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Single LM317 based  MPPT Simulator Circuit

Single LM317 based MPPT Simulator Circuit

To make this simple MPPT circuit we first modify a standard LM317 power supply circuit into a buck converter then configure it with a solar panel for implementing an MPPT function.

Modifying a LM317 Power Supply into an MPPT Solar Optimizer

In our previous article we learned how a standard LM317 power supply could be transformed into an inductor based efficient variable buck converter power supply circuit.

In this article we analyze how the same circuit design could be enhanced into an effective MPPT circuit by a adding an LDR/LED optocoupler and an opamp voltage follower circuit stages.

The complete circuit diagram of the proposed MPPT circuit using LM317 buck converter can be witnessed in the following image:

Single LM317 based MPPT Circuit


The figure illustrates the discussed MPPT circuit, the LM317 and its associated components form a basic buck converter circuit whose output can be varied by simply varying a resistor across C2.

In our previous power supply design we saw a pot being positioned in parallel with C2 for enabling the variable output voltage feature, however since the present design is supposed to perform an automatic MPPT, this pot could be seen replaced with an LDR/LED opto coupler.

How the Circuits Works

The LED LDR opto coupler is a simple homemade device wherein a red LED and LDR are sealed face to face inside a tiny light proof enclosure.

The LDR leads here can be seen connected in parallel with C2,while the LED is integrated with the output of an opamp voltage follower circuit stage.

The input of the opamp can be seen hooked up with the solar panel through a 10k preset.

The idea here is to make sure that as the solar panel voltage increases, the opto LED intensity also increases, which in turn causes the resistance of the LDR to drop.

The dropping resistance causes the buck PWM is narrow its pulses thereby preventing the output voltage to rise, but nevertheless ensuring a proportionate rise in current for the connected load.

In one of my earlier post we understood that in any buck converter design the output from the converter depends on the PWM and the input voltage.

That implies if the solar voltage tends to increase, the buck output could get affected and begin increasing proportionately. This could in turn cause overloading of the panel and degrade the efficiency of the panel.

The present  LM317 MPPT design takes care of this situation through the LED/LDR device and the LM317 variable resistor feature, and combines the two features in conjunction with an opamp voltage follower to develop an effective self adjusting PWM based MPPT circuit.

The adjustment of the opamp 10k preset appears to be quite simple.

How to Adjust the LM317 MPPT Preset

At optimal sunlight, the 10k preset is adjusted such that the output from the buck converter produces a voltage on par with the load voltage specification.

For example suppose the load is a 12v battery, in that case the 10K preset is adjusted to produce around 14.4V.

Once this is done, from here on the output could be assumed to self adjust in response to the sun shine...meaning now as the sun shine increases the LM317 buck converter self adjusts and narrows the PWM at the base of Q1 inhibiting any rise in voltage, but in the process the inductor L1 and C4 makes sure that the excess sunshine is transformed into a proportionate amount of extra current for the battery to enable a faster charging.

Conversely if the sun shine deteriorates, the PWM tends to widen, causing the voltage for the battery to self adjust automatically maintaining the 14,4V level...... albeit with a proportionate amount of reduction in the current.

The self optimizing functionality is carried out throughout the day ensuring the most effective outcome from the panel for the connected load.



About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

70 thoughts on “Single LM317 based MPPT Simulator Circuit”

  1. Hello sir, please how can I prevent high voltage going to the battery if the Q1 becomes faulty, because I noticed if the Q1 short circuit, the input voltage is same as output voltage.

    2. the ideal way if to connect battery first before the panel, how can I prevent the solar controller from start charging if connected otherwise.

    thanks sir

  2. Good evening sir. According to the circuit diagram, i got to know that the circuit can provide an output voltage of 1.8-32 V. Can i get to know the specifications of the solar panel to be used for the proper working of the circuit. I mean to say that what is the range of input voltage and current this circuit can handle.

    Please do reply sir.

  3. Good day sir, please what’s the diff between the above circuit and 555based buck converter circuit for mppt, thanks

  4. Hi sir,
    I’m using your lm317 mppt design for an academic research and need an assistance on components selection with mathematical backups.

    • Hi thakib, you can refer to the datasheet of the IC LM317C you may find the technical details there, beyond this I do not have any further data.

  5. Hello, you can post the editing of the circuit for the PV panel with the following data: Pmp 200Wp, Vmp 78.3V, Imp 2.55A, Voc 99.7V

  6. good day sir.Im having a problem with my thesis on the solar charger circuit because i tried so charger circuits but the power drop is pretty big.My solar panel’s current output is 1.1amps and when it goes to the charger circuit..the output becomes 100milliamps..Can you give me a 24volts solar charger circuit sir with less power loss.

