The proposed solar optimizer circuit can be used for getting the maximum possible output in terms of current and voltage from a solar panel, in response to the varying sun light conditions.
A couple of simple yet effective solar panel optimizer charger circuit are explained in this post. The first one can be built using a couple of 555 ICs and a few other linear components, the second optin is even simpler and uses very ordinary ICs like LM338 and op amp IC 741. I have explained the procedures.
Circuit Objective
As we all know, acquiring highest efficiency from any form of power supply becomes feasible if the procedure doesn't involve shunting the power supply voltage, meaning we want to acquire the particular required lower level of voltage, and maximum current for the load which is being operated without disturbing the source voltage level, and without generating heat.
Briefly, a concerned solar optimizer should allow its output with maximum required current, any lower level of required voltage yet making sure the voltage level across the panel stays unaffected.
One method which is discussed here involves PWM technique which may be considered one of the optimal methods to date.
We should be thankful to this little genius called the IC 555 which makes all difficult concepts look so easy.
Using IC 555 for the PWM Conversion
In this concept too we incorporate, and heavily depend on a couple of IC 555s for the required implementation.
Looking at the given circuit diagram we see that the entire design is basically divided into two stages.
The upper voltage regulator stage and the lower PWM generator stage.
The upper stage consists of a p-channel mosfet which is positioned as a switch and responds to the applied PWM info at its gate.
The lower stage is a PWM generator stage. A couple of 555 ICs are configured for the proposed actions.
How the Circuit Functions
IC1 is responsible for producing the required square waves which is processed by the constant current triangle wave generator comprising T1 and the associated components.
This triangular wave is applied to IC2 for processing into the required PWMs.
However the PWM spacing from IC2 depends on the voltage level at its pin#5, which is derived from a resistive network across the panel via the 1K resistor and the 10K preset.
The voltage between this network is directly proportional to the varying panel volts.
During peak voltages the PWMs become wider and vice versa.
The above PWMs are applied to the mosfet gate which conducts and provides the required voltage to the connected battery.
As discussed previously, during peak sunshine the panel generates higher level of voltage, higher voltage means IC2 generating wider PWMs, which in turn keeps the mosfe switched OFF for longer periods or switched ON for relatively shorter periods, corresponding to an average voltage value that might be just around 14.4V across the battery terminals.
When the sun shine deteriorates, the PWMs get proportionately narrowly spaced allowing the mosfet to conduct more so that the average current and voltage across the battery tends to remain at the optimal values.
The 10K preset should be adjusted for getting around 14.4V across the output terminals under bright sunshine.
The results may be monitored under different sun light conditions.
The proposed solar panel optimizer circuit ensures a stable charging of the battery, without affecting or shunting the panel voltage which also results in lower heat generation.
Note: The connected soar panel should be able to generate 50% more voltage than the connected battery at peak sunshine. The current should be 1/5th of the battery AH rating.
How to Set up the Circuit
- It may be done in the following manner:
- Initially keep S1 switched OFF.
- Expose the panel to peak sunshine, and adjust the preset to get the required optimal charging voltage across the mosfet drain diode output and ground.
- The circuit is all set now.
- Once this is done, switch ON S1, the battery will start getting charged in the best possible optimized mode.
Adding a Current Control Feature
A careful investigation of the above circuit shows that as the mosfet tries to compensate the falling panel voltage level, it allows the battery to draw more current from the panel, which affects the panel voltage dropping it further down inducing a run-away situation, this may seriously hinder the optimizing process
A current control feature as shown in the following diagram takes care of this problem and prohibits the battery from drawing excessive current beyond the specified limits. This in turn helps to keep the panel voltage unaffected.
RX which is the current limiting resistor can be calculated with the help of the following formula:
RX = 0.6/I, where I is the specified minimum charging current for the connected battery
A crude but simpler version of the above explained design may be built as suggested by Mr. Dhyaksa using pin2 and pin6 threshold detection of the IC555, the entire diagram may be witnessed below:
No Optimization without a Buck Converter
The above explained design works using a basic PWM concept which automatically adjusted the PWM of a 555 based circuit in response to the changing sun intensity.
Although the output from this circuit produces a self adjusting response in order to maintain a constant average voltage at the output, the peak voltage is never adjusted making it considerably dangerous for charging Li-ion or Lipo type batteries.
