The application of solar photovoltaic (PV) energy sources is gaining popularity thanks to the worries of global warming and relativeness of cost implications. A lot of engineers feel solar power is a tempting industry because of its ‘green energy’ ideology. The circuit in this experiment shows it can handle up to 5 A of current from a simple solar panel that output, not more than 75 watts. A charging system is known as ‘pulse-time modulation’ is presented in this circuit design.
MOSFET Control
The current flow from the solar panel to the battery is governed by an N-channel MOSFET, T1. This component works without any heat sink that would eliminate any heat generated during operation because its RD-S(on) rating only 0.024 Ω. Furthermore, a Schottky diode D1 holds the battery from discharging into the solar panel when the sun is down. It also ensures backwards polarity protection to the battery. From the circuit layout of this 5 amp solar controller, you can see the lines with a red hue which means these are possibly higher current paths.
The charge controller will not consume current from the battery. Instead, it is powered by the solar panel. During the night, the charge controller efficiently enters the standby mode. Once daylight is present, the battery starts charging the moment the solar panel gathers enough voltage and current.
How the Circuit Works
The battery terminal potential is split by resistor R1 and trimpot P1. As a result, the voltage configures the charge state for the controller. The most significant component of the charger controller is IC1, a type TL431ACZ voltage reference device that equips an open-collector error amplifier.
The battery detection voltage is steadily compared to the TL431’s built-in reference voltage. IC1 will ensure conduction of the MOSFET as long as P1’s configured level is lower than the internal reference voltage. Once the battery begins to receive the charge, its terminal voltage will begin to rise. The moment the battery achieves the charge-state threshold, the output of IC1 falls to less than 2 V. This will cause the MOSFET to turn off and thereby halting all current flow into the battery. Once T1 is switched off, LED D2 will not illuminate anymore.
There is no alternate path assigned in the regulator IC and because of that, the output of IC stays low as soon as the current flow to the battery is blocked. This also prevents the MOSFET to initiate conduction although the battery voltage drops. Lead-acid battery chemistry requires float charging thus you will need a simple oscillator to govern that operation. The ones used in this experiment takes advantage of the negative resistance in transistors.
A common NPN transistor type 2SC1815 is the one we have implemented in this 5 amp solar controller circuit. Once the LED goes out, R4 begins to charge a 22-µF capacitor (C1) until the voltage is sufficient to affect the emitter-base junction of T2 to drop. At this stage, the transistor will rapidly turn on and discharges the capacitor via R5. The voltage drop across R5 is enough to move T3. As a result, this modifies the reference voltage setting. The MOSFET now attempts to charge the battery again and once the battery voltage achieves the charged point one more time, the cycle repeats. The 2SC1815 transistor performed well while operating this experiment compared to other transistors which may be more aggressive in terms of switching efficiencies.
While the battery becomes fully charged, the oscillator’s ‘on’ interval becomes shorter as the ‘off’ time stays long. This is dictated by the timing components namely R4 and C1. Practically, a pulse of current is delivered to the battery that decreases over