A diode is a semiconductor device that only allows current to flow in one direction. When current flows through a silicon diode there is a small potential difference across the diode of around 0.7V. A diode has a maximum current rating and a maximum reverse breakdown voltage rating - exceeding either of these will damage the diode.
A diode has two leads called the anode and the cathode and current can only flow from the anode to the cathode. Typical silicon diodes include the small 1N4148 signal diode which can take a current of around 200mA and the slightly more robust 1N4001 which has a maximum current rating of 1A and a reverse breakdown voltage of 50V. In both cases the cathode is marked with a ring on the body of the diode.
When a diode is connected so that current can flow from the anode to the cathode it is forward biased as shown in the diagram.
In this example the forward bias voltage across the diode is 0.7V and the potential difference across the bulb is 5.3V.
As current flows through the diode and there is a 0.7V potential difference it will dissipate power and get warm. If too much current flows then the diode will be damaged. All diodes have a maximum forward bias current.
When a diode is connected so that no current can flow it is reverse biased as shown.
No current flows in the circuit and the potential difference across the bulb is 0V. Therefore, the (reverse) potential difference across the diode is 6V. If the reverse bias voltage is too high the diode will be damaged. All diodes have a maximum reverse bias voltage.
The transfer characteristics of a device describe how the current changes as the potential difference changes.
In reverse bias the current remains zero even when the potential difference increases to several volts.
In forward bias the current is initially zero when the potential difference is less than 0.6V and then rapidly increases as the potential difference increases to 0.7V. Note that the forward bias potential difference does not exceed 0.7V, if current flows through the diode, the potential difference is around 0.7V. This means that a diode is almost always used in series with other components.
The diode is a non-ohmic component. It has a very high resistance when V < 0.6V and the resistance rapidly falls to a very low value when V > 0.6V being approximately 0Ω when V = 0.7V.
A diode, or several diodes, can be used to convert A.C. in to D.C. by only allowing current to flow in one direction. Rectifier diodes can usually handle large forward bias currents and have a reverse breakdown voltage greater than the peak voltage of the A.C. being rectified. See the power supply page for more details.
When any device containing a coil of wire, such as a relay, electric motor or solenoid, is turned off a very large reverse voltage is generated for a very short time. This voltage is called the back e.m.f. and can be several hundred volts from even small relays and motors. The back e.m.f. can easily damage logic gates, Op-Amps and transistors.
A diode placed in reverse bias across the device can protect the rest of the circuit. When the device is operating normally the diode is reverse biased and does not conduct (and therefore has no effect what so ever). When the device is turned off the large back e.m.f. momentarily forward biases the diode, the diode conducts and limits the back e.m.f. to 0.7V above the power supply voltage. Instead of the control circuit being subject to hundreds of volts, it now only has to cope with 12.7V which is much more reasonable.
The fact that the potential difference across a diode is almost constant at 0.7V when a current flows means that it can make a useful voltage reference. In the diagram the output voltage is always 0.7V even when the input voltage varies. This could be used as the reference voltage in a comparator circuit.
Consider a circuit where the ±13V output from a comparator circuit needs to be connected to a logic circuit working at 5V. The output of the comparator is ±13V but the input to the logic circuit needs to be either 5V or 0V.
When the output from the comparator is +13V the diode is forward biased and conducts. The potential divider is used to create a 5V Logic signal (Logic 1). The voltage drop across the diode is 0.7V and so the potential divider must reduce 12.3V to 5V.
The potential difference across the two resistors are in the ratio 7.3:5 and therefore a 73kΩ and a 50kΩ resistor are used. In reality values from the E24 series such as 68kΩ and 47kΩ would be used.
When the output from the comparator is -13V the diode is in reverse bias. No current flows and the output voltage is 0V (Logic 0).
When we talk about a diode we are almost invariably talking about a silicon diode. Diodes are also made from a semiconductor called Germanium. A Germanium diode has a very low forward bias potential difference of around 0.1V making them ideal for use in very low voltage circuits.
A typical application of Germanium diodes is as the demodulator in a simple A.M. radio. See the radio page.
© Paul Nicholls
Electronics Resources by Paul Nicholls is licensed under a Creative Commons Attribution 4.0 International License.