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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Solar cell tester. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources You can use solar cells just like any other power source. Each of them is designed to maintain a certain current strength at a given voltage. However, unlike conventional power supplies, the output characteristics of a solar cell depend on the amount of incident light. For example, an incoming cloud can reduce output power by more than 50%.
Moreover, not all cells deliver the same power under the same lighting conditions, even if the cells are identical in size and design. Deviations in technological regimes can lead to a noticeable spread in the output currents of the elements of one batch. These factors must be taken into account when designing and manufacturing structures with solar cells. Therefore, if you want to get the most out of photovoltaic converters, you need to check all the elements. To better understand what parameters are subject to verification, we first consider the characteristics of a silicon solar cell. Characteristics of the photoelectric converter Whenever working with any power source, it is necessary to imagine what is the relationship between voltage and current, as well as their dependence on the load. In most cases, the relationship is determined by Ohm's law. Unfortunately, silicon solar cells are non-linear devices and their behavior cannot be described by a simple formula. Instead, a family of easy-to-understand curves (Figure 1) can be used to explain the characteristics of the element.
100 mW / cm2 correspond to the energy illumination created by the direct flux of solar radiation on the earth's surface and at sea level at noon in a clear sky; 75 mW/cm2 correspond to 3/4; 50 mW/cm2 - 1/2; 25 mW/cm2 is 1/4 of this illumination. It is possible to study the current-voltage characteristics (Fig. 1) in more detail using the circuit shown in Fig. 2. The circuit measures the output voltages and current flowing through a variable resistive load. We will assume that the light intensity remains constant during the measurement. First, use the potentiometer to set the maximum resistance value. In this case, in fact, there is no current in the circuit and the resulting output voltage can be considered equal to the no-load voltage, which is the voltage that the element generates when no load is connected to it. It is about 600 mV (0,6 V). The magnitude of this voltage may vary slightly when moving from one element to another in one batch and from one manufacturer to another. As the resistance of the resistor decreases, the element is loaded more and more. As with a conventional battery, this causes an increase in current consumption. At the same time, the output voltage drops slightly, as it should with an unregulated power supply. So far, this is not surprising. Then something strange happens. A situation is reached where the output current no longer increases as the load resistance decreases. Nothing can cause an increase in current, not even a short circuit. In practice, this current is rightly called the short-circuit current. In essence, the solar generator has become a source of direct current. The question arises: what about the voltage? The voltage will constantly decrease in proportion to the increase in load.
As soon as the load resistance becomes zero, the voltage drops to zero. By the way, a short circuit of the photoelectric converter does not lead to its failure. The amount of current that an element can develop depends on the intensity of the light. For the first measurement, we arbitrarily chose the highest level of irradiance, which corresponds to the upper curve (Fig. 1). Each subsequent curve was obtained on the same element with a gradual decrease in light intensity. power curve If it was necessary to plot the output power versus voltage, then the result could be something similar to that shown in Fig. 3. At one end of the graph, there is maximum current at zero voltage. Of course, no power is released at this point due to the lack of voltage. At the other end of the graph, there is a maximum voltage at zero current, resulting in no power being released either. Between these two limits, during operation of the photovoltaic converter, power is released in the load, and the peak power is released only at one point. It is in it that the combination of all factors ensures the selection of the greatest energy from the solar cell. The peak power corresponds to a voltage of about 450 mV (0,45 V), which coincidentally coincided with the inflection of the current curve shown in Fig. 1. The fact that the family of current curves has the same shape means that we will always get the maximum power at the same voltage, regardless of the brightness of the sun. Of course, the actual power will depend on the intensity of solar radiation at a given time, however, the peak power will be observed at the same voltage. Thus, in order to correctly evaluate the quality of a silicon solar cell, it is necessary to load it so that the output voltage is 0,45 V, and then measure the output power. This method is effective not only for comparing elements with each other under the same conditions, but also for assessing the quality of an individual element.
Development of the tester scheme As already mentioned, for testing solar cells, you can use the circuit shown in Fig. 2. By the way, this is a quick and easy way, according to which, after connecting the element to the specified circuit, you just need to set the appropriate voltage using a potentiometer and take readings from devices that measure voltage and current. By multiplying voltage and current, you can get the amount of power. However, all elements are slightly different, and therefore the resistances corresponding to the peak power of individual elements will also be different. And in accordance with this, it is necessary to change the load resistance each time in order to restore the required operating voltage. In addition, the energy generated by the solar cell is completely dissipated in the potentiometer, causing it to heat up and become unstable. The root solution to this problem would be to replace the load resistor in the circuit. What could be better than a transistor? This is a great replacement. In this particular application, the transistor can be thought of as a dynamic resistance. A small transistor base current, set as shown in fig. 4 causes a significant change in the collector current. The base current actually changes the resistance of the transistor, which in turn is used as a load for the solar cell.
Unfortunately, the transistor has the same drawback as the potentiometer, i.e., the need to adjust the base current when changing the element under test. This operation is not difficult with a small number of elements, but suppose you need to check 30, 40 or more elements. This will take too long. It would be nice to find a way to automatically adjust the base current without having to manually set it each time. It would be highly desirable to have a parallel voltage regulator. A parallel voltage regulator is a regulator that is surrounded by a feedback loop that uses the input voltage to control the base current. Regardless of the initial input voltage, the shunt regulator changes its shunt resistance so that the output voltage is maintained at the desired level. The principle of operation of the circuit As a result, we arrive at the scheme shown in Fig. 5, which uses an op-amp to control the base current of the transistor. The 220 ohm resistor serves to limit the base current. The regulator compares the input voltage from the photovoltaic converter with a reference voltage. Usually, a zener diode circuit is used as a reference voltage source. However, in our case, a zener diode with an extremely low stabilization voltage, preferably below 1 V, would be required. Unfortunately, zener diodes for such voltages are either very sensitive to temperature changes or expensive (usually both). On the other hand, a forward biased silicon diode can serve as an excellent low voltage reference.
