What do they do?
- Capacitors are components that can store electrical pressure (Voltage) for long periods of time.
- When a capacitor has a difference in voltage (Electrical Pressure) between its two leads it is said to be charged.
- A capacitor is charged by forcing a one way (DC) current to flow through it for a short period of time.
- It can be discharged by letting an opposite direction current flow out of the capacitor.
- Consider for a moment the analogy of a water pipe that has a rubber diaphragm sealing off each side of the pipe as shown in Figure 1.
- If the pipe had a plunger on one end, as shown in Figure 1, and the plunger was pushed toward the diaphragm, the water in the pipe would force the rubber to stretch out until the force of the rubber pushing back on the water was equal to the force of the plunger.
- You could say the pipe is charged and ready to push the plunger back. In fact, if the plunger is released it will move back to its original position.
- The pipe will then be discharged or with no charge on the diaphragm.
- Capacitors act the same as the pipe in Figure 1.
- When a voltage (Electrical Pressure) is placed on one lead with respect to the other lead, electrons are forced to “pile up” on one of the capacitor’s plates until the voltage pushing back is equal to the voltage applied. The capacitor is then charged to the voltage.
- If the two leads of that capacitor are shorted, it would have the same effect as letting the plunger in Figure 9 move freely.
- The capacitor would rapidly discharge and the voltage across the two leads would become zero (No Charge).
What would happen if the plunger in Figure 1 was wiggled in and out many times each second?
- The water in the pipe would be pushed by the diaphragm then sucked back by the diaphragm.
- Since the movement of the water (Current) is back and forth (Alternating) it is called an Alternating Current or AC.
- The capacitor will therefore pass an alternating current with little resistance.
- When the push on the plunger was only toward the diaphragm, the water on the other end of the diaphragm moved just enough to charge the pipe (transient current).
- Just as the pipe blocked a direct push, a capacitor clocks direct current (DC).
- An example of alternating current is the 50 cycle (50 wiggles each second) current produced when you plug something into a wall outlet.
How are they made?
- There are many different types of capacitors used in electronics.
- Each type is made from different materials and with different methods.
- Capacitors are also made to handle different amounts of electrical pressure or voltage.
- Each capacitor is marked to show the maximum voltage that it can withstand without breaking down.
- All capacitors contain the same fundamental parts, which consist of two or more conductive plates separated by a nonconductive material.
- The insulating material between the plates is called the dielectric.
- The basic elements necessary to build a capacitor are shown in Figure 2.
THE METAL FOIL CAPACITOR
- Perhaps the most common form of capacitor is constructed by tightly winding two foil metal plates that are separated by sheets of paper or plastic as shown in Figure 3.
- By picking the correct insulating material the value of capacitance can be increased greatly, but the maximum working voltage is usually lowered.
- For this reason, capacitors are normally identified by the type of material used as the insulator or dielectric.
- Consider the water pipe with the rubber diaphragm in the center of the pipe.
- The diaphragm is equivalent to the dielectric in a capacitor.
- If the rubber is made very soft, it will stretch out and hold a large amount of water, but it will break easily (large capacitance, but low working voltage).
- If the rubber is made very stiff, it will not stretch far, but will be able to withstand higher pressure (low capacitance, but high working voltage).
- By making the pipe larger and keeping the stiff rubber we can achieve a device that holds a large amount of water and withstands a high amount of pressure (high capacitance, high working voltage, large size).
- These three types of water pipes are illustrated in Figure 4.
- The pipes follow the rule that the capacity to hold water, (Capacitance) multiplied by the amount of pressure they can take (Voltage) determines the size of the pipe.
- In electronics the CV product determines the capacitor size.
What is it?
- The dielectric (rubber diaphragm in the water pipe analogy) in a capacitor is the material that can withstand electrical pressure (Voltage) without appreciable conduction (Current).
- When a voltage is applied to a capacitor, energy in the form of an electric charge is held by the dielectric.
- In the rubber diaphragm analogy the rubber would stretch out and hold the water back.
- The energy was stored in the rubber.
- When the plunger is released the rubber would release this energy and push the plunger back toward its original position.
- If there was no energy lost in the rubber diaphragm, all the energy would be recovered and the plunger would return to its original position.
- The only perfect dielectric for a capacitor in which no conduction occurs and from which all the stored energy may be recovered is a perfect vacuum.
- The DIELECTRIC CONSTANT (K) is the ratio by which the capacitance is increased when another dielectric replaces a vacuum between two plates.
- Table 1 shows the Dielectric Constant of various materials.
THE VARIABLE CAPACITOR
- To make a variable capacitor, one set of stationary aluminum plates are mounted to a frame with a small space between each plate.
- Another set of plates are mounted to a movable shaft and designed to fit into the space of the fixed plates without touching them.
- The insulator or dielectric in this type of variable capacitor is air.
- When the movable plates are completely inside the fixed plates, the device is at minimum capacitance.
- The shape of the plates can be designed to achieve the proper amount of capacitance versus rotation for different applications.
- An additional screw is added to squeeze two insulated metal plates together (Trimmer) and thus set the minimum amount of capacitance.
How is it calculated?
The amount of charge a capacitor can hold (capacitance) is measured in Farads. In practice, one farad is a very large amount of capacitance, making the most common term used micro-farad or one millionth of a farad. There are three factors that determine the capacitance that exist between two conductive plates:
- The bigger the plates are (Surface Area), the higher the capacitance. Capacitance
(C) is directly proportional to Area (A).
- The larger the distance is between the two plates, the smaller the amount of capacitance. Capacitance (C) is indirectly proportional to distance (d).
- The larger the value of the dielectric constant, the more capacitance (Dielectric constant is equivalent to softness of the rubber in our pipe analogy). The capacitance (C) is directly proportional to the Dielectric Constant (K) of the insulating material.
From the above factors, the formula for capacitance in Farads becomes:
Where C = Capacitance in Picofarads (Farad x 10-12)
K = Dielectric Constant
A = Area of one Plate in square inches
N = Number of Plates
d = Distance between plates in inches
CAPACITOR VALUES AND MARKINGS
- The older styles of capacitors were marked with colored dots or rings similar to resistors.
- In recent years, the advances in technology have made it easier to print the value, working voltage, tolerance, and temperature characteristics on the body of the capacitors.
- Certain capacitors use a dielectric that requires markings to insure one lead is always kept at a higher voltage than the other lead.
- Figure 5 shows typical markings found on different types of capacitors. Table 2 gives the standard values used and the different methods for marking these values.
- Capacitor markings vary greatly from one manufacturer to another as the above table shows.
- Voltages may be marked directly (200V) or coded (2D).
- The value of capacitance may be marked directly on the part as shown in columns 4 and 5 (note that .001μF and 1000μF have the same marking, but the difference in size makes the value obvious).
- The number 102 may also be used to represent 1000 (10+2 zeros).
- In some instances the manufacturer may use an R to represent the decimal point.
- The tolerance is usually printed directly on the capacitors.
- When tolerance is omitted, the standard tolerance is assumed to be +80% to –20% for electrolytics.
- Capacitance change with temperature is coded in parts per million per degree C or by atemperature graph.
- See manufacturers specifications for complete details.
- Figure 6 shows the schematic symbols used to represent capacitors.
- The + symbol indicates that the capacitor is polarized and the lead marked with the + sign must always have a higher voltage than the other lead.
- The curved plate, plate with sides, and minus sign also indicate the capacitor is polarized and these leads must always be at a lower voltage than the other lead.
- The arrow crossing through the capacitor indicates of capacitance is variable
1.Bunty B. Bommera
2.Dakshata U. Kamble