5V regulated solar battery power supply circuit analysis

This circuit is powered by a solar panel, which provides you with a pure regulated DC voltage of 5V. The solar cell power supply circuit consists of an oscillation transistor and a voltage stabilizing transistor. When sunlight is bright enough to produce a voltage higher than 1.9v, the solar panel charges the battery. A diode is needed between the panel and the battery because when it's really not illuminated it leaks about 1mA from the battery. The voltage regulating transistor is designed to limit the output voltage to 5v. This voltage will probably be maintained within the circuit's capabilities, which is about 10mA.

The oscillator transistor should be of the high current type as it will be on for an extremely limited time to saturate the core of the transformer. This energy is then released in the form of high-voltage pulses. These pulses are then passed to the electrolytic and displayed as a 5v supply with approximately 10mA capability. If the current increases to 15mA, the voltage drops to about 4v.

Wire the transformer to ensure it provides positive feedback. The transistor turns on through the 1k resistor, which creates an expanded flux within the core. The flux cuts off the turns of the secondary winding and creates a voltage that increases the turn-on voltage and the transistor turns on more. The transistor is fully conductive and the current through the mains supply becomes maximum. The core becomes saturated, and while the flux is indeed at a maximum, it doesn't actually expand the flux, so the secondary produces no voltage (only the voltage and current provided by the battery).

The voltage and current going into the base of the transistor is reduced, which reduces the current through the mains supply. The flux now begins to collapse, which creates a voltage on the secondary of opposite polarity. This turns off the transistor and the magnetic flux quickly collapses and creates a high voltage. This voltage is passed through the diode and charges the electrolyte. The circuit operates at approximately 50kHz and pulses briefly charge the electrolyte.

The 15k resistor has a 3k3 "trimmer" resistor that allows you to adjust the output to 5v or just above 5v. The microcontroller will run on voltages up to 5.5v, but some will freeze at 5.6v, so be careful. Monitor the output voltage at the connections of the 15k resistor (and 3k3) as well as the 2k2 resistor. The voltage at this point is exactly 0.63v (630mV), at which voltage the regulator transistor turns on and robs the oscillator transistor of its "on" voltage.

When a load is placed on the output of the circuit, the voltage across the electrolyte drops and the regulator turns off slightly. This allows the oscillator transistor to run "harder" and send pulses of energy to the electrolyte to charge it. If the load is removed, the current consumption of the circuit is approximately 3.5ms. This could be quiescent current in the circuit.

Output current is limited because each mA requires approximately 5mA from the battery. At 15mA output, the current required by the battery is approximately 75mA. That's why we need a high current capable transistor for the oscillator. The BC547 transistor doesn't work because it really can't pass high current.

The solar panel will provide around 10–15mA in bright sunlight, so any load on the output should be as small as possible. An example is information logging, where the micro is active for a short period of time and then goes into "sleep" mode.