Coupling and Decoupling

 
Coupling and Decoupling

Decoupling capacitors When designing a circuit, many novice engineers and hobbyists take a stable and well regulated power supply for granted, only to find out that their circuits don’t perform as expected during testing, or after the assembly is already complete. Analog circuits such as audio amplifiers or radios may produce a strange hum or a crackling noise audible in the background, and digital circuits such as microcontrollers may become unstable and unpredictable. The reason for this underperformance often lies in the fact that the input voltage is rarely stable in practice. Instead, when viewed with an oscilloscope, a DC power supply often shows many glitches, voltage spikes and AC voltage components. What is a decoupling capacitor? A decoupling capacitor acts as a local electrical energy reservoir. Capacitors, like batteries, need time to charge and discharge. When used as decoupling capacitors, they oppose quick changes of voltage. If the input voltage suddenly drops, the capacitor provides the energy to keep the voltage stable. Similarly, if there is a voltage spike, the capacitor absorbs the excess energy. Decoupling capacitors are used to filter out voltage spikes and pass through only the DC component of the signal. The idea is to use a capacitor in such a way that it shunts, or absorbs the noise making the DC signal as smooth as possible. Because of this, decoupling capacitors are also called bypass capacitors, since they bypass the power source when needed. They can be regarded as small uninterruptible power supplies dedicated to a single circuit board, or even a single component on a board. It is not uncommon to have a single capacitor for each integrated circuit used. As a matter of fact, in digital systems, almost all capacitors on the board may be used for decoupling. Power supply decoupling Decoupling capacitors [… read more]

Filter capacitor

 
Filter capacitor

Filter capacitors Capacitors are reactive elements, which make them suitable for use in analog electronic filters. The reason for this is that the impedance of a capacitor is a function of frequency, as explained in the article about impedance and reactance. This means that the effect of a capacitor on a signal is frequency-dependent, a property that is extensively used in filter design. Analog electronic filters are used to perform a predefined signal processing function. An example of such a function is a low-pass filter (LPF), which passes through low frequencies, but blocks high frequencies. Another example is the high-pass filter (HPF), which passes through high frequencies but blocks low frequencies. These are some basic filter types which can be combined to create other more complicated filters, such as band-pass or notch filters. Electronic filters can be realized in many different ways. They can be made using analog components only, such as capacitors, inductors, resistors, transistors, and operational amplifiers. They can also be realized using digital technology – digital signal processing circuits that consist of a specialized computer or microcontroller and software appropriate for the application. Analog filters are further divided into passive and active filters. Active filters use amplifying circuits and components such as transistors and opamps, while passive filters use resistors, inductors and capacitors exclusively. The advantage of passive filters is that no power source is needed apart from the processed signal itself, while the advantage of active filters is reduced size and cost. Line filters A special subset of electronic filters are line filters. They are used to suppress electrical noise coming from the power supply line. There are many sources of power line noise which make the power supply voltage fluctuate at various frequencies. Some noise sources, such as air conditioners, refrigerators, heaters and other large [… read more]

Parasitic Inductance

 
Parasitic Inductance

What is inductance? Electric inductance is a property of all conductors. A change in the current flowing through the conductor creates (induces) a voltage in that conductor, as well as all nearby conductors. The induced voltage opposes the change in the current that induced the voltage. Inductance is a consequence of two laws of physics. Firstly, a constant current flowing through a conductor creates a constant magnetic field. Secondly, a variable magnetic field induces a voltage in all nearby conductors, including the conductor which was used to create the magnetic field in the first place. When these two laws are combined, the resulting effect is inductance. Just like resistors are used to introduce a desired resistance in a circuit, and like capacitors are used to introduce a desired capacitance, inductors are electrical elements used to introduce a desired amount of inductance into the circuit. The inductance formula for an ideal solenoid (a coil of wire) wound around a cylindrical body of material is given as:     where L is the inductance, µ is the magnetic permeability of the material used in the inductor, A is the cross-sectional area of the coil and l is the length of the solenoid (not the length of the wire, but the longitudinal dimension of the coil). An ideal capacitor has no resistance and no inductance, but has a defined and constant value of capacitance. The unit used to represent inductance is henry, named after Joseph Henry, an American scientist who discovered inductance. Parasitic inductance Parasitic inductance is an unwanted inductance effect that is unavoidably present in all real electronic devices. As opposed to deliberate inductance, which is introduced into the circuit by the use of an inductor, parasitic inductance is almost always an undesired effect. There are few applications in which parasitic inductance is [… read more]

