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]

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]

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]

Ceramic Capacitor

 
Ceramic Capacitor

What are ceramic capacitors? A ceramic capacitor uses a ceramic material as the dielectric. Ceramics were one of the first materials to be used in the producion of capacitors, as it was a known insulator. Many geometries were used in ceramic capacitors, of which some, like ceramic tubular capacitors and barrier layer capacitors are obsolete today due to their size, parasitic effects or electrical characteristics. The types of ceramic capacitors most often used in modern electronics are the multi-layer ceramic capacitor, otherwise named ceramic multi-layer chip capacitor (MLCC) and the ceramic disc capacitor. MLCCs are the most produced capacitors with a quantity of approximately 1000 billion devices per year. They are made in SMD (surface-mounted) technology and are widely used due to their small size. Ceramic capacitors are usually made with very small capacitance values, typically between 1nF and 1µF, although values up to 100µF are possible. Ceramic capacitors are also very small in size and have a low maximum rated voltage. They are not polarized, which means that they may be safely connected to an AC source. Ceramic capacitors have a great frequency response due to low parasitic effects such as resistance or inductance. Ceramic capacitor definition A ceramic capacitor is a capacitor which uses a ceramic material as the dielectric. The two most common types are multi-layer ceramic capacitors and ceramic disc capacitors. Characteristics Precision and tolerances There are two classes of ceramic capacitors available today: class 1 and class 2. Class 1 ceramic capacitors are used where high stability and low losses are required. They are very accurate and the capacitance value is stable in regard to applied voltage, temperature and frequency. The NP0 series of capacitors has a capacitance thermal stability of  ±0.54% within the total temperature range of -55 to +125 °C. Tolerances of the nominal capacitance [… read more]