12V LDO Solar Charge Controller

This Low Dropout Voltage (LDO) solar charge controller uses a simple differential amplifier and series P channel MOSFET linear regulator –their compatibility seems like a marriage made in heaven. Voltage output is adjustable. It is mainly intended for charging 12V lead-acid batteries.

Solar Charge Controller Specifications

  • Solar panel rating: 50W (4A, 12V nominal) (open circuit voltage: 18 to 20V)
  • Output voltage range: 7 to 14V (adjustable) (not recommended for 6V applications)
  • Max power dissipation: 16W (includes power dissipation of D3)
  • Typical dropout voltage: 1.25V @ 4A
  • Maximum current: 4A (current limiting provided by solar panel characteristics)
  • Voltage regulation: 10mV (no load to full load)
  • Battery discharge: 1mA (Chinese controls discharge at typically 5mA)
  • LED indicators:
    • RED: Solar panel active
    • GREEN: Series regulator limiting current (fully charged or topping off)
  • Reverse battery protection: Control shuts down if battery is inadvertently connected reverse

Schematic of 12V Solar Charge Controller Circuit

12V LDO Solar Charge Control schematic

Bill of Materials



Dropout Voltage

The input voltage exceeds the input voltage by 1.25V when charging at the maximum rate –the lower, the better. Low Dropout Voltage (LDO) is the catch phrase for anything under approximately 2V. This could potentially be reduced to below 1V by making D3 a schottky rectifier.

Current Limiting

Current limiting is provided by the solar panel –it is not a commonly understood fact that the solar panel tends to be a constant current device. For this reason, a solar panel can withstand a short circuit. Therefore, the control does not need current limiting.

Float Charge of Lead-Acid Batteries

This control charges the battery at a constant voltage and also maintains a charged battery (float charge). The float charge voltage specification is a little lower than the charge voltage, so to accommodate both voltages, a compromise is reached by simply reducing the voltage slightly –that is how ALL automotive systems operate. To obtain maximum charge in a 12V battery, set the control to 14 to 14.6V. Automotive systems further reduce voltage to 13 to 13.5V in order to accommodate high temperature operation as the battery is usually located in the hot engine compartment –battery has a negative thermal coefficient of voltage.

Voltage Adjustment

To set the voltage, disconnect the battery and connect a 1K dummy load resistor to the output. The resistor is necessary to shunt potential MOSFET leakage current as well as the green LED current.

LDO Solar Charge Control Circuit Operation

R4 and D1 form a 6V shunt zener voltage reference. Q1 & Q2 make up the classic differential amplifier that amplifies the difference between the reference voltage and the feedback voltage from the arm of potentiometer R6. The output is taken from the collector of Q1 and drives the gate of P Channel MOSFET Q3. Differential voltage gain is probably in the order of 100 to 200. For best performance, I selected Q1 & Q2 for matched hFE. As the feedback voltage increases at the arm of R6, Q2 turns on harder and steals some of the emitter current away from Q1. The collector current of Q1 follows the emitter current and drops less voltage across R1 thus reducing Vgs of Q3 and turning it off. C2 provides frequency compensation to prevent the amplifier from oscillating.

Q3 is dormant unless the battery is connected reverse –should this happen, Q3 turns on and reduces the reference voltage input to zero thus turning Q1 & Q3 and preventing damaging battery current.

D3 prevents the battery voltage from appearing across an inactive solar panel.

Thermal Management

This is a linear series regulator that dissipates significant power when the pass transistor is both conducting current and dropping voltage simultaneously –during maximum charge rate when the voltage drop is low, the heatsink runs warm –when the battery is fully charged and there is low charge current, the heatsink is cold –but when the battery starts to top off at maximum voltage, the heatsink runs very hot –such is the nature of a linear regulator. At 4A, Q3 drops 3.3V (assuming solar panel voltage is 18V)(the remaining 0.7V is the D3 voltage drop. P = 4A * 3.3V = 13.2W. The heatsink is rated at 3.9°C/W, so heatsink temperature rise = 13.2W * 3.9°C/W = 51.5°C. Adding the 25°C ambient temperature results in a heatsink temperature of 76.5°C. While this may seem very HOT to the touch, it is still cool to the transistor that is rated for a junction temperature of 175°C.

