The article discusses a simple low drop or zero drop solar charger circuit without microcontroller which can be modified in many different ways as per user preference. The circuit does not depend on microcontroller and can be built even by a layman.

What is a Zero Drop Charger

A zero drop solar charger is a device which ensures tat the voltage from the solar panel reaches the battery without undergoing any kind of drop either due to resistance or semiconductor interference such as diodes etc in line.The circuit here uses a mosfet as a switch for ensuring minimum drop of voltage received from the attached solar panel.

Moreover the circuit has a distinct advantage over other forms of zero drop charger designs, it does not unnecessarily shunt the panel making sure the panel is allowed to operate at its highest efficiency zone.

Let's understand how these features could be achieved through this novel circuit idea designed by me.

How the Circuit Functions

Referring to the proposed zero drop voltage regulator charger circuit diagram we see a rather straightforward configuration consisting of an opamp and a mosfet as the main active ingredients.

Here the opamp is wired as a comparator wherein its non-inverting pin is positioned as the voltage sensor via a voltage divider stage made by R3 and R4.

The inverting pin is as usual rigged as the reference input using R2 and the zener diode.

Assuming the battery to be charged is a 12V battery, the junction between R3 and R4 is calculated such that it produces 14.4V at a certain optimal input voltage level which may be the open circuit voltage of the connected panel.

On applying the solar voltage at the shown input terminals, the mosfet initiates with the help of R1 and allows the entire voltage across its drain lead which finally reaches the R3/R4 junction.

The voltage level is instantly sensed here and if in case it's higher than the set 14.4V, switches ON the opamp output to a high potential.

This action instantly switches OFF the mosfet making sure no further voltage is allowed to reach its drain.

However in the process the voltage now tends to fall below the 14.4V mark across the R3/R4 junction which yet again prompts the opamp output to go low and in turn switch ON the mosfet.

The above switching goes on repeating rapidly which results in a constant 14.4V at the output fed to the battery terminals.

The use of the mosfet ensures an almost zero drop output from the solar panel.

D1/C1 are introduced for maintaining and sustaining a constant supply to the IC supply pins.

Unlike shunt type regulators, here the excess voltage from the solar panel is controlled by switching OFF the panel, which ensures zero loading of the solar panel and allows it to operate at its most efficient conditions, quite like an MPPT situation.

The zero drop solar charger circuit without microcontroller can be easily upgraded by adding an auto cut off, and an over current limit features.

Circuit Diagram

Simple Zero Drop Solar Charger Circuit without Microcontroller

Parts List

R1,R2 = 10K
R3,R4 = use an online potential divider calculator for fixing the required junction voltage
D2 = 1N4148
C1 = 10uF/50V
C2 = 0.22uF
Z1 = should be much lower than the selected battery over charge level

IC1 = 741
Mosfet = as per the battery AH and the solar voltage.

Adding a Current Control Feature

The above circuit looks quite an efficient design however lacks a current control feature.

The diagram below shows how the above circuit can be upgraded with a current control feature by simply adding a BC547 transistor stage across the inverting input of the opamp.

R5 can be any low value resistor such as a 100 ohm.

R6 determines the maximum allowable charging current to the battery which may be set by using the formula:

R(Ohms) = 0.6/I, where I is the optimal charging rate (amps) of the connected battery.

Note: A P-channel mosfet may not work correctly, therefore the mosfet in the above zero drop solar circuits must be replaced with N-channel mosfets as shown in the folowing diagrams. Readers are requested to do the modifications as per the following diagrams.


Simple Zero Drop Solar Charger Circuit without Microcontroller and current controlled


Finalized Solar zero drop battery charger circuit:

As per the suggestion of "jrp4d"the above explained designs needed some serious modifications for operating correctly. I have presented the finalized, corrected working designs for the same through the below shown diagrams:

According to "jrp4d":

Hi - I've been messing about with Mosfets (voltage control circuits) and I don't think either circuit will work except where the line in voltage is only a few volts large than the target battery voltage. For anything where the line in is much more than the battery the mosfet will just conduct because the control circuit can't control it.

In both circuits its the same problem, with P-channel the op-amp cant drive the gate high enough to turn it off (as observed by one post) - it just passes the line voltage straight thru to the battery. In the N channel version the op-amp can't drive the gate low enough because its operating at a higher voltage than the -ve line in side.

Both circuits need a driving device operating at the full line in voltage, controlled by the op-amp

The corrected versions are shown below as per the above suggestion:

Using NPN BJT or N-channel mosfet:

zero drop solar charger circuit without microcontroller

In the above figure the NPN power transistor could be a TIP142, or a IRF540 mosfet .....and please Remove D1 as it's simply not required

Using PNP transistor or P-mosfet


In the above figure, the power transistor could be a TIP147 or a IRF9540 mosfet, the transistor associated with R1 could be a BC557 transistor......and please Remove D1 as it's simply not required.


How to Set up the zero drop solar charger circuit

It's very easy.

  1. Do not connect any supply at the mosfet side.

  2. Replace the battery with a variable power supply input and adjust it to the charging level of the battery which is supposed to be charged.

  3. Now carefully adjust the pin2 preset until the LED just shuts off....flick the preset to and fro and check the LED response it should also blink ON/OFF correspondingly, finally adjust the preset to a point where the LeD just shuts off completely....seal the preset.

  4. Your zero drop solar charger is ready, and set.

You can confirm the above by applying a much higher input voltage at the mosfet side, you'll find the battery side output producing the perfectly regulated voltage level that was previously set by you.

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