The diode has a number of applications in electronic circuits. One application you may be familiar with is a rectifier. A rectifier converts alternating current (AC) to direct current (DC). Alternating current periodically changes direction while direct current only flows in one direction. "The most common function of a diode," according to Wikipedia, "is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking current in the opposite direction (the reverse direction)." It’s easy to demonstrate how the diode will let current flow in one direction but block the flow of current in the opposite direction.

Parts needed:

1 Base Grid (11” x 7.7”) # 6SC BG
1 Battery Holder (2-AA) # 6SC B1
1 Diode 1N4001 # 6SC D3
1 Slide Switch # 6SC S1
1 6V Lamp Socket (With Bulb) # 6SC L2
4 Conductor with 2-snaps # 6SC 02

Build the circuit shown.

When you switch the circuit on you can think of it as current flowing from the positive side of the battery (marked with a "+" sign) to ground (marked with a "-" sign) and you can think of the flow of current as kinetic energy that can be used to do useful work such as light up an incandescent bulb.(1) So, when you switch the circuit on, why doesn't the light bulb light up? Because the diode is blocking the flow of electricity.

Switch the circuit off and change the diode from the reverse direction to the forward direction.

Switch the circuit on and the light bulb lights up. Now the diode is no longer blocking the flow of electricity and current can flow from the positive (+) terminal on the battery through the circuit to ground (-).

Now let's take a look at the Light Emitting Diode (LED). The light emitting diode functions much the same as an ordinary diode, but also lights up when enough current is flowing through it in the forward direction (forward-biased). LEDs are often used in electronic devices to indicate that the device is switched on, but they can serve many practical purposes.

Parts needed:

1 Base Grid (11” x 7.7”) # 6SC BG
1 Battery Holder (2-AA) # 6SC B1
1 Red LED # 6SC D1
1 Slide Switch # 6SC S1
1 6V Lamp Socket (With Bulb) # 6SC L2
4 Conductor with 2-snaps # 6SC 02

Build the circuit shown.

When you switch the circuit on...nothing happens. Why doesn't the light LED light up? Because the LED is reversed and blocking the flow of electricity.

Switch the circuit off and turn the LED around.

Now you can switch the circuit on and with the LED in the forward-biased direction, it lights up.