Nonlinear Conduction | Ohm's Law | Electronics Textbook
The most important diode characteristic is its current-voltage (i-v) relationship. In order to exaggerate a few important points on the plot, the scales in both the. Unlike a resistor, the amount of current through a diode will depend upon 'which When the voltage is applied this way round it tends to pull the free electrons and The shape of this exponential curve depends upon various factors which the actual current/voltage relationship depends upon the detail of how the diode . Required practical - investigate current - voltage graphs To investigate the relationship between current and potential difference for a resistor, bulb and diode. When forward-biased, the diode's resistance is very large at low potential.
In an incandescent lamp the kind employing the principle of electric current heating a thin filament of wire to the point that it glows white-hotthe resistance of the filament wire will increase dramatically as it warms from room temperature to operating temperature. If we were to increase the supply voltage in a real lamp circuit, the resulting increase in current would cause the filament to increase temperature, which would in turn increase its resistance, thus preventing further increases in current without further increases in battery voltage.
The phenomenon of resistance changing with variations in temperature is one shared by almost all metals, of which most wires are made. For most applications, these changes in resistance are small enough to be ignored. In the application of metal lamp filaments, the change happens to be quite large. It is by no means the only example. The straight-line plot of current over voltage indicates that resistance is a stable, unchanging value for a wide range of circuit voltages and currents.
Resistors, which are manufactured to provide a definite, stable value of resistance, behave very much like the plot of values seen above. The plot is no longer a straight line. It rises sharply on the left, as voltage increases from zero to a low level. As it progresses to the right we see the line flattening out, the circuit requiring greater and greater increases in voltage to achieve equal increases in current.
We could say that the resistance here is nonlinear, increasing with increasing current and voltage. The nonlinearity is caused by the effects of high temperature on the metal wire of the lamp filament.
Another example of nonlinear current conduction is through gases such as air. At standard temperatures and pressures, air is an effective insulator. Once ionized, air and other gases become good conductors of electricity, allowing electron flow where none could exist prior to ionization.
That is our unit charge. And the units for voltage in general is volts. Now, let's think about what would happen if we now open the bottom of this pipe. So we open this up. Well, the water's immediately gonna drop straight down. That potential energy is gonna be converted to kinetic energy.
And you could look at a certain part of the pipe right over here, right over here. And you could say, well, how much water is flowing per unit time? And that amount of water that is flowing through the pipe at that point in a specific amount of time, that is analogous to current. Current is the amount of charge, so we could say charge per unit time. Q for charge, and t for time.
And intuitively you could say, how much, how much charge flowing, flowing past a point in a circuit, a point in circuit in a, let's say, unit of time, we could think of it as a second. And so you could also think about it as coulombs per second, charge per unit time. And the idea of resistance is something could just keep that charge from flowing at an arbitrarily high rate. And if we want to go back to our water metaphor, what we could do is, we could introduce something that would impede the water, and that could be a narrowing of the pipe.
And that narrowing of the pipe would be analogous to resistance. So in this situation, once again, I have my vertical water pipe, I have opened it up, and you still would have that potential energy, which is analogous to voltage, and it would be converted to kinetic energy, and you would have a flow of water through that pipe, but now at every point in this pipe, the amount of water that's flowing past at a given moment of time is gonna be lower, because you have literally this bottleneck right over here.
So this narrowing is analogous to resistance.
How much charge flow impeded, impeded. And the unit here is the ohm, is the ohm, which is denoted with the Greek letter omega. So now that we've defined these things and we have our metaphor, let's actually look at an electric circuit. So first, let me construct a battery.
So this is my battery. And the convention is my negative terminal is the shorter line here. So I could say that's the negative terminal, that is the positive terminal.
Associated with that battery, I could have some voltage. And just to make this tangible, let's say the voltage is equal to 16 volts across this battery.
And so one way to think about it is the potential energy per unit charge, let's say we have electrons here at the negative terminal, the potential energy per coulomb here is 16 volts. These electrons, if they have a path, would go to the positive terminal. This means that the energy required for an electron or hole to be able to cross the barrier is eVd.
The 'square law' model assumes that the current, when forward biassed, is proportional to the square of the applied voltage. The 'corner' model assumes that the current is zero for any voltage below Vd but rises when we try to apply a voltage greater than this.
In effect, the diode is viewed as a sort of 'switch' which is open when we apply low or negative voltages but which closes when we try to apply a voltage equal to or greater than Vd. This means that it is impossible to get a voltage larger than this across the diode.
BBC - GCSE Bitesize: The diode
In this way of looking at things, Vd is called the corner voltage. The 'one way' model simplifies things even more by assuming that the corner voltage is so small that we can regard it as being zero! Although this is a drastic simplification, for many practical purposes, it is the only thing we need to remember about a diode. That is, it behaves like a switch which is open no current when we apply volts on way and closed as much current as we like when we try to apply volts the other way.