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]

Motor starting capacitor

 
Motor starting capacitor

Motor capacitors AC induction motors, also known as asynchronous motors, use a rotating magnetic field to produce torque. Three-phase motors are widely used because they are reliable and economical. The rotating magnetic field is easily achieved in three-phase asynchronous motors because the phase angle offset between the individual phases is 120 degrees. However, single-phase AC motors require external circuitry which creates the phase angle offset in order to produce a rotating magnetic field. This circuitry can be realized using advanced power electronics, or more simply using a motor capacitor. The video below shows an easy to understand explanation of the working principle of the AC induction motor. AC single-phase induction motors Single-coil AC induction motors AC induction motors usually use two or more coils to generate a rotating magnetic field, which produces torque on the rotor. When a single coil is used, it will generate a pulsating magnetic field, which is enough to sustain rotation, but not sufficient to start the motor from a standstill. Motors with a single coil have to be started by using an external force, and can rotate in either direction. The direction of the rotation depends on the external force. If the motor was started in a clockwise direction, it will continue to rotate and build up speed in the clockwise direction, until it reaches a maximum speed which is defined by the power source frequency. Similarly, it will continue rotating counter-clockwise if the initial rotation was counter-clockwise. These motors are not practical due to their inability to reliably start rotation on their own. Start capacitor AC induction motors One way to improve on the single coil design is by using an auxiliary coil in series with a motor starting capacitor. The auxiliary coil, also called starting coil, is used to create an initial rotating [… 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]