Low Dropout Voltage Regulator Using 5 Transistors

Circuit Description:

The low dropout regulator is used to obtain maximum current from a solar panel while charging a typical lead acid 12 volt battery. It provides about the same current as if the panel was connected directly to the battery. The circuit is a discrete version of the LM1084 which is a 5 amp adjustable, 3 terminal, low dropout regulator available for about $2.50. The regulator voltage for the battery is set to 13.6 which is near full charge. The voltage could be a little higher at 14.1 but this would require temperature compensation so the voltage falls somewhat as the temperature rises. Using 13.6 without compensation doesn't present a temperature problem.


Transistors Q1,Q2 form a differencial pair where Q1 monitors the battery voltage and Q2 sits at a reference voltage set by the white LED. Resistors R3,R4 form a voltage divider so that the input to Q1 will be the same as Q2 when the battery is at 13.6 and the regulator will shut down to some minimum current to maintain 13.6 volts on the battery. The white LED voltage is about 2.7 volts.

Resistor R5 (330 ohm) sets the current for the transistor pair (Q1,Q2) to about 6 milliamps since the voltage across R5 will be the reference 2.7 minus the e/b drop of 0.6. This gives us I=E/R = (2.7-0.6)/330 = 6.4 milliamps. When the battery is significantly lower than 13.6, Q1 will be off and 6.4 milliamps will flow through Q2 and R6 producing a voltage across R6 (750) of E=IR = .0064 * 750 = 4.8 volts.

Q3 is used as a buffer for the voltage across R6 and also provides current to the pass transistors Q4,Q5. The emitter/base junction of Q3 will drop about 0.6 volts so the voltage on the emitter will be 4.8 minus 0.6 = 4.2 and the current through R8 (330) will be I=E/R = 4.2 / 330 = 12.7 milliamps. This should be sufficient to drive Q4,Q5 at 5 amps or more assuming a minimum gain of at least 20 for Q4 and Q5. Resistor R9 (750) is used to insure some minimum current is required to turn on Q4. This works out to about 1 milliamp. Resistor R10 (750) serves as a pullup to get the circuit started when a battery is not connected. The regulator could be used as a 13.6 volt power supply without the battery connected.


Dropout voltage measured 0.82 when the input was 13.86 and output was 13.04 and charge current was 1.92 amps.

The gain (hFE) of Q5 (2N3442) measured about 45 at 2 amps with less than 1 volt between emitter and collector. It should work at 5 amps with a gain of maybe 20, but I didn't try it. The published spec is hFE=20 minimum at 3 amps with VCE = 4 volts. The value of R8 could be lower or R6 higher to increase the drive current to Q4 if required.

R1 and R2 represent the internal resistance of the battery and the panel. If the panel (no load) voltage is 19 and the charge current is 2 amps, and the battery voltage is 13, and the dropout voltage is 0.82, then the panel's internal resistance (R2) would be R=E/I = 19 - (13 + 0.82) / 2 = 2.6 ohms.

When the battery is near full charge and the current is say 200 milliamps, the panel voltage will be around E = 19 - IR = 19 - (0.2 * 2.6) = 18.48 and the drop across Q5 will be about 18.48 - 13.6 = 4.88 volts.

But these values are approximate since the panel impedance is not constant.

Parts List:
Q1,Q2 --- 2N3906 or most any small signal PNP.
Q3 --- 2N3904 or most any small signal NPN.
Q4 --- 2N2905A or similar medium power (500mA) PNP
Q5 --- 2N3442 or 2N3055, high power NPN
One White LED (2.7 volt)
D1 --- 1N4148 or any small silicon diode
R3 --- Approximately 560 ohms. Adjust this resistor for the desired battery voltage.
R5, R8 --- 330 ohms
R6, R9, R10 --- 750 ohms
R4, R7 --- 2.2K

18 watt, 12 volt, 1.5 amp Solar Panel purchased used in 1994.

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