Capacitors (Part 1)
Transistors and Transistor Circuits
Okay, we're in Section 7.3 and we're looking at the subject of Capacitors. Capacitors are another fundamental building block in electronic circuits. Certain characteristics of a capacitor are similar to resistors and inductors. In other ways, they are unique. They are used in nearly every electronic system. They can be defined as the ability to store electrical energy in an electrostatic field. They are devices designed to have a certain capacitance.
Their Basic Construction. A capacitor consists of two conductors called 'plates' separated by an insulator called the 'dielectric'. If we look at this illustration, we can see here is one of the plates right here and here's the other plate. These two plates are separated by a piece of material called the dielectric and it is technically an insulator. Various materials are used. The materials that are used will impact the capacitance significantly. Notice we have two leads here. This is where it will be connected to the circuit and down we have a schematic symbol for a capacitor.
Charges and Electric Fields
A voltage applied to the plates of the capacitor will charge the plated of the capacitor according to the polarity of the applied voltage. If I can draw a little circuit here, I'll draw a battery and will label this as the positive terminal and this as the negative and let's see … make that a little more, there we go and then we'll come over here and draw a capacitor. It's like that. The charge on the capacitor will be the same as that on the supply. If you had a positive here and it's connected to the top plate of the capacitor, that side would be positive. The negative side will be connected to the bottom plate and that side would be negative.
The capacitor will charge according to the polarity of the applied voltage. As the charges on the plate accumulates, current flow is reduced through the charging circuit. In the next two slides, we're going to look at the process of charging a capacitor. Now if you look at this first image right here, we will note there's an applied voltage, there's a resistor. These two lines here are representative of the capacitor plates. Here we have a voltmeter. You'll notice that the voltmeter is reading zero volts and the circuit is open.
When the switch is closed, current flows from the negative plate of the power supply to the bottom plate of the capacitor. Electrons accumulate on the bottom plate. Electrons on the top plate are attracted to the positive side of the power supply leaving a deficiency of electrons i.e. a positive charge here. This accumulation of charges is called charging a capacitor. These electrons are accumulating here. What electrons are here are moving to the positive plate. We're developing an electrostatic field in between the plates of this capacitor.
You'll note that when the switch is closed this begins to develop. You notice that the meter is developing a charge. This does not happen instantaneously. Depending on the size of the capacitor and the current flowing in the circuit, this can take varying amounts of time. But we have the process of the charge beginning. Keep in mind that the current is flowing from here to here. There's movement of current from here to here. No current is moving through the capacitor.
When fully charged, current stops moving and the cap is charged. The charged cap now opposes the power supply. Here we have the 10v supply here. We charge the capacitor and now the polarity across the capacitor is as strong here. With a meter, we would measure 10 volts. Notice we have a strong electrostatic field that is contained in that dielectric material that we alluded to earlier.
Okay. The switch is open and there's no path for discharge. Now, the capacitor was charged here on this screen. The switch is open and we have this charge and it is stored in this electrostatic field. There is no current movement and there is no path to discharge this capacitor. The charge will remain indefinitely as it has no discharge path. The charges eventually leak off due to imperfections in the dielectric. The time to charge the capacitor would vary with the size of the cap. So after the charge has taken place, here we see the volt meter, we have the meter across the cap. We see the 10-volt charge but in this case, there is no path for current or port discharge.
Discharging a Capacitor
The next slide here we're going to look at discharging a capacitor and we'll be looking at this slide and the next one. You'll notice here we have the strong charge across this capacitor. There's an electrostatic field that has been developed, but you'll notice the circuit is open. There is no … the charge remains. It does not discharge. You'll notice the voltage across the resistor is zero volts because the circuit is open.
Okay. In this slide, what we're going to do … the switch is closed providing a path for current flow. Notice the switch here is closed. Voltage is measured on the VOM. The electron movement is highest after the switch is closed and gradually discharges. The moment this is closed we have a large surge of current we will measure 10v initially, but then as the cap discharges you'll notice the meter is moving and the capacitor is discharging. The capacitor has completely discharged and no electrostatic field remains. Here the cap has discharged completely over the passage of time. Now the meter is reading zero and there's no electrostatic field.
Units of Measure
The unit of measure for a capacitor is the 'farad'. A capacitor has a capacitance of 1F. When a potential difference of 1V will charge it with 1C of electricity i.e. 1A. If we have this capacitor and we put 1V across it, it is capable of holding 1C of electricity i.e. electrons, we will say that it has one … it is a 1F capacitor.
Now, generally values of a capacitor are in the sub-farad range and I would say very sub-farad range. Common values here is a 27pF capacitor. Here's one that is 0.05mF. What does this mean? Well, when you say 27pF, you're referring to 27*10-12 and 0.06mF is 0.06*10-6. Capacitors typically are in very small values, much smaller than 1F.
Factors affecting Capacitance
The physical characteristics of a capacitor determine its capacitance. Three primary factors determining the value of a capacitor are and we're going to look at plate area, plate separation, and dielectric material. Here we see the plates. Here we see the dielectric material. Here we see the separation. Let's Look at these three factors a little more in depth.
First of all, Plate Area. Doubling the plate area will double the charge size if all other factors remain the same. Here we have the plate area. If we make this plate area larger, if we double it, we will double the size of the capacitor.
Plate separation. If we double the distance between the plates, all other factors remaining the same, we will have only one-half as much electrostatic intensity. The plate size is directly proportional to the capacitance. If we separate them the distance… the wider the distance we're going to have an inversely proportional relationship, meaning that as the distance has increased, the capacitance decreases.
Finally, Dielectric Material. Permittivity is a measure of a material's ability to concentrate an electrostatic field. The ability to concentrate an electrostatic field. What we're going to find is that there are many different types of dielectric materials. Some are far better than others at concentrating an electrostatic field. It's ability to hold a field of electrostatic field.
Now, the concept of permittivity is similar to that of permeability. Remember in permeability we talked about with transformers that the core material had an ability to transfer a magnetic field. Well, an electrostatic field and permittivity is a similar concept. Permittivity is similar to permeability. Permittivity is measured as dielectric constant k. The value of capacitance is directly related to k.
Now there is a simulation at this particular website. Please go over there. I'm going to move over there at this time. This is addressing factors affecting capacitance. What we have here is a simulation of a large capacitor. What we're going to be able to do here is to change the dielectric material, change the plate area and change the distance between the plates. As we do that, we're going to see how is the capacitance of this device impacted?
First of all, let's look at the dielectric material. We're starting out with paper in between. Let's see as we change that, let's see. Let's change the air first. Now, notice with air our capacitance is 44.27mF. Air is a poor dielectric material. Let's switch to paper. Notice it jumped up significantly to 154mF, to Bakelite up to 212mF and if we go to Mica you notice it jumped all the way up to 239mF. Mica was far by the best. I'm going to leave it on paper just because I think it shows up best on the screen.
Now, let's change the plate area. Notice the plate area now is very large. Let's go in and change that. I'm going to take it down to the smallest I can take 0.02sqmters. The plate area… now notice the value of capacitance is 30.99mF. As this goes up, notice it jumped to 61mF and then to 123mF and finally to 154mF. Again the plate area is directly proportional to the capacitance.
Finally, the distance. Now, this is the distance in between the plates. If we go in and change that distance, we'll notice we're going to… we start out at 154mF where the distance is very wide. Notice as I make it smaller that the capacitance goes up significantly. Notice that 0.001M the capacitance has jumped up all the way up to over 3,000mF. When we have it at 0.02M notice it drops all the way down to 154mF. Okay. This has served as a start for capacitors. We will have another section here. This is completing 7.3 A.