      • i’ve used your automatic charger regulator using lm338 with comaparator lm 741 and voltage regulator 78L12..for all the resistors in your design,i choose their power at 1watt..At that time its not the sunny but i’ve series a 2 pcs 12volts 20watts solar panel to add their voltages.the resulting output voltage on the solar panel was 15 volts but when i tap in on the charger that i made..The resulting output of the charger that i’ve measured was 3.43 volts and then when i measure the current with my multimeter.It was 17.56milliamps only.Can you give me a advice if what should i do?may battery is 12volts7ah and my thesis proposal was 24volts input in the buckboost converter that will give an output of 0 to 48 volts dc..Please help me on this sir..Thanks a lot

        • my plan is to series a 2 pcs 12 volts battery so that it will result a 24 volts dc output sir.So that’s why i’d like to ask a help from you if you have a 24 volts solar charger circuit with less power loss.I think the comparator and regulator consume so much power that’s why a big power loss resulted on my solar charger.Sorry for my bad grammar because its already midnight here and i’m so sleepy right now.

        • you mean to say your LM338 IC regulator give 3.43V when a 15V is applied at the input? that’s not possible unless you have done some mistake in the LM338 circuit. How did you adjust the LM338 pot? even a 7812 will give you a clean 12V with a 15V input.

          also without sunlight the panel current will be quite minimal, so make sure to check the panel current first before connecting to the LM38 circuit.

          first confirm the LM338 output, after that you can go ahead with the controller cut off setting…do the procedures step-wise.

          • Thanks so much. it’s working great, but the tip147 is getting too hot at high current gain.please how can I combine it in parallel. Thanks once again

            • Glad it’s working Glory, TIP147 can handle upto 10 amps, but transistors can begin heating even when only 20% of the rated current is applied across them, so a large heatsnk becomes essential for these devices.
              If you want to put them in parallel you can simply join their respective terminals in parallel, but make sure to mount all of them over a common heatsink.


            • Thanks sir for your response, I think I got a wrong result earlier, where I said I had current gain from the mppt design, I am getting the following result presently.

              At 2.02pm weather partly cloudy, I measured the Solar voltage without any connection, I got 18.53v

              Pwm-1.74A the controller I bought
              Mppt -1.46A. I Set the output voltage at 14.4v

              Please why could this be, what can I do. Thanks

            • Hi Glory, which MPPT are you referring to, is it the above one?
              The above design is totally dependent on the optimization of the coil parameters and the frequency, its performance can be improved by experimenting and optimizing these parameters to the most effective levels.

            • To optimise the frequency how do I change the capacitor, with c2 of 0.01uf I got frequency of 155hz. What should be the range of frequency to get optimum result.thanks.

            • Thanks so much for your educative site and well organised system. This is the comparison result BTW my bought pwm charger and the built mppt.

              Presently the weather is cloudy @Vin-19.12V
              Pwm- Iin- 2.79Amps, Iout-2.52Amps
              Mppt-Iin-2.08Amps, Iout-2.89Amps

              Please sir, I want to ask,
              1. why is the Iin 2.08amps for the mppt lesser than that of the bought pwm
              2. What can I do to solve this.

              Thanks for the good work sir, God will bless you more and more abundantly.

            • Hi Glory,

              where did you measure the input voltage, is it across the solar panel? But anyhow it is the output that becomes important, and since you are getting higher amps with the MPPT that looks much efficient. The drop at the input voltage could be due to the loading and switching of the inductor of the MPPT buck converter.

            • Please sir give me the values of the components I need to change for 24v battery load with 300w/24v panel, because I tried some changes, increasing the Q, L and R2 but the resistors are getting burnt. Thanks sir.

            • Glory, make sure the voltage does not exceed 25V, in this condition you can reduce the 22 ohm to 10 ohm, increase LED resistor to 1K, and reduce R2 value to 0.1 ohms. Rest can be as is.

              However I am not sure about L1, you may have to experiment with it by initially trying 1000uH inductance value

            • OK no problem, I just forgot that the IC LM317HV is rated to handle upto 60V, just make sure Q1 is also selected appropriately

            • Good day sir, thanks for your update, what is the function of the voltage follower and how does it work in this circuit.

            • Thanks Glory, the voltage follower opamp acts as a buffer between the panel and the LDR, and helps to provide an equivalent level of supply to the LED in response to the panels varying voltage and ensures a safe operation of the LED

            • Good day sir, how can I improve the mppt efficiency, what are the factors I should work on, the losses are great.

            • Glory, the above circuit is a basic design it does not have any room for further improvements, …except the initial adjustments, and the component selections

      • Hello Swag, you said for higher panel rating 250w just change Q, L , R2 but the maximum output current for lm317 is 1.5A. Please how is this compatible with high panel wattage

        • Hello Glory, the IC LM317 is used only for implementing the buck converter circuit, the output current is controlled by the PNP, therefore the PNP becomes solely responsible for the output current delivery.

  7. Pls is this. Circuit tested n can I use it for my 40watts solr pannel and what's the maximum amps of battery it can charge

    • you can use it with a 40 watt panel. the circuit can be upgraded to charge any battery simply by modifying the L1 and Q1 accordingly

  8. amazing,, im thinkin` same ckt before the last time youve shown the LM317 as BuckConv, very impressive.. thanks author & Sir/Engr Swag(Hitman). The cheapest way to have MPPT alternative, how cool!

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