For the Future

A 6V version –while this control may be adjusted down to 7V for charging 6V batteries, the performance is marginal, but will function at reduced current. A 6V version is on the drawing board.


Perf board –sorry, no circuit board artwork at the time of publication.

12V LDO Solar Charge Control Photo


Join the conversation!

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  • amitkamboj8195gmail-com

    What is the output current from this charge controller and specify the baster AH rating also which can be charged ?

    • Jim Keith

      The output current is identical to the solar panel output current. e.g. if a 20W solar panel has an output current of 2.5A in full sunlight, the charge current will be 2.5A. Regarding the battery AH capacity, it is generally best if the battery can be fully charged within one day or less. e.g. A 2.5A solar panel can fully charge a 10AH battery in 4hours–this is ideal because it can charge the battery on a partly cloudy day. However, consider the load because daylight discharge subtracts from the charging current and may prevent full charge. Also, it is acceptable to use a much larger battery if the solar panel can normally keep it charged–this assumes that the daily percentage of discharge is generally low so that it can deliver power over the period of several cloudy days.

  • alexis

    Do you have a specific recommended part # for D3? I see 4A is recommended, what would be the recommended voltage in this case? I presume over 14 v.

    • Jim Keith

      Actually, D3 need not have a higher current rating than the solar panel output. A 50V PIV rating is all that is necessary because it sees no more than the battery voltage. However, most diodes are in the order of 400 to 600V. For a part #, I recommend the 6A4 or 6A6 (400 & 600V respectively). At a 6A current rating, it is well over the 4A indicated, but it is a standard device that costs between 0.1 and $0.40. I do not see an inexpensive 4A rectifier.

  • madhukar27

    Hi Jim
    Madhukar from India here.
    I plan to implement this circuit with a 12V,10W solar panel for charging a 12V 4.7 AH battery. Any changes to be made?
    Also what calculation was used to decide the value of resistors and capacitance ?

    please mail me the documentation for this project at


  • ravi159951

    thanks for replying sir
    i want to know that can we use basic buck regulator to charge 12v,6A/h battery form 40w panel
    or we use 12V LDO Solar Charge Controller…? which is better

    • Jim Keith

      Hello ravi159951, The buck regulator has marginally higher efficiency at the cost of higher complexity. MPPT takes the buck regulator to the next level, but the efficiency improvement remains marginal. For low power applications like this, I favor the simple linear regulator described above.

  • Mister Bill

    I also need a design for 24 Volt, but for 35 Ah batteries.
    Jim, could you please help us on this?


  • irfanisarwadgmail-com

    Hello Sir
    Its Irfan Bangalore(Karnataka)
    I wanted the solar charge controller design for 24 volt 14 Ah batteries.
    It would be very great full if any helps me out with this.

  • rakeshozaheregmail-com

    Hello Jim,

    I am Rakesh Oza.I am from Rajkot(Gujarat,India).
    I have tested and verified the design.
    It is working fine.
    Thank you very much for sharing the design and detailed description.
    I have learned very much from your sharing.

    • Jim Keith

      Hello ravi159951,
      Start by reading this primer:

    • ravi159951

      hye plz help me, i have to make a solar charge controller 12v what can i do?
      what are the things required for it plz help me

    • rakeshozaheregmail-com

      I have connected 20watt solar panel with mentioned circuit to charge 12Volt/7Ah battery.I am using this battery for 6watt/12volt LED bulb(1 no.) to be used for 10hours at night.Battery discharge current is around 450miliampere.During daytime this battery again gets charged within 4-5 hours.This solar panel is charging battery with approximately 1 Ampere.You can also use 15watt solar panel.

    • farhan901224

      hai! it interesting to know that u managed using this circuit. may i know the current rating and how long it take to full charge the battery ?

  • Slawomir

    Hey everybody.
    Got a question for those who a more into that.
    Jim wrote that the battery is charged with constant voltage. And what about charge current? How high it could be?
    Another one.
    Regulator reduce the charging voltage to float voltage automatically or I should do it by myself?
    Thank you for your respond.

  • tucson100gmail-com

    Hi Jim,
    What is the limiting factor for the 20 volt limit from the solar panel?
    The panels I’m using can peak at about 22 volts with no load, do you think it would be a problem?

  • aasimzia

    i am using IRF9630. it is working fine but i wanna ask its vgs=-+ 20 and solar panel open voltage is 21. will it work?

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