Trimmer Capacitor

 

What are trimmer capacitors? Trimmer capacitors are variable capacitors which serve the purpose of initial calibration of equipment during manufacturing or servicing. They are not intended for end-user interaction. Trimmer capacitors are almost always mounted directly on the PCB (Printed Circuit Board), so the user does not have access to them, and set during manufacturing using a small screwdriver. Due to their nature, trimmer capacitors are cheaper than full sized variable capacitors and rated for many fewer adjustments. Trimmer capacitors are used to initially set oscillator frequency values, latencies, rise and fall times and other variables in a circuit. Should the values drift over time, these trimmer capacitors allow repairmen to re-calibrate equipment when needed. There are two types of trimmer capacitors: air trimmer capacitor and ceramic trimmer capacitor. Trimmer capacitor definition A trimmer capacitor is a variable capacitor used for initial calibration and recalibration of equipment. It is commonly mounted directly on a PCB and accessed only by professional repairmen, not the end-user. Characteristics Voltage rating, capacitance range, polarity Trimmer capacitors can be rated for voltages up to 300 volts, although voltage ratings of up to 100 volts are much more common. Since trim caps are variable capacitors, they come in a capacitance range rather than a single capacitance value. The minimum capacitance is usually between 0.5 pF and 10 pF, while the maximum capacitance is usually between 1 pF and 120 pF. The actual capacitance value can be varied between the minimum and maximum capacitance values for a given trimmer capacitor, but it can never be set to zero. It is worth noting that trimmer capacitors are not polarized. Tolerances and accuracy Trimmer capacitors do not boast a good capacitance value tolerance. Sometimes, the tolerances can be as high as -0 to +100%. This means that a trimmer [… read more]

Mica Capacitor

 
Mica Capacitor

What are mica capacitors? Mica is a group of natural minerals. Silver mica capacitors are capacitors which use mica as the dielectric. There are two types of mica capacitors: clamped mica capacitors and silver mica capacitors. Clamped mica capacitors are now considered obsolete due to their inferior characteristics. Silver mica capacitors are used instead. They are made by sandwiching mica sheets coated with metal on both sides. This assembly is then encased in epoxy in order to protect it from the environment. Mica capacitors are generally used when the design calls for stable, reliable capacitors of relatively small values. They are low-loss capacitors, which allow them to be used at high frequencies, and their value does not change much over time. Mica minerals are very stable electrically, chemically and mechanically. Because of its specific crystalline structure binding, it has a typical layered structure. This makes it possible to manufacture thin sheets in the order of 0.025-0.125 mm. The most commonly used are muscovite and phlogopite mica. The first has better electrical properties, while the second has a higher temperature resistance. Mica is delved in India, Central Africa and South America. The high variation in raw material composition leads to high cost needed for inspection and sorting. Mica doesn’t react with most acids, water, oil and solvents. Mica capacitor definition Silver mica capacitors use mica as the dielectric. They have great high-frequency properties due to low resistive and inductive losses, and are very stable over time. Characteristics Precision and tolerances The minimum tolerance for silver mica capacitor values can be as low as ±1%. This is much better than practically all other types of capacitors. In comparison, certain ceramic capacitors can have tolerances of up to ±20%. Stability Mica capacitors are very stable and very accurate. Their capacitance changes little over [… read more]

Polymer Capacitor

 

