The Diode: An Electronic One-Way Valve

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The Idea

A diode is an electronic device that lets current flow in only one direction. Diodes are found in electronic circuits and form the basis for more complicated devices, such as transistors and integrated circuits. LEDs (light-emitting diodes) and solar cells are diodes.

Unlike the resistors we studied in previous experiments, diodes do not follow Ohm's law. They are called nonlinear devices, which gives them properties that are useful in a wide variety of electronic applications.

What You Need

• diode
• DC power supply
• voltmeter
• ammeter
• jumper wires

Method

1. Set up the circuit, as shown in Figure 116-1. This consists of the positive terminal of the power supply connected to the positive end of the diode (identified by the longer of the two leads). The ammeter is connected in series with the diode and, together, they are attached to the power supply. The voltmeter is connected to the two terminals of the diode.
2. Start with the power supply at the lowest level and make sure the voltage and current meters read zero.
3. Very slowly, walk the voltage up, taking current and voltage readings at each step. Continue until the current suddenly goes up significantly higher than previous levels. Do not allow too much current to flow or the diode can be damaged.

Expected Results

The relationship between voltage and current is not linear.

As the voltage increases, a threshold is reached where a small increase in voltage results in a huge increase in current.

This relationship between current and voltage is exponential.

Why It Works

The current that passes through a diode is related to the voltage applied across it by the diode equation:

I = IoeqV/kT

where I is the current and V is the voltage (q and k are constants, T is the diode temperature, and Io is a property of the diode).

This equation shows that the current increases exponentially as the voltage is increased. At first the change is slow. But after about 0.7 V, the diode offers little resistance to the flow of current.

Current-voltage characteristics

Plot the current versus voltage on a linear plot. Its shape is exponential with a "knee" around 0.7 V defining a region where a small increase in voltage causes a very large increase in current. If you plot the log of the current versus the voltage, you should get a straight line at least over a major part of the data range. If voltage is a logarithmic function of current, current is an exponential function of voltage. Plotting this confirms the nonlinear behavior of the diode characteristic.

Voltage in the reverse direction

What happens if you swap the two leads of the diode? This applies a voltage in the opposite direction as in the previous case. As a one-way valve, the diode does not allow any measurable current to flow in the reverse direction. If you try to force the issue and continue to increase the voltage (going the wrong way), you may (depending on the diode) reach a condition called the breakdown voltage. When this happens, the opposition to the current flow breaks down and the diode allows the current to flow. In many diodes, this is a reversible condition, which can be used to establish a set voltage level in a circuit. Going into breakdown may damage some diodes, so be careful if you try to measure this in your circuit.

The Point

A diode is a nonlinear device. A small increase in voltage produces a large increase in current, which grows exponentially with voltage.