A solar cell is simply a large photodiode operated in its photovoltaic mode. In its most common application, it is used to convert light energy to electrical energy, and it thereby serves as a source of de power. Unlike the photodiode, the solar energy converter is usually constructed in the form of a circular wafer with a very thin layer of P material at its surface. Light penetrates the thin P layer to the PN junction where excess minority carriers are generated, as in the photodiode. Silicon is the semiconductor material most commonly used in solar-cell construction. Figure 18-32 shows the solar-cell symbol and its connection to a fixed load resistance RL• Remember that the positive load current It shown here is actually the reverse current through the PN junction .
The characteristic curves of a solar cell have the same appearance as photodiode characteristics in the photovoltaic region, shown in Figure 18-26. With reference to It and VI. in Figure 18-32, these curves are simply plots of the photodiode equation
for different values of II” i.e., for different values of light intensity. A typical set of curves for a 3-i/l,-diameter cell, with axes relabeled as It and VI., is shown in Figure 18-33. A nominal value for the solar intensity at noon on the earth’s surface on a clear day is 100 mWfcm2, which is often referred to as one Sill! in solar-power technology.
The intersection of each curve with the It.-axis in Figure 18-33 is the short circuit current, corresponding to a given light intensity. Of course, zero power is delivered to the load when the load is a short. The intersection of each curve with the VI.-axis in Figure 18-s- 33 is the open-circuit voltage, V”, corresponding to a given light intensity. Again, zero power is delivered when the load is open. On each curve of constant light intensity, there is a point where the power, PI. = VJ/., is maximum. This point is on the knee of the curve, and maximum load power is
intersects that point of maximum power. In Figure 18-33 it can be seen that’a load resistance of 0.67 fl results in maximum power when the light intensity is 80 mW/ ern’. However, a load resistance of 1.66 fl is required to achieve maximum power at 15 In solar-power applications, solar cells are connected in series/parallel arrays. Parallel connections increase the total current that can ‘be supplied by the array, and series connections increase the voltage
Example 18-14
An. array of solar cells is to be constructed so it will deliver 4 A at 6 V under a certain light intensity. If each cell delivers 0.8 A at 0.6 V under the given intensity, how many cells are required and how should they be connected?
Solution. To obtain a 6-V output from cells that produce 0.6 Veach, we require
6/0.6 = 10 series-connected cells.
The current in a series string of cells is the same as the current in anyone cell:
0.8 A. To obtain 4 A, we require 4/0.8 = 5 parallel paths, each of which contributes
0.8 A.
Figure 18-35 shows specifications for a commercially available array of 36 series-connected solar cells, the Solavolt MSVM401O. Notice the dependence of the characteristics on temperature as well as solar intensity
Light-Activated SCR (LASCR)
A light-activated SCR (LASCR) is constructed like a conventional SCR except that the reverse-biased NP junction in the center or the structure is exposed to light (see Figure IR-5). Light energy causes reverse current to flow across the junction in the same manner as it Goes in a photodiode. Recall that the SCR will regen eratively switch to its on state if the leakage current hecomes sufficiently large. Thus, the LASCR can be triggered on if a sufficiently intense light ralls on the exposed junction. Since the leakage current also increases with temperature, the LI\SCR turns on at lower light levels when temperature increases. Most LASCRs have an accessible gate terminal so they can be controlled conventionally by an external circuit.
Figure 18-36 shows the schematic symbol for an LASCR and a typical application. When switch S, is dosed, the lamp illuminates the LASCR and triggers it on, thus allowing current to flow in the load. Momentary pushbutton switch S2 is used to reset the LASCR by reducing its current below the holding current. The principal advantage of this arrangement is the complete electrical isolation it provides between the control circuit and the load.
Light-Emitting Diodes (LEOs)
When current flows through a forward-biased PN junction, free electrons cross from the N side and recombine with holes on the P side. Recall from Chapter 2 that free electrons are in the conduction band and therefore have greater energy than holes, which are in the valence band. When an electron in the conduction band recombines with a hole in the valence band, it releases energy as it falls into that lower energy state. The energy is released in the form of heat and light. In some materials, such as silicon, most of the energy is converted to heat, while in
The difference in energy between the conduction band and the valence band is called the energy gap (see Figure 2-3), and its value, which depends on the material, governs the wavelength of the emitted light:
This wavelength is in the infrared region of the light spectrum. Since infrared is not visible and since most energy is released as heat, silicon is not used. in tl e fabrication of light-emitting diodes. For the same reasons, germanium is not used. Instead, gallium arsenide (GaAs), gallium phosphide (GaP), and gallium arsenide phosphide (GaAsP) Me commonly used. The color of the light emitted by these
materials can be controlled by the type and degree of doping. Red, green, and yellow LEOs are commonly available.
Figure 18-37 shows typical manufacturer’s specifications for a line of that the forward voltage drop across an LEO IS considerably greater than that across a silicon or germanium diode. The drop is typically between 2 and 3 V, depending on forward current. Note also that the maximum permissible reverse voltage is relatively small, 5 V in this case
Example 18-15
figure 18-38 shows an LED driver circuit designed to illuminate the LED when the ransis 01 is turned on (saturated) by a 5-V positive illpu-i ui;~ 0″” •. ~;lIU the values of RB and Rc that should be used if the LED is to be illuminated by 20 mA of forward current. Assume that the forward drop across the LED is 2.5 V and that the silicon transistor has /3 = 50.
Solution.
When the transistor is saturated,
LEDs arc used in visual displays of all kinds. The principal disadvantage of the LED is that ‘1 draws considerable current in comparison to the types of low power circuits with which it is typically used. For that reason, LEDs are no longer widely used in such lcw-power devices as calculators and watches. In some applications, power is conserv ed by pulsing the LEOs on and off at a rapid rate, rather than supplying them’ with a steady drive current. The LEOs appear to be continuously illuminated because of the eye’s persistence, that is, its maintenance of the perception of light during the short intervals during which the LEDs are off.
A very popular use for LEOs is in the construction of seven-segment displays, such as those illustrated in the specification sheet shown in Figure 18-39. Notice that each of the seven segments is an LED identified by a letter from A through G. By illuminating selected segments, any numeral from 0 through can be displayed. For example, the numeral 3 is displayed by illuminating only those LEDs
corresponding to segments A, B, C, D, and G. The seven-segment display must be driven from logic circuitry (called a decoder) that supplies current to the correct combination of segments, depending on the numeral to he displayed. In some displays, the decoder circuitry is built-in. In Figure IX-39, note that both COI//l1/01lanod < (TI L~ (2) and common-cathode (TII.~ 13) configurations are available. In the TIL~12. a positive voltage is connected to the common anode, so selected LEDs arc illuminated hy making their respective cathodes low (0 volts). In the TIL313, the common cathode is held low and LEOs are illuminated by making their respective anodes high, VIII..:1 LED configurations are available for hexadvcimul displays (0 through I) an.l A through F) and for alphanumeric displays (numerals and all alphabetic