What are polymer capacitors? Polymer capacitors are capacitors which use conductive polymers as the electrolyte. They use solid polymer electrolytes instead of liquid or gel electrolytes that are found in ordinary electrolytic capacitors. By using solid electrolyte, the electrolyte drying is completely avoided. Electrolyte drying is one the factors that limit the lifetime of ordinary electrolytic capacitors. There are several types of polymer capacitors, including aluminium polymer capacitors, polymerized organic semiconductors and conductive polymer capacitors. In most cases, polymer capacitors can be used as direct replacements for electrolytic capacitors, as long as the maximum rated voltage is not exceeded. The maximum rated voltage of solid polymer capacitors is lower than the maximum voltage of classical electrolytic capacitors: usually up to 35 volts, although some polymer capacitors are made with maximum operating voltages of up to 100 volts DC. Polymer capacitors have a number of qualities superior to ordinary electrolyte capacitors: longer lifetime, higher maximum working temperature, better stability, lower equivalent series resistance (ESR) and a much safer failure mode. These qualities come at a price of lower maximum voltage rating and a narrower capacitance range, as well as a higher cost compared to wet electrolyte capacitors. This type of capacitor is not that new: production started in the 1980s and since then, they have been used in many applications including server motherboards and computer graphic accelerator cards. Polymer capacitor definition A polymer capacitor is a capacitor which uses solid polymers as the electrolyte. They have a number of superior qualities including a safer failure mode, lower losses and a longer lifetime than electrolytic capacitors. Characteristics Equivalent series resistance Compared to ordinary electrolytic capacitors, polymer capacitors have a lower equivalent series resistance. This allows polymer capacitors to withstand higher ripple currents during normal operation. A ripple current is the AC component [… read more]

Film Capacitor

 
Film Capacitor

What are film capacitors? Film capacitors are capacitors which use a thin plastic film as the dielectric. This film is made extremely thin using a sophisticated film drawing process. Once the film is manufactured, it may be metallized or left untreated, depending on the needed properties of the capacitor. Electrodes are then added and the assembly is mounted into a case which protects it from environmental factors. They are used in many applications because of their stability, low inductance and low cost. There are many types of film capacitors, including polyester film, metallized film, polypropylene film, PTFE film and polystyrene film. The core difference between these capacitor types is the material used as the dielectric, and the proper dielectric must be chosen according to the application. PTFE film capacitors, for example, are heat-resistant and used in aerospace and military technology, while metallized polyester film capacitors are used in applications that require long term stability at a relatively low. Cheaper plastics are used if cost is a bigger concern than performance. Film capacitor definition A film capacitor is a capacitor that uses a thin plastic film as the dielectric. They are relatively cheap, stable over time and have low self-inductance and ESR, while some film capacitors can withstand large reactive power values. Characteristics Film capacitors are widely used because of their superior characteristics. This capacitor type is not polarized, which makes them suitable for AC signal and power use.  Film capacitors can be made with very high precision capacitance values, and they retain that value longer than other capacitor types. This means that the aging process is generally slower than in other capacitor types, such as the electrolytic capacitor. Film capacitors have a long shelf and service life, and are very reliable, with a very low average failure rate. They have [… read more]

Supercapacitor

 

What are supercapacitors? Supercapacitors are electronic devices which are used to store extremely large amounts of electrical charge. They are also known as double-layer capacitors or ultracapacitors. Instead of using a conventional dielectric, supercapacitors use two mechanisms to store electrical energy: double-layer capacitance and pseudocapacitance. Double layer capacitance is electrostatic in origin, while pseudocapacitance is electrochemical, which means that supercapacitors combine the workings of normal capacitors with the workings of an ordinary battery. Capacitances achieved using this technology can be as high as 12000 F. In comparison, the self-capacitance of the entire planet Earth is only about 710 µF, more than 15 million times less than the capacitance of a supercapacitor. While an ordinary electrostatic capacitor may have a high maximum operating voltage, the typical maximum charge voltage of a supercapacitor lies between 2.5 and 2.7 volts. Supercapacitors are polar devices, meaning they have to be connected to the circuit the right way, just like electrolyte capacitors. The electrical properties of these devices, especially their fast charge and discharge times, are very interesting for some applications, where supercapacitors may completely replace batteries. Supercapacitor definition A supercapacitor is a specially designed capacitor which has a very large capacitance. Supercapacitors combine the properties of capacitors and batteries into one device. Characteristics Charge time Supercapacitors have charge and discharge times comparable to those of ordinary capacitors. It is possible to achieve high charge and discharge currents due to their low internal resistance. Batteries usually take up to several hours to reach a fully charged state – a good example is a cell phone battery, while supercapacitors can be brought to the same charge state in less than two minutes. Specific power The specific power of a battery or supercapacitor is a measure used to compare different technologies in terms of maximum power output [… read more]

