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Gentle Battery Regulator Schematic Circuit Diagram

This small but very effective circuit protects a lead-acid battery (12-V solar battery or car battery) against overcharging by a solar module when the incident light is too bright or lasts too long. It does so by energising a fan, starting at a low speed when the voltage is approximately 13.8 V and rising to full speed when the voltage exceeds 14.4 V (full-charge voltage). The threshold voltage (13.8 V) is the sum of the Zener diode voltage (12 V), the voltage across the IR diode (1.1 V), and the baseemitter voltage of the 2N3055 (0.7 V).


  1. Zener Diode (D1): 12V
  2. Infrared Diode (IR Diode): Voltage drop: 1.1V
  3. NPN Transistor (Q1): 2N3055 (or equivalent)
  4. Diode (D2): 1N4001 or similar
  5. Resistor (R1): Value to limit base current (calculated based on the transistor’s current gain)
  6. Potentiometer (Pot1): Variable resistor to adjust the sensitivity
  7. Fan (F1): 12V DC Fan
  8. Battery: 12V Lead-Acid Battery (e.g., solar or car battery)
  9. Solar Module: Connected to charge the battery

Circuit Description:

The circuit uses a Zener diode, an infrared diode, and an NPN transistor (2N3055) to regulate the charging of a lead-acid battery. Here’s how it works:

  1. Zener Diode (D1): Acts as a voltage reference. The breakdown voltage of the Zener diode is 12V.
  2. Infrared Diode (IR Diode): Voltage drop is 1.1V. This, along with the Zener voltage, sets the initial threshold voltage at 13.1V (12V from Zener + 1.1V from IR diode).
  3. Transistor (Q1 – 2N3055): The base-emitter junction voltage of the 2N3055 is around 0.7V. When the total voltage (Zener voltage + IR diode voltage) exceeds 13.8V, the transistor begins to conduct.
  4. Resistor (R1): Limits the current flowing into the base of the transistor. Its value is chosen to limit the base current and is calculated based on the desired base current and the current gain of the transistor.
  5. Potentiometer (Pot1): Allows the sensitivity of the circuit to be adjusted. It can be used to fine-tune the voltage threshold at which the circuit activates the fan.
  6. Diode (D2): Provides a path for the current when the fan is turned off. This diode prevents backflow of current through the fan when it’s not running.
  7. Fan (F1): 12V DC Fan. The fan starts at a low speed when the voltage is approximately 13.8V and reaches full speed when the voltage exceeds 14.4V (full-charge voltage).

Gentle Battery Regulator Schematic Circuit Diagram

Gradual Switching Characteristic in the Circuit

Unlike circuits utilizing relays or IC amplifiers, this circuit exhibits a smooth switching behavior, eliminating problems like relay chatter and constant switching near the ‘hard’ switching point. Notably, it consumes zero current (auto power-off) below 13 V. During construction, special attention should be given to the polarization of Zener and IR diodes. Additionally, the transistor necessitates a heat sink to dissipate heat, especially when the fan operates below 14 V. An effective heat sink can be fashioned from a galvanized bracket, readily available at DIY shops.

Adaptation for Higher Power Modules

The specified component values are tailored for a 10-W solar module. Should a higher-power module be employed, a motor with a corresponding higher power rating becomes imperative. Exploiting the positive temperature coefficient of the lamp filament, the circuit ensures optimal functionality. At low voltages, the filament’s resistance is low, gradually increasing with voltage. This variation curtails the fan speed, preventing an annoying noise level. Additionally, the lamp serves as a protective measure. If one’s finger inadvertently enters the fan blades, the lamp swiftly absorbs most of the power, illuminating brightly. This action significantly reduces the fan’s torque. Any standard 10-W or 20-W car headlight (or two 25-W headlights in parallel) can function effectively as the lamp.


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