Moreover the above circuit is not equipped to convert the excess voltage from the panel into a proportional amount of current for the connected lower voltage rated load.
Adding a Buck Converter
I tried to rectify this condition by adding a buck converter stage to the above design, and could produce an optimization that looked very similar to an MPPT circuit.
However even with this improved circuit I could not be entirely convinced regarding whether or not the circuit was truly capable of producing a constant voltage with trimmed down peak level and a boosted current in response to the various sun intensity levels.
In order to be entirely confident regarding the concept and to eliminate all the confusions I had to go through an exhaustive study regarding buck converters and the involved relation between the input/output voltages, current, and the PWM ratios (duty cycle), which inspired me to create the following related articles:
Calculating Voltage, Current in a Buck Inductor
The concluding formulas obtained from the above two articles helped to clarify all the doubts and finally I could be perfectly confident with my previously proposed solar optimizer circuit using a buck converter circuit.
Analyzing PWM Duty Cycle Condition for the Design
The fundamental formula which made things distinctly clear can be seen below:
Vout = DVin
Here V(in) is the input voltage which comes from the panel, Vout is the desired output voltage from the buck converter and D is the duty cycle.
From the equation it becomes evident that the Vout can be simply tailored by "either" controlling the duty cycle of the buck converter or the Vin....or in other words the Vin and the duty cycle parameters are directly proportionate and influence each others values linearly.
In fact the terms are extremely linear which makes the dimensioning of an solar optimizer circuit much easier using a buck converter circuit.
It implies that when Vin is much higher (@ peak sunshine) than the load specs, the IC 555 processor can make the PWMs proportionately narrower (or broader for P-device) and influence the Vout to remain at the desired level, and conversely as the sun diminishes, the processor can broaden (or narrow for P-device) the PWMs again to ensure that the output voltage is maintained at the specified constant level.
Evaluating the PWM Implementation through a Practical Example
We can prove the above by solving the given formula:
Let's assume the peak panel voltage V(in) to be 24V
and the PWM to be consisting a 0.5 sec ON time, and 0.5sec OFF time
Duty cycle = Transistor On time / Pulse ON+OFF time = T(on) / 0.5 + 0.5 sec
Duty cycle = T(on) / 1
Therefore substituting the above in the below given formula we get,
V(out) = V(in) x T(on)
14 = 24 x T(on)
where 14 is the assumed required output voltage,
therefore,
T(on) = 14/24 = 0.58 seconds
This gives us the transistor ON time which needs to be set for the circuit during peak sunshine for producing the required 14v at the output.
How it Works
Once the above is set, the rest could be left for the IC 555 to process for the expected self-adjusting T(on) periods in response to the diminishing sunshine.
Now as the sunshine diminishes, the above ON time would be increased (or decreased for P-device) proportionately by the circuit in a linear fashion for ensuring a constant 14V, until the panel voltage truly falls down to 14V, when the circuit could just shut down the procedures.
The current (amp) parameter can be also assumed to be self adjusting, that is always trying to achieve the (VxI) product constant throughout the optimization process. This is because a buck converter is always supposed to convert the high voltage input into a proportionately increased current level at the output.
Still if you are interested to be entirely confirmed regarding the results, you may refer to the following article for the relevant formulas:
Calculating Voltage, Current in a Buck Inductor
Now let's see how the final circuit designed by me looks like, from the following info:
As you can see in the above diagram, the basic diagram is identical to the earlier self optimizing solar charger circuit, except the inclusion of IC4 which is configured as a voltage follower and is replaced in place of the BC547 emitter follower stage. This is done in order to provide a better response for the IC2 pin#5 control pinout from the panel.
Summarizing the Basic Functioning of the Solar Optimizer
The functioning may be revised as given under:IC1 generates a square wave frequency at about 10kHz which could be increased to 20kHz by altering the value of C1.
This frequency is fed to pin2 of IC2 for manufacturing fast switching triangle waves at pin#7 with the help of T1/C3.
The panel voltage is suitably adjusted by P2 and fed to the IC4 voltage follower stage for feeding the pin#5 of the IC2.
This potential at pin#5 of IC2 from the panel is compared by pin#7 fast triangle waves for creating the correspondingly dimensioned PWM data at pin#3 of IC2.