Tantalum Capacitor

 
Tantalum Capacitor

What are tantalum capacitors? Tantalum capacitors are a subtype of electrolytic capacitors. They are made of tantalum metal which acts as an anode, covered by a layer of oxide which acts as the dielectric, surrounded by a conductive cathode. The use of tantalum allows for a very thin dielectric layer. This results in a higher capacitance value per volume, superior frequency characteristics compared to many other types of capacitors and excellent stability over time. Tantalum capacitors are generally polarized, which means that they may only be connected to a DC supply observing the correct terminal polarity. The downside to using tantalum capacitors is their unfavorable failure mode which may lead to thermal runaway, fires and small explosions, but this can be prevented through the use of external failsafe devices such as current limiters or thermal fuses. Technology advances allow tantalum capacitors to be used in a wide variety of circuits, often found in laptops, automotive industry, cell phones and others, most often in the form of surface mounted devices (SMD). These surface mount tantalum capacitors claim much less space on the printed circuit board and allow for greater packing densities. Tantalum capacitor definition Tantalum capacitors are electrolytic capacitors which use tantalum metal for the anode. They are polarized capacitors with superior frequency and stability characteristics. Characteristics General characteristics Tantalum capacitors are made with capacitance values ranging from 1nF all the way to 72mF and they are much smaller in size than aluminum electrolytic capacitors of the same capacitance. The voltage rating for tantalum capacitors varies from 2V to more than 500V.  They have an equivalent series resistance (ESR) ten times smaller than the ESR of aluminum electrolytic capacitors, which allows for larger currents to pass through the capacitor with less heat generated. Tantalum capacitors are very stable over time and [… read more]

Electrolytic Capacitor

 
Electrolytic Capacitor

What are electrolytic capacitors? An electrolytic capacitor is a type of capacitor that uses an electrolyte to achieve a larger capacitance than other capacitor types. An electrolyte is a liquid or gel containing a high concentration of ions. Almost all electrolytic capacitors are polarized, which means that the voltage on the positive terminal must always be greater than the voltage on the negative terminal. The benefit of large capacitance in electrolytic capacitors comes with several drawbacks as well. Among these drawbacks are large leakage currents, value tolerances, equivalent series resistance and a limited lifetime. Electrolytic capacitors can be either wet-electrolyte or solid polymer. They are commonly made of tantalum or aluminum, although other materials may be used. Supercapacitors are a special subtype of electrolytic capacitors, also called double-layer electrolytic capacitors, with capacitances of hundreds and thousands of farads.  This article will be based on aluminum electrolytic capacitors. These have a typical capacitance between 1µF to 47mF and an operating voltage of up to a few hundred volts DC. Aluminum electrolytic capacitors are found in many applications such as power supplies, computer motherboards and many domestic appliances. Since they are polarized, they may be used only in DC circuits. Electrolytic capacitor definition An electrolytic capacitor is a polarized capacitor which uses an electrolyte to achieve a larger capacitance than other capacitor types. Reading the capacitance value In the case of through-hole capacitors, the capacitance value as well as the maximum rated voltage is printed on the enclosure. A capacitor that has “4.7μF 25V“ printed on it has a nominal capacitance value of 4.7μF and a maximum voltage rating of 25 volts, which is never to be exceeded. In the case of SMD (surface mounted) electrolytic capacitors, there are two basic marking types. The first one clearly states the value in microfarads [… read more]