6V Solar Charge Shunt Regulator Schematic

6V Solar Charge Shunt Regulator

A shunt regulator technique may be effectively employed in regulating the charging voltage of a lead-acid battery. Although it has a practical power limitation (< 20W or so), it has the notable advantage of being an LDO (low dropout) regulator (zero voltage dropout in this case) - this is in contrast with the typical series regulator that often has a dropout voltage in the order of 1 to 3V. Note that 6V solar panels often have minimal voltage headroom that tends to make it difficult to obtain the full charge voltage of 7.2V.
The shunt regulator is connected across the load rather than in series with the load. It regulates the voltage by shunting excess current away from the solar panel. In order for a shunt regulator to function, it must work against high source resistance – in this case, the solar panel is ideal, having a very high source resistance. Shunt regulators tend to be less efficient than series regulators because they dissipate the most power when the output is at minimum – however, in this case the power dissipated is the excess solar panel power that would be otherwise unused.

The circuit is a simple, but practical solar charge regulator – generally simpler than the typical series regulator.


6V Solar Charge Shunt Regulator Schematic

Circuit function

D10 is a schottky battery isolation diode – it is vital in this application so that the actual battery charge never gets shunted by the regulator. The low forward voltage of D10 (approx 0.25V @ low current) means that the solar panel must generate only 0.25V over the battery voltage. The shunt regulator consists of two transistors (Q1 & Q2) in the composite NPN/PNP connection. This has the current gain of a Darlington with the input characteristics of a bipolar transistor. Q2 dissipates approx. 3.5W, so a small heatsink is required. D2 in series with zener D3 (6.8V) and the Vbe of Q1 make up the voltage reference – when the voltage exceeds 7.45V, Q1 & Q2 turn on so that the current is shunted. I had to trim up the voltage of the zener slightly by adding D2 because the zener was running on the low side. The shunt current path is through D4 through D8 – these diodes dissipate approximately half of the power thus taking it away from Q2. D4 through D8 also drop sufficient voltage to turn on LED D9, thus indicating at least 75% charge level.


The circuit may be simplified by eliminating the two LEDs and limiting resistors. D4 though D8 then may also be eliminated. This reduces the component count from 15 to only 6.


In the protoboard photo there is an error – I had the green LED connected to common rather than the cathode of D8.

For the future

Extracting excess power from a shunt regulator for useful purposes


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  • Richard

    Thank you for sharing your knowledge with us!

    What protection features does your circuit have?

    I am also trying to build one of my own for a 5v battery. What modifications would you recommend I do if I were to use your circuit and make it compatible with a 5v battery?

  • John Bilash

    Just a note to thank for taking the time to post these projects.

    I made some slight modifications to the 6V shunt regulator above, and am using it to limit charging of a 12V supercapacitor–from a small array of PV mini-modules. Instead of using the D4-D8 diodes, I’m able to shunt the current through the LED indicator path (using a leftover high-power capable LED).

    It’s just a fun little, thing: I’m using the supercap to run a wireless remote that controls one of those AC appliance switches. The PV box sits on my desk. When the supercap loses some juice, it can be recharged via the PV using my desk lamp. It generally doesn’t lose too much of the charge, so recharging only takes about a minute. The remote will never need a battery again. 🙂

    You’re very kind to share your knowledge in such a gracious way.

  • Muhammad Zaim

    Hi,sir can you send a Schematic diagram, PCB layout and list of component for this circuit.

  • ajit

    Hi, sir i used diode 1n4007 & for 220 ohms 3 resistance in series having same value, but my zeiner diode getting hot i cant touch with bare hand. Pls suggest….

    • Jim Keith

      Zener D3 will get very hot and/or fail if the remainder of the circuit does not function properly –check and recheck wiring carefully. Add 47Ω resistor in series with D3 to limit current before testing again. Also, Q1 may be subject to failure if Q2 does not sink current. Good luck.

  • Manjit

    Hi Jim,

    When D9 led turns on, indicating full charging, still voltage of battery keeps increasing in my case. I have double checked my circuit, but could not find the solution to stop it.

    • Jim Keith

      A slight increase is possible (100mV or so) due to the forward characteristics of the output diode.
      Disconnect the battery and measure the output voltage –perhaps add a dummy load of 1K or so. This will tell you if there is a gross problem. Recheck transistor pin-outs against spec sheets. Good luck.