In the previous article, we looked at various types of capacitors. Now, let's discuss the selection of a capacitor for a particular application. Generally, selecting a capacitor is not a difficult task unless you have specific circuit requirements. Engineers often have a derived nominal capacitance on hand for a circuit or need to use capacitance with an IC or active component. Most ICs (like 555, microcontroller ICs, etc.) recommend capacitance values specified in their datasheets for different applications.
Unless there are specific circuit requirements and the required capacitance is in Picofarads, a ceramic capacitor can be used. If the required capacitance is in Nanofarad, MLC (Multilayer Ceramic) capacitors can be blindly reliable. If the required capacitance is in Microfarads, aluminum electrolytic capacitors are a common choice. For a wider temperature range and robustness, glass and mica capacitors can be used.
Besides rated capacitance, rated voltage is the second most important parameter that must essentially be taken into consideration. The nominal voltage of the capacitor must always be at least 1.5 times or twice the maximum voltage that can be found in the circuit. Capacitors are not as reliable as resistors. They are easily damaged when the applied voltage approaches their maximum rating.
If a circuit has specific requirements, many other factors will need to be considered. Different types of capacitors are preferable for specific circuits and applications. The preferred applications of different types of capacitors are summarized in the following table:
In addition to the suitability of different capacitors for specific applications, other important factors that may be considered include the following:
- Tolerance – It must be checked whether the operation of the circuit depends on the precision capacitance. A capacitor with the lowest tolerance should be used if narrow capacitance is required. A capacitor's capacitance will never vary beyond its rated tolerance unless it is damaged due to excessive voltage or environmental conditions.
- Operating temperature range and temperature coefficient – If the circuit is temperature sensitive or the capacitance is not expected to vary beyond a limit over a range of temperatures, its operating temperature range and temperature coefficient must be considered. The extent of the change in capacitance must be calculated based on the temperature coefficient and the temperature curve. The temperature sensitivity of a circuit can also be addressed by using positive and negative temperature coefficient capacitors together. In this case, the maximum variation of capacitance over a range of temperatures must be calculated.
- Frequency dependence – Many capacitors have their capacitance dependent on frequency and may not be suitable for a specific range of frequencies. Depending on the circuit, the dependence of capacitance on frequency must essentially be considered.
- Operating Losses – Operating losses can be an important factor where circuits require energy efficiency (such as battery-operated circuits). For such circuits, careful selection of capacitors must be made considering their dissipation factor (typical power loss in percentage), dielectric absorption, leakage current or insulation resistance, and self-inductance. All these losses must be minimized to improve the efficiency and battery life of the circuit.
- Ripple Current and Pulse Voltages – These are very important checks. The circuit must be manipulated for pulsating voltages and maximum ripple current. A capacitor with appropriate ripple current and working voltage should be chosen.
- Polarity and reverse voltage – If an electrolytic capacitor is used in the circuit, it must be connected in the correct direction. Its reverse voltage rating must be at least twice the reverse voltage possible on that branch of the circuit.
Standard Capacitor Values
Capacitors are also available in standard values according to the E series, like resistors. To learn more about standard values of resistors, capacitors, inductors and Zener diodes, check out the following article, “Basic Electronics 08 – Value Reading, Tolerance and Power of Resistors”.
There are fewer standard values for capacitors compared to resistors. Generally, capacitors are only available in the E-6 series of standard values (10, 15, 22, 33, 47, and 68) followed by a specified number of zeros.
Series and parallel combination of capacitors
It may not be possible to obtain the exact desired capacitance value in the standard E series. In such cases, a combination of capacitors in series or parallel can be used to obtain the desired capacitance in the circuit. When capacitors are connected in series, the equivalent capacitance is given by the following equation:
1/C Series = 1/C 1 + 1/C 2 + 1/C 3 + . . . .
When capacitors are connected in parallel, the equivalent capacitance is given by
C Parallel =C 1 +C 2 +C 3 + . . . .
The equation for a series combination of capacitances is derived from the fact that the sum of voltage drops across all series-connected capacitances will equal the applied voltage, while the current through them will remain the same. The equation for a series combination of capacitances is derived as follows:
V Total =V C1 +V C2 +V C3 + . . . .
1/C Series * ∫i.dt = 1/C 1 * ∫i.dt + 1/C 2 * ∫i.dt + 1/C 3 * ∫i.dt + . . .
1/C Series = 1/C 1 + 1/C 2 + 1/C 3 + . . . .
The equation for parallel combination of capacitances is derived from the fact that the sum of currents through all parallel-connected capacitances will equal the total current, while the voltage across them will remain the same. The equation for the parallel combination of capacitances is derived as follows:
I = i1 + i2 + i3 + . . . .
C Parallel * dV/dt = C 1 * dV/dt + C 2 * dV/dt + C 3 * dV/dt + . . . . .
C Parallel =C 1 +C 2 +C 3 + . . . .
Reading resistor packs
In the past, color codes and different types of numerical codes were used to indicate value, tolerance and working voltage of capacitors. Today, the capacitance, tolerance and working voltage are printed on the body of the capacitors or indicated by the codes of standard BS1852 or BS EN 60062. In these coding systems, the value, tolerance and working voltage of the capacitor are indicated by numeric codes of two or three digits followed by a letter. The capacitance value is always indicated in Picofarads. If it is a two-digit code, it is the direct value of the capacitance in Picofarads, and if it is a three-digit code, the first two digits indicate a number (Series E-6), and the third digit indicates a multiplier giving the value final capacitance in Picofarads. A letter can be used to indicate the capacitor tolerance. The tolerance indicated by different letters has been summarized in the following table:
For example, if 47F is printed on a capacitor, it means its capacitance value is 47 pF and its tolerance is one percent. Similarly, if 472J is printed on a capacitor, it means its capacitance value is 4700 pF or 4.7nF and its tolerance is five percent. Letter codes for commonly available capacitances are listed in the following table:
Ceramic capacitors have additional codes, consisting of a digit between two letters, to indicate temperature range and temperature coefficient. The letters and digits of these codes have the following indications:
The voltage rating is indicated by a number that expresses the working voltage in Volts. Likewise, the number '50' indicates an operating voltage of 50V.
In the next article, we will discuss supercapacitors.