Diode D1, the forward bias on which is set by resistor R1, determines the voltage range of the regulator, limiting the voltage across the "calibration" adjusting resistor. The reference voltage from the slider of this potentiometer is fed to the non-inverting input of the amplifier. The voltage of the photoelectric converter is applied to the inverting input of the amplifier through the resistor R3. Resistor R4 sets the gain of the operational amplifier (in this case, it is 100). Due to its peculiarity, the op-amp tries to equalize the voltage on its inverting and non-inverting inputs by controlling the current flowing through the shunt regulator transistor Q1. The transistor reduces the input voltage to such a value that it becomes equal to the voltage at the tap of the resistor VR1. This voltage can be adjusted between 0-0,7 V. However, a transistor cannot realistically have zero resistance, which is required to bring the voltage down to zero. No matter how hard you try, the transistor will still have a small residual voltage of about 150 mV. This limits the regulation range within 0,15-0,7 V. Control devices The voltage on the solar cell is measured with a voltmeter M1, and the current flowing through the shunt transistor is measured with an ammeter M2. Power (in watts) is determined by multiplying the readings of both devices. The voltmeter is connected directly to the element. It is a 1 mA panel meter with a series limiting resistor that allows it to display 1 V at full scale. On the other hand, an operational amplifier is used together with the ammeter M2 to measure the current. The circuit is designed so that the emitter current of transistor Q1 must flow through resistor R13. This current corresponds to the current generated by the solar cell. When current flows through resistor R13, a small voltage drop is created. It is amplified by a differential amplifier whose inverting and non-inverting inputs are energized through resistors R6 and R7, respectively. The gain value is controlled by resistors R8-R10. Resistor R8 is permanently connected between the output and the inverting input. Its resistance is 3 MΩ, and the corresponding gain is 300. When a current of 13 mA flows through the resistor R100, the output voltage of the amplifier is 1 V. The output voltage of the differential amplifier is measured with a voltmeter identical to M1. This instrument is calibrated in units of current. In our case, a voltage of 1 V corresponds to a current of 100 mA. When resistor R8 is connected in parallel with resistor R10, the gain decreases to 60. In this case, a voltage of 1 V at the output of the amplifier corresponds to a current of 500 mA flowing through R13. Thus, we have expanded the range of measured currents, covering values of 100-500 mA. Similarly, when resistor R9 is connected in parallel to resistor R8, currents in the range of 0-3 A can be measured. Tester design Although the solar cell tester can be made in any way, printed wiring is highly recommended. The printed circuit board is shown in fig. 6. Place the details of the circuit according to fig. 7 and solder them, observing the polarity of the semiconductors. Note that the shunt transistor Q1 is located on the foil side of the board. The transistor must be carefully screwed to a large copper pad that acts as a heat sink. In this case, it is not required to isolate the transistor case.
Ideally resistors R6 and R7 should form a matched gift. However, precision resistors are expensive and difficult to obtain. Therefore, I recommend taking a small group of 10 kΩ resistors and measuring them with a digital multimeter. It doesn't take long to find two matching resistors. The remaining components can be used as resistors R2 and R3. On the other hand, resistor R13 is not an ordinary resistor. I doubt that you will be able to find such a resistor in a general store. But it can be made from a piece of wire 10 cm long and 0,26 mm in diameter, which is usually used for windings. Wind the wire around the frame (pencil) so that the resulting coil fits exactly on the board. The accuracy of current measurement depends on the accuracy of selecting the value of the resistor R13. In order to increase accuracy, you can start with a piece of wire a little longer than 10 cm and shorten it, controlling the amount of current using the M2 ammeter. The two gauges, the "calibration" control and the range selector, are housed with the printed circuit board in any suitable housing. When connecting these components, polarity must be observed. To power the instrument, two 12-volt supplies with positive and negative leads and a common ground wire are required. The type of power sources and the magnitude of the voltage are not critical. If desired, the tester can be powered using two 9-volt batteries for transistor receivers. A diagram of one of the possible power sources is shown in fig. 8.
Probably the hardest thing to find or make is a holder with a contact device for solar cells. Here you need to show some imagination yourself. A flat aluminum plate a little larger than the cell itself can serve as a good electrode to make a connection to the cell's rear contact, while a volt-ohmmeter probe will make excellent contact to the front of the cell. To automate testing, you may need to buy or make a special clamp. As I said, it will take a little imagination and understanding of what exactly is needed.
Working with the tester The tester is very easy to use. It is necessary to connect the element to the circuit, illuminate it and take readings. The back contact of the element is a positive electrode and is connected to the positive input of the tester. The current collection grid on the front surface of the element is a negative electrode and is connected to the grounded output of the tester. It is necessary to ensure reliable contact with the electrodes of the element. Since we are dealing with a fairly small voltage, even a small contact resistance can lead to a significant difference in readings. To ensure a reliable connection, it is necessary that the contacts are pressed well enough against the element. However, excessive pressure should be avoided as the elements are very thin, brittle and break easily! This is where a well-designed element contact device comes in handy. The "calibration" regulator sets the operating voltage at which the power is measured. It is usually set once at 450 mV. However, if necessary, the operating voltage can be changed. In short, if you have a tester, you can not guess about the parameters of the elements, but measure them. Author: Byers T.
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