By David Herres
Capacitance is everywhere. Electronic circuits make use of this phenomenon of the physical universe because of it can be used to separate dc from high-frequency signals, to store electrostatic energy, to block reverse-polarity current flow, and for many other applications. The other side of the coin is that unwanted, or parasitic, capacitance becomes increasingly problematic at the higher frequencies necessary for contemporary high-bandwidth connectivity.
Engineers have developed techniques for mitigating parasitic capacitance in transmission lines by, for example, employing clever innovations such as category cabling with high twist rates in conjunction with differential transmission. On circuit boards, practices such as use of slender, tapered electrodes that less resemble flat plates help reduce parasitic effects.
Along with inductance, capacitance is a defining element in characteristic impedance. This property of any transmission line becomes increasingly important as the frequency or total length of the line rises. At today’s awesome bit rates, even circuit board traces must be designed with parasitics in mind. In repair work it is essential to avoid changing the length or routing of point-to-point wiring, lest the characteristic impedance be altered resulting in impedance mismatch.
An ideal capacitor, as its schematic symbol suggests, consists of two closely-spaced flat metal plates with leads attached, separated by an insulating material. The capacitance is directly proportional to the area of the plates and inversely proportional to the distance between them. If the total area of the plates doubles, the capacitance doubles. If the distance between them is cut in half, the capacitance also doubles. Other factors go into determining capacitance as well such as the dielectric constant of the material that separates the plates.
Capacitance is generally a permanent property, whether the device of interest sits on a shelf in the warehouse or is deployed in circuit. One glaring exception is the behavior of electrolytic capacitors. They can lose capacitance at high and low temperatures and over a period of time, especially if not used.
Sometimes capacitance in electrolytic capacitors can be restored by the application of a steady dc voltage. That’s because the dielectric in an electrolytic capacitor is created by an electrochemical process when the right voltage is applied. This dielectric layer is extremely thin (it is usually an oxide that forms on the plate serving as the anode), making possible large capacitance values. Because of this property, electrolytics find use in motor start and/or run circuits, in power supplies to filter and remove ripple from the rectified current, and in variable-frequency drives (VFDs).
Tantalum capacitors are one kind of electrolytic, notable for their small size, low dc leakage, and stable capacitance. But they are expensive, so their use is limited to applications where these qualities are needed such as in cell phones and aerospace.
Unlike capacitance, capacitive reactance depends upon the circuit in which the capacitor is used. Specifically, the capacitive reactance depends on frequency. The capacitor has its greatest capacitive reactance when the applied voltage is dc. It drops linearly as frequency rises.
In real-life situations, parasitic capacitance often arises between conductors running in parallel, as between two wires in a cable. The lower capacitive reactance at higher frequencies tends to shunt out higher frequency signals, which is what makes high-speed connectivity problematic.
Capacitive reactance is given by Xc = ½ πfC
where Xc = capacitive reactance, ohms; f = frequency, Hz; and C = capacitance, farads. Note that capacitive reactance is expressed in ohms and conforms to Ohm’s law, the power equations, and all circuit theorems.
One of the capacitor’s most useful properties is its ability to tune a circuit with great precision to a single frequency, excluding all others, or to let a circuit exclude a single frequency while passing the rest of the spectrum. If a capacitor and inductor sit in series, they will block all frequencies except for their resonant frequency and the frequencies very near it, at which the combination will conduct. If they sit in parallel, they will block the resonant frequency and conduct all others.
An oscilloscope can be used in conjunction with a signal generator to determine the capacitance of an unknown device. Those using Tektronix equipment can connect a 1 kΩ precision resistor in series with the device under test. Two TPP 1000 voltage probes connected to separate oscilloscope channels are connected across the resistor. An AFG 2021 arbitrary function generator and a DPO 4104 oscilloscope comprise the test setup.
Directions for testing a ceramic capacitor and an inductor may be found in an application note available at www.tek.com.
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