At peak sun shine P2 is appropriately adjusted such that IC2 generates the broadest possible PWMs and as the sun shine begins diminishing, the PWMs proportionately gets narrower.
The above effect is fed to the base of a PNP BJT for inverting the response across the attached buck converter stage.
Implies that, at peak sunshine, the broader PWMs force the PNP device to conduct scantily {reduced T(on) time period}, causing narrower waveforms to reach the buck inductor...but since the panel voltage is high, the input voltage level {V(in)} reaching the buck inductor is equal to the panel voltage level.
Thus in this situation, the buck converter with the help of the correctly calculated T(on) and the V(in) is able to produce the correct required output voltage for the load, which could be much lower than the panel voltage, but at a proportionately boosted current (amp) level.
Now as the sun shine drops, the PWMs also become narrower, allowing the PNP T(on) to increase proportionately, which in turn helps the buck inductor to compensate for the diminishing sunshine by raising the output voltage proportionately...the current (amp) factor now gets reduced proportionately in the course of the action, making sure that the output consistency is perfectly maintained, by the buck converter.
T2 along with the associated components form the current limiting stage or the error amplifier stage. It makes sure that the output load is never allowed to consume anything above the rated specs of the design, so that the system is never rattled and the solar panel performance is never allowed to divert from its high efficiency zone.
C5 is shown as a 100uF capacitor, however for an improved outcome this might be increased to 2200uF value, because higher values will ensure better ripple current control and smoother voltage for the load.
P1 is for adjusting/correcting the offset voltage of the opamp output, such that pin#5 is able to receive a perfect zero volts in the absence of a solar panel voltage or when the solar panel voltage is below the load voltage specs.
The L1 specification may be approximately determined with the help of the info provided in the following article:
How to Calculate Inductors in SMPS Circuits
Solar Optimizer using Op Amps
Another very simple yet effective solar optimizer circuit can be made by employing a LM338 IC and a few opamps.
So I have explained the proposed circuit (solar optimizer) with the help of the following points:The figure shows an LM338 voltage regulator circuit which has a current control feature also in the form of the transistor BC547 connected across adjustment and ground pin of the IC.
Opamps Used as Comparators
The two opamps are configured as comparators. In fact many such stages may be incorporated for enhancing the effects.
In the present design A1's pin#3 preset is adjusted such that the output of A1 goes high when the sun shine intensity over the panel is about 20% less than the peak value.
Similarly, A2 stage is adjusted such that its output goes high when the sunshine is about 50% less than the peak value.
When A1 output goes high, RL#1 triggers connecting R2 in line with the circuit, disconnecting R1.
Initially at peak sun shine, R1 whose value is selected a lot lower, allows maximum current to reach the battery.
Circuit Diagram
When sunshine drops, voltage of the panel also drops and now we cannot afford to draw heavy current from the panel because that would bring down the voltage below 12V which might entirely stop the charging process.
Relay Changeover for Current Optimization
Therefore as explained above A1 comes into action and disconnects R1 and connects R2. R2 is selected at a higher value and allows only limited amount current to the battery such that the solar voltage does not crash below 15 vots, a level that's imperatively required at the input of LM338.
When the sunshine falls below the second set threshold, A2 activates RL#2 which in turn switches R3 to make the current to the battery even lower making sure that the voltage at the input of the LM338 never drops below 15V, yet the charging rate to the battery is always maintained to the nearest optimum levels.
If the opamp stages are increased with more number of relays and subsequent current control actions, the unit can be optimized with even better efficiency.
The above procedure charge the battery rapidly at high current during peak sunshines and lowers the current as the sun intensity over the panel drops, and correspondingly supplies the battery with the correct rated current such that the it gets fully charged at the end of the day.
What Happens with a Battery Which may not be Discharged?
Suppose in case the battery is not optimally discharged in order to go through the above process the next morning, the situation may be fatal to the battery, because the initial high current might have negative affects over the battery because it's yet to discharged to the specified ratings.
To check the above issue, a couple of more opamps are introduced, A3, A4, which monitor the voltage level of the battery and initiate the same actions as done by A1, A2, so that the current to the battery is optimized with respect to the voltage or the charge level present with the battery during that period of time.
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