Q factor

 
Q factor

Q Factor definition The Q factor of a capacitor, also known as the quality factor, or simply Q, represents the efficiency of a given capacitor in terms of energy losses. It is defined as: where QC is the quality factor, XC is the reactance of the capacitor, C the capacitance of the capacitor, RC is the equivalent series resistance (ESR) of the capacitor, and ω0 is the frequency in radians at which the measurement is taken. In an AC system, the Q factor represents the ratio of energy stored in the capacitor to the energy dissipated as thermal losses in the equivalent series resistance. For example, a capacitor that is capable of storing 2000 joules of energy while wasting only 1 joule has a Q factor of 2000. Since Q is the measure of efficiency, an ideal capacitor would have an infinite value of Q meaning that no energy is lost at all in the process of storing energy. This is derived from the fact that the ESR of an ideal capacitor equals zero. The Q factor is not a constant value. It changes significantly with frequency for two reasons. The first reason is the obvious ω0 term in the above equation. The second reason is that ESR is not a constant value with regard to frequency. The ESR varies with frequency due to the skin effect, as well as other effects related to the dielectric characteristics. A related term, called the dissipation factor(DF), is sometimes defined in capacitor datasheets instead of the Q-factor. In AC circuits the DF is simply the reciprocal value of Q. Why is the Q factor important? Most applications do not have to take the Q factor into serious consideration, and standard capacitors may be used in those applications. However, the Q factor is one [… read more]

Impedance and Reactance

 
Impedance and Reactance

Impedance and reactance An element in a DC circuit can be described using only its resistance. The resistance of a capacitor in a DC circuit is regarded as an open connection (infinite resistance), while the resistance of an inductor in a DC circuit is regarded as a short connection (zero resistance). In other words, using capacitors or inductors in an ideal DC circuit would be a waste of components. Yet, they are still used in real circuits and the reason is that they never operate with ideally constant voltages and currents. As opposed to constant voltage circuits, in AC circuits the impedance of an element is a measure of how much the element opposes current flow when an AC voltage is applied across it. It is basically a voltage to current ratio, expressed in the frequency domain. Impedance is a complex number, which consists of a real and an imaginary part: where Z is the complex impedance. The real part R represents resistance, while the imaginary part X represents reactance. Resistance is always positive, while reactance can be either positive or negative. Resistance in a circuit dissipates power as heat, while reactance stores energy in the form of an electric or magnetic field. Impedance of a resistor Resistors in AC circuits behave the same way they do in DC circuits. Basically, the impedance of a resistor consists only of the real part, which is equal to the resistance of the resistor. Therefore, the impedance of a resistor can be expressed as: where Z is the impedance, and R is the resistance of the resistor. It is obvious that a resistor has no reactance, and can therefore store no energy. Also, when a voltage is applied across the resistor, the current flowing through the resistor will be in phase with the voltage, [… 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]