Equipment:

• peg board - between shelves 12 and 13
• diodes mounted on banana-jack cables - shelf 11C
• Variac - shelf 3E
• suicide cord - shelf 11C
• light bulb on magnetic stand - front of shelf 14
• two magnets - side of shelf 14
• oscilloscope - in front of shelf 13
• 3 banana-to-banana cables
• 1 scope cable, alligator-to-BNC
• video camera and holder - file cabinet between shelves 3 and 4

Theory: A diode will pass current in only one direction. A set of 4 diodes can be used to make a full-wave rectifier, with current only flowing in one direction through the circuit. The silicon diodes used here have a voltage drop of 0.6 V, something that's easy to see using this setup.

Description: A small light bulb is used here. It can be mounted on the peg board using the two magnets. The AC voltage is provided by the suicide cord, which is plugged into the variac. Start with the variac at zero; the variac setting should never exceed 10 V.

The simplest way to arrange the four diodes to begin with is to plug the black plugs of two diodes into the black socket of the light bulb. Then take the red plugs of the two remaining diodes and plug them into the red socket of the light bulb. Use one square on the peg board, with the light bulb in the middle. The two free black plugs should go to the bottom left and right corners; the two free reds should go to the top left and right corners.

The oscilloscope should be looking at the voltage across the light bulb. To show un-rectified AC, connect the suicide cord across just the light bulb. Turn up the variac from zero until the bulb is glowing brightly; several volts is enough to do this. Leave the variac setting here from now on. To move to a half-wave rectified signal, move the ends of the suicide cord to the far ends of either diagonal. Note the peak voltage on the scope compared to what it was before (you should see the 0.6 V drop), and note that the bulb is dimmer.

To show rectified AC, move one red plug from the top left to the bottom left, and the other red plug from top right to bottom right. The ends of the suicide cord will now be at the bottom left and right corners. You should see a full-wave rectified signal on the scope; again, note the signal amplitude and the intensity of the light.
In a small room, the students can view the scope directly. In a larger room, the video camera can show the scope screen on a TV monitor, or all 6 monitors in room 107.

Setup time: 20 minutes

References: (available in the demo room)

• Freier and Anderson, "A Demonstration Handbook for Physics", Eo-10, page E-58.