage).
If the voltage applied across the capacitor becomes too great, the dielectric will break down (known as electrical breakdown) and arcing will occur between the capacitor plates resulting in a short-circuit. The working voltage of the capacitor depends on the type of dielectric material being used and its thickness.
The DC working voltage of a capacitor is just that, the maximum DC voltage and NOT the maximum AC voltage as a capacitor with a DC voltage rating of 100 volts DC cannot be safely subjected to an alternating voltage of 100 volts. Since an alternating voltage that has an RMS value of 100 volts will have a peak value of over 141 volts! (2 x 100).
Then a capacitor which is required to operate at 100 volts AC should have a working voltage of at least 200 volts. In practice, a capacitor should be selected so that its working voltage either DC or AC should be at least 50 percent greater than the highest effective voltage to be applied to it.
Another factor which affects the operation of a capacitor is Dielectric Leakage. Dielectric leakage occurs in a capacitor as the result of an unwanted leakage current which flows through the dielectric material.
Generally, it is assumed that the resistance of the dielectric is extremely high and a good insulator blocking the flow of DC current through the capacitor (as in a perfect capacitor) from one plate to the other.
However, if the dielectric material becomes damaged due excessive voltage or over temperature, the leakage current through the dielectric will become extremely high resulting in a rapid loss of charge on the plates and an overheating of the capacitor eventually resulting in premature failure of the capacitor. Then never use a capacitor in a circuit with higher voltages than the capacitor is rated for otherwise it may become hot and explode.

Introduction to Capacitors Summary

We have seen in this tutorial that the job of a capacitor is to store electrical charge onto its plates. The amount of electrical charge that a capacitor can store on its plates is known as its Capacitance value and depends upon three main factors.
  • Surface Area – the surface area, A of the two conductive plates which make up the capacitor, the larger the area the greater the capacitance.
  • Distance – the distance, d between the two plates, the smaller the distance the greater the capacitance.
  • Dielectric Material – the type of material which separates the two plates called the “dielectric”, the higher the permittivity of the dielectric the greater the capacitance.
We have also seen that a capacitor consists of metal plates that do not touch each other but are separated by a material called a dielectric. The dielectric of a capacitor can be air, or even a vacuum but is generally a non-conducting insulating material, such as waxed paper, glass, mica different types of plastics etc. The dielectric provides the following advantages:
  • The dielectric constant is the property of the dielectric material and varies from one material to another increasing the capacitance by a factor of k.
  • The dielectric provides mechanical support between the two plates allowing the plates to be closer together without touching.
  • Permittivity of the dielectric increases the capacitance.
  • The dielectric increases the maximum operating voltage compared to air.
Capacitors can be used in many different applications and circuits such as blocking DC current while passing audio signals, pulses, or alternating current, or other time varying wave forms. This ability to block DC currents enables capacitors to be used to smooth the output voltages of power supplies, to remove unwanted spikes from signals that would otherwise tend to cause damage or false triggering of semiconductors or digital components.
Capacitors can also be used to adjust the frequency response of an audio circuit, or to couple together separate amplifier stages that must be protected from the transmission of DC current.
At DC a capacitor has infinite impedance (open -circuit), at very high frequencies a capacitor has zero impedance (short-circuit). All capacitors have a maximum working voltage rating, its WV DC so select a capacitor with a rating at least 50% more than the supply voltage.

There are a large variety of capacitor styles and types, each one having its own particular advantage, disadvantage and characteristics. To include all types would make this tutorial section very large so in the next tutorial about The Introduction to Capacitors I shall limit them to the most commonly used types.