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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  • 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?


    wat are the exact specifications of battery which we can use?

  • angelus-freakgmail-com

    is there an alternative for fqp27p06?

    • Jim Keith

      The FQP27P06 is the device of choice–RDSon = 0.07Ω, DigiKey has 24,000 in stock @ $1.34. However there are other acceptable choices. In the DigiKey search engine, use these search terms: MOSFET, P-channel metal oxide, TO-220 package, Standard (gate). Then sort for lowest RDSon. Then select one that suits your needs; e.g. if you need 4A, look for something with an RDSon 0.05Ω to 0.11Ω–if all you need 1 or 2A, you can go with a higher RDSon–note that forward voltage drop is a major consideration, and not simply power dissipation (P = I²R). Then look at the breakdown voltages available; e.g. 40 to 100V is good. Per the search engine, the following devices are acceptable for 4A:

      IRF9Z34NPBF, IXTP26P10T, FDP4020P, ZXM64P035L3, IXTP24P085T, IRF5210PBF, IRF5305PBF, IXTP52P10P

      From the above list, some are in stock, some not in stock, some obsolete etc. Also, ebay can be a good source for one or two pieces. Good luck!

  • Nexum

    has anyone got a solder through PCB design layout of the board i.e. Gerber file. it would be much appreciated. Thanks in advance

  • aasimzia

    checked on PCB?

  • Nexum

    Hi Keith,
    it’s great too see such a simple yet effective controller circuit. Could you please send any documentation or info not presented here? Thanks in advance

  • Mister Bill

    Hi Jim, Thanks for posting this circuit. I plan on building it, but I need to modify it for 24V operation.

    What changes are required to input a 24V 4A solar array and charge a 24V 35Ah battery bank?

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