Simplest Low Drop Solar Charger Circuit

Circuit Explanation:

This low drop solar panel charger circuit is going to be used to accomplish optimum current from a solar panel system whilst charging a conventional lead acid 12 volt battery. It gives you approximately the identical current as though the solar panel was attached straight to the battery. The circuit is a discrete equivalent form of the LM1084 which is basically a 5 amp variable, 3 terminal, low dropout regulator available in the market for around $3. The regulator voltage for the battery is determined to 13.6 that could be in the vicinity of full charge. The voltage could possibly be somewhat over at 14.1 however this would demand temperature stabilization which means that the voltage collapses a tad bit as the temperature goes up. Employing 13.6 with no stabilization need not include a temperature issue.

Simplest Low Drop Solar Charger Circuit



Transistors Q1, Q2 is wired as a differential couple where Q1 detects the battery voltage and Q2 is positioned at a reference voltage fixed by the white colored LED. Resistors R3, R4 are cinfigured a voltage divider to ensure that the input to Q1 can be the just the same as Q2 while the battery is at 13.6 as well as the regulator is going to turn off to certain nominal current to keep up 13.6 volts on the battery. The white LED voltage is around 2.7 volts.

Resistor R5 (330 ohm) fixes the current for the transistor set (Q1, Q2) to approximately 6 milliamps because the voltage across R5 will likely be the reference 2.7 – 0.6 (e/b drop of ). This offers us I=E/R = (2.7-0.6)/330 = 6.4 milliamps. Whenever the battery is noticeably below 13.6, Q1 is going to be off and 6.4 milliamps is going to move via Q2 and R6 delivering a voltage across R6 (750) of E=IR =.0064 * 750 = 4.8 volts.

Q3 is designed as a barrier for the voltage across R6 as well as guarantees current to the bypass transistors Q4,Q5. The emitter/base junction of Q3 will probably shake off around 0.6 volts as a result the voltage on the emitter will likely be 4.8 – 0.6 = 4.2 as well as the current all through R8 (330) is going to be I=E/R = 4.2 / 330 = 12.7 milliamps. This certainly will be adequate to operate Q4,Q5 at 5 amps or perhaps higher when lowest possible amount gain of at least 20 for Q4 and Q5. Resistor R9 (750) is employed to confirm a little some current needs to switch on Q4. This functions to approximately 1 milliamp. Resistor R10 (750) acts as a pullup to secure the circuit initialization any time a battery is not plugged in. The regulator could be utilized in the form of a 13.6 volt power supply with no the battery hooked up.


Drop-out voltage checked 0.82 as soon as the input had been 13.86 and output had been 13.04 and charge current had been 1.92 amps.

The gain (hFE) of Q5 (2N3442) checked roughly 45 at 2 amps with under 1 volt between emitter and collector. This must run at 5 amps with a gain of possibly 20, although I could not check it out. The posted spec is hFE=20 minimum at 3 amps with VCE = 4 volts. The significance of R8 can certainly be lesser or R6 larger to augment the drive current to Q4 in case demanded.

R1 and R2 signify the inner resistance of the battery as well as the solar panel. In case the panel (no load) voltage is nineteen and the charge current is two amps, and the battery voltage is thirteen, along with the drop-out voltage is 0.82, subsequently the panel’s internal resistance (R2) could very well be R=E/I = 19 – (13 + 0.82) / 2 = 2.6 ohms.

As soon as the battery is in close proximity to full charge along with the current is suppose 200 milliamps, the panel voltage is going to be around E = 19 – IR = 19 – (0.2 * 2.6) = 18.48 so the drop throughout Q5 is going to be approximately 18.48 – 13.6 = 4.88 volts.

However these datas are rough assumptions given that the panel impedance is not really persistent.

Parts List for the low drop solar panel charger circuit:
Q1, Q2 = 2N3906 or the majority of small signal PNP.
Q3 = 2N3904 or the majority of small signal NPN.
Q4 = 2N2905A or comparable medium power (500mA) PNP
Q5 = 2N3442 or 2N3055, high power NPN
One White LED (2.7 volt)
D1 = 1N4148 or just about any small silicon diode

R1, R2 = N/A (see text)
R3 = Approximately 560 ohms. Fine-tune this resistor to get the preferred battery voltage.
R5, R8 = 330 ohms
R6, R9, R10 = 750 ohms
R4, R7 = 2.2K