LED (Light Emitting Diode)
The light emitting diode (LED) has been used for years as a signal lamp, as number indicator and as a light emitting transmitter in an opto-coupler. In recent years the LED is becoming ever more popular as a light source. The operating principle of the LED rests on the release of energy by direct recombination of electrons and holes in a PN-semiconductor. This energy is released in the form of electromagnetic radiation. In general this is:
with: W = energy (eV)
h = Planck constant = 4.133 × 10-15 eVs
f = frequency of electromagnetic waves in Hz.
In the specific case of a semiconductor we find: W = Wg = energy gap or band gap between the valence and conduction band. This energy gap is also temperature dependent. Si has a W = 1.12eV at 0 K (= -273°C) and this becomes 1.1 eV at 300K (room temperature). The wavelength of the released radiation is given by:
Herein c = 300,000km/s = speed of light.
To have continuous radiation we require continuous recombination to occur between electrons from the conduction band and holes from the valence band. This continuous process can be obtained by a forward polarised PN-junction (e.g. with IF = 20mA).
To obtain visible light (λ = 400 to 700nm, see fig. 2) must lie between 1.77 and 3.1eV. The semiconductor material most often used for the construction of LED’s is 3-5 crystals with the following configuration: • group 3: Ga, Al • group 5: As, P, N.
The different combinations and impurities lead to different wavelengths of the radiated light. The spectrum of a galliumphosphide - LED (GaP) can be yellow ( λ = 575nm) to green (λ = 560nm). Gallium-arsenidephoshide (GaAsP) emits yellow ( λ = 590nm) to red light ( λ = 650nm). GaAs has an energy gap of about 1.43eV so that λ ≈ 900nm and GaAs can operate as an infrared transmitter (IRED = infrared emitter). Fig 1 shows the curves that accompany a GaAsP - LED.
Figure 1. Curves of a GaAsP LED
- With nominal current the forward voltage drop of a LED is between 1.3 to 3V depending on the type of semiconductor and the impurities.
- With the passing of time LED’s start to age: the radiated power decreases. High currents and high ambient temperatures have a disastrous effect on the aging process. A load operates classically with IF = 20mA, but if a lower light intensity is required an IF = 5 or 10mA is used.
- Fig. 1d shows a LED as indicator. With V = 12V ; IF = 10mA and VF = 1.6V the value of the current limiting resistor becomes:
In fig. 2 we draw:
- The standard human eye curve prepared by the Commission Internationale de l'Eclair (the CIE-curve). This curve not only provides the sensitivity of the eye at different wavelengths but also the lumens/watt conversion of the light source at that particular frequency.
- The spectral curve of a Ga-As transmitter (LED).
- The wavelength of maximum sensitivity of other LED-materials.
- The sensitivity of a silicon detector (diode, transistor). It is clear why Ga-AS LED’s are used with an (Si-) transistor or diode as opto-coupler elements.
Figure 2. Human eye curves and spectral sensitivity of LEDs and Si-detectors
Optical couple elements
The inverse current through a PN-junction also changes through exposure. This property is used by photo-diodes and photo-transistors. In a photo-transistor the base collector diode is exposed to light. A combination of LED and photo transistor is a so called optocoupler (opto-isolator or couple element: see fig. 3a).
Figure 3. Optocoupler with LED and photo transistor
Information is transmitted via an optical route, without galvanic contact between in and output. We send a current IF (e.g. 10mA) through the LED (light emitting diode). This LED (transmitter) will convert IF in radiated light that via a translucent material hits a detector (e.g. a photo transistor) (fig. 3b). The detector transforms the light into electric current IC (with a connected transistor).
The current transfer ratio is important: (= 20 to 600%).
Sender and receiver can be constructed in a DIP-housing. Equally easily they can be connected via fibre optic cables from a few meters to a few kilometers in length. For very large distances a laser can be used as transmitter. The larger the distance between the LED and photo transistor, the larger the break over voltage (1 to 10kV) between input and output. In the case where an opto coupler is used as detection element to record fast current and voltage changes the special attention needs to be paid to the choice of type. An opto-coupler is a slow element.
In addition there is capacitive feedback between output and input that can cause problems at high switching frequencies. In addition to much used types with LED - photo transistor and LED photo - Darlington’s there are also a number of other combinations possible such as LED - photo diode, LED – photo thyristor, LED - photo triac, LED - photo FET, etc...
Photovoltaic cells convert visible and invisible light into an emf. The currently used cell is in fact a Si-diode. In fig. 4a we have drawn such a diode with the contact potential between metal and silicon and the diffusion voltage of the barrier layer in no-load conditions. If light hits the junction free pairs of electrons and holes appear. The minority charge carriers (electrons in the P-area) are attracted to the positive potential (N-area) so that Vd reduces. Since the contact potential between metal and silicon does not change, a photo - emf (approximately 0.5V) results across the diode depending on the luminous flux ϕ.
Figure 4. Si-photo-voltaic cell
From fig. 2 it follows that Si-photo-voltaic cells are sensitive for light with a wavelength of between 400 and 1100nm. These cells are especially sensitive for red and infrared light. A logarithmic scale links photo-emf (V) and illuminance (lux).
If we connect a resistor Rb across a photo-voltaic cell then we can determine the operating point of this circuit (fig. 5). From the product of the photo-emf and the current produced we find the power that the cell gives. In the example of fig. 5 we find a maximum power of 27.8mW with a load resistor of 2.52 kΩ . We always attempt to load a PV - cell with an optimal resistor so that maximum power is obtained at the existing illumination. This operating point is indicated as the MPP (maximum power point).
Figure 5. Optimum operating point of a loaded cell
Note the PV-cell can be short circuited and then has a current of ISC . In fig. 5 this short circuit current ISC = 0.15 mA and the open circuit photovoltaic emf is about 0.45V. To increase the voltage and required current we form batteries of photovoltaic cells. These are called solar batteries or sun panels. These days they are being more and more promoted as a clean energy source. Photovoltaic cells are used in control circuits, light measurement of visible and almost infrared light, power calculators and photo-electric relays, etc.