The circuit consists of an oscillator transistor and a regulator transistor.
The solar panel charges the battery when sunlight is bright enough to produce a voltage above 1.9v. A diode is required between the panel and the battery as it leaks about 1mA from the battery when it is not illuminated.
The regulator transistor is designed to limit the output voltage to 5v. This voltage will be maintained over the capability of the circuit, which is about 10mA.
Theoscillator transistor must be a high-current type as is is turned on for a very short period of time to saturate the core of the transformer.
This energy is then released as a high-voltage pulse.
These pulses are then passed to the electrolytic and appear as a 5v supply with a capability of about 10mA. If the current is increased to 15mA, the voltage drops to about 4v.
The transformer is wired so that it gives POSITIVE feedback.
The transistor turns on via the 1k resistor and this produces expanding flux in the core.
The flux cuts the turns of the secondary winding and produces a voltage that ADDS to the turn on voltage and the transistor is turned on MORE. The transistor gets fully turned ON and the current through the primary becomes a maximum. The core becomes saturated and although the flux is a maximum, it is not expanding flux and thus the secondary produces no voltage (only the voltage and current supplied by the battery).
The voltage and current into the base of the transistor is reduced and this reduces the current through the primary.
The flux now begins to collapse and this produces a voltage in the secondary of an opposite polarity.
This turns the transistor OFF and the magnetic flux collapses quickly and produces a high voltage.
This voltage is passed through the diode and charges the electrolytic.
The circuit operates at approx 50kHz and the pulses quickly charge the electrolytic.
The 15k resistor has a 3k3 “trimmer” resistor to enable you to adjust the output to exactly 5v or slightly above 5v. Microcontrollers will work up to 5.5v but some will freeze at 5.6v, so be careful.
The output voltage is monitored at the join of the 15k resistor (and 3k3) and the 2k2 resistor. The voltage at this point is exactly 0.63v (630mV) and at this voltage the regulator transistor turns ON and robs the oscillator transistor with “turn-on” voltage.
When a load is placed on the output of the circuit, the voltage across the electrolytic drops and the regulator turns off slightly. This allows the oscillator transistor to operate “harder” and send pulses of energy to the electrolytic to charge it. If the load is removed, the current consumption for the circuit is about 3.5mA. This is the quiescent current for the circuit.
The output current is limited as each mA requires about 5mA from the battery.
At 15mA output, the current required from the battery is about 75mA. That’s why we need a high-current capability transistor for the oscillator. A BC 547 transistor will not work, as it is not capable of passing a high current.
The solar panel will deliver about 10 – 15mA on bright sunlight, so any load on the output must be as small as possible.
An example is data logging, where the micro is active for short periods of time, then goes into “sleep” mode.
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