Diodes and Transistors

Diodes

Symbol for the diode:File:Diode symbol.jpg

We can take advantage of the properties of a p-n junction to make a diode, which is an electrical component that only allows current flow in one direction. A diode made of silicon needs about 0.7V across it in order to conduct. At about -100V, the diode will fail and the current will force its way though.

Applications of Diodes

Half-Wave Rectifier

A half-wave rectifier will cut off half of a sine wave, leaving only the positive or negative side.

The schematic for a simple rectifier:

The graph for half-wave rectifier:

Flyback Diode

An approximate model of a DC motor is a resistor and inductor in series. If we suddenly break the circuit to switch off the motor, the inductor will continue to try and push current though, resulting in a sudden spike in voltage ($v=L*di/dt$). Mechanically, this is like trying to bring the velocity of a certain moving mass to zero, instantly. We can solve this problem by adding a diode, as shown:

This way, the current can flow through the diode and dissipate in the resistor. Yet, the diode prevents a short circuit from occurring when the switch is closed.

Peak Detector

We can make a peak detector if we hook up our circuit like this:

The graph of the input and output voltages looks like this:

Each time the input voltage dips, the diode prevents the capacitor from draining.

There are two problems with this circuit: First, the voltage of the peak must be greater than the voltage drop, or we won't detect anything. Second, the circuit has a very low impedance, and the capacitor drains a lot of current. A better peak detector can be built with op-amps.

Voltage Clamp

A voltage clamp will limit the output voltage, which is useful when we need to protect circuits from high voltages.

In this ciruit, if $V_{i}n$ exceeds $V_{m}ax$, the diode will conduct and force the voltage to stay at $V_{m}ax$. At lower voltages, the diode does not conduct, and the voltage is not affected.