Figure I. Schematic far the MRP4 Solar Conîml Circuir ure I. The heart of the project is an LM324 op amp, Only three of the tour amplifiers in the chip arc used in this project.

To determine the state of charge of the battery, we monitor its terminal voltage. Resistors Rt and R2 divide ihe battery's voltage by three- Amplifier D buffers this voltage before passing it along to amplifier A. This amplifier is configured as a voltage comparator. The buttery's terminal voltage, now divided by three, is compared to the state-of-charge reference voltage.

The state of charce (SOC) is determined by a 7HL05 three-terminal voltage regulator Resistor R12 keeps a constant load on the regulator to improve its stability. Trimmer Rl4. along with R13. sets the state of charge by dropping Lhe regulated +5 volts slightly- This is our state-of-charge set point. The state-of-charge set point is three limes the value. If you want the controller to turn off the charging current at 14.3 volts, then the SOC voltage at pin #2 of UIA would be 4.766 volts. The highest you can set the state of charge is 15 volts, That's the full output of the 78L05 regulator limes three.

Even though the battery sense is divided by one-percent resistors, there may be some final adjustment needed to the SOC trimmer for the exact state-of-charge voUage at the battery terminals. The circuit is protected against reverse polarity by a 1N4Ü02 diode.

When the array starts to produce energy, all the array's power goes into the battery via the blocking diode. As the battery becomes full, its terminal voltage will rise. When the terminal voltage readies the state-of-charge set by Ri4, the comparator switches states. Some of the output is fed back io the sense line. This raises the voUage of the sense line up slightly. By doing so. we introduce some hysteresis to the comparator.

At the same time, the output also is squared up by the ihird amplifier Its output drives the shunt transistor fully on. Since the transistor is fully saturated, it shorts the array to ground. This stops the battery from charging. The blocking diode prevents the battery from being shorted to ground when the transistor turns on.

With the array shorted to ground, the terminal voUage of the battery begins Lo drop. But, because of the hysteresis, the terminal voltage drops below that of the state of charge. Depending on the value of the resistor in the hysteresis loop, the batter} voltage will drop a volt or so. When the terminal voltage drops below the hysteresis threshold, the comparator switches off. This reverses the action of the transistor switch, turning it off. Once again, full array current is allowed to flow into the battery and the process is repeated. The battery is then protected fror:: overcharge by shorting out the array when lhe state of charge has been reached.

The charging LED will then flash on and off as the shuni transistor shorts the array io ground. The charging LED gets its

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