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How to Add a Diode to Reduce Voltage

by Phil Lamass

A diode conducts electricity in one direction, from its positive terminal (anode) to its negative terminal (cathode). A diode will not fully conduct electricity until the voltage across it reaches a specific value called its "forward voltage." For most small signal silicon diodes this value is approximately 0.7 volts. A voltage applied across a diode will be reduced by an amount equal to the forward voltage. This is called the "forward voltage drop." Forward voltage drops of connected diodes are added together. For example, the voltage drop across two diodes connected in series is equal to the sum of their forward voltages --- approximately 1.4 volts for silicon diodes.

Wire the Series Diode Circuit

Step 1

Connect the anode of one of the diodes to the supply strip at the top of the breadboard.

Step 2

Plug the cathode of the diode --- indicated by a dark band on the body of the diode --- into the main part of the breadboard.

Step 3

Connect the anode of the second diode to the cathode of the first diode.

Step 4

Connect one lead of the resistor to the cathode of the second diode.

Connect the other lead of the resistor to the ground strip at the bottom of the breadboard.

Measure the Diode Forward Voltage Drops

Step 1

Connect the negative terminal (black wire) of the battery holder to the ground strip.

Step 2

Connect the positive terminal (red wire) of the battery holder to the supply strip.

Step 3

Check your wiring.

Step 4

Place four fresh batteries into the battery holder.

Step 5

Attach the negative probe of the voltmeter to the ground strip.

Step 6

Attach the positive probe of the voltmeter to the supply strip and note the voltage indicated by the voltmeter.

Step 7

Attach the positive probe of the voltmeter to the cathode of the first diode --- closest to the supply strip. Note the voltage indicated by the voltmeter. It should be approximately 0.7 volts lower than the value in Step 6.

Attach the positive probe of the voltmeter to the cathode of the second diode. Note the voltage indicated by the voltmeter. It should be approximately 1.4 volts lower than the value in Step 6.

Items you will need

  • 4 AA batteries and appropriate battery holder
  • 220-ohm Resistor
  • 2 small signal diodes (for example 1N914)
  • Solderless breadboard
  • Volt meter

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