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What is the function of a diode?
Dec 21, 2017

Diodes offer us these additional functions :

It can act as a temperature sensor. When a small current of a few milliamps is passed through a typical silicon diode, a junction voltage of around 700mV will appear across the diode at around 20 Deg. C. For every deg C increase, the junction voltage will drop by around 2mV

It can act as a voltage limiter to protect sensitive circuits from damage due to static or other high voltage electrical noise. For example, a diode is often connected across a DC relay coil, with its cathode to the + of the coil. When the coil de-energises, the collapsing magnetic field will tend to generate a 'back EMF' of several hundred volts, if the diode is not present. This could destroy the transistor which controls the relay. By having the diode across the coil, the back EMF is limited to less than 1 volt.  A pair of diodes are often used in the input circuitry of signal amplifiers, to clamp the input signal to little more than the V+ or V- supply voltage levels within the amplifier. One diode will have its cathode connected to V+ and its anode connected to Signal input. The other diode will have its anode conected to V- and its cathode connected to Signal input.

It can act as a voltage controlled variable capacitor giving a variation of several tens of pF as the reverse bias voltage connected across it is varied. Some types are specially engineered to exploit this effect better, and these are known as Varicap diodes.

A standard silicon diode can be used as a simple voltage regulator with a voltage of around 700mV. Several diodes can be connected in series to create regulated voltages of 700mV x n where n is the number of diodes in series.  A specially developed version of the diode gives us many more voltage options, and is known as the Zener diode.

A special diode is the Gunn diode, which can be used to generate low power microwaves, at a power of a few mW. These are often used in doppler speed detectors and some types of intruder alarms. The frequency of oscillation of these diodes is controlled by mounting the diode in a cavity, whose dimensions detemine the wavelength of oscillation. These diodes rely on a special effect within the semiconductor known as negative resistance, where at a certain bias condition, increasing the bias voltage gives a decrease in current. 

Diodes can also be used as AF or RF signal switches , which can be controlled by passing a DC current through them. Special versions for this function are known as PIN diodes.

Another common diode, developed to have special properties is the LED, or light emitting diode. These have developed rapidly in the past few years and offer efficient alternatives to incandescent and fluorescent lights. They are also very small and are available is a wide range of colours, so can be arranged to create high resolution video screens and advertising displays. Some versions are made to emit invisible infrared wavelengths, and these are often found in TV remote controls, which send a beam of modulated infrared light to the TV, containing instruction data, such as change volume, change channel etc.

A cousin of the LED is the Laser Diode. This produces a pure, coherent beam of light which can be precisely focussed into a narrow beam. Laser LEDs are found in DVD and BluRay devices, telecomms equipment, pollution detection sensors, range finders and lecture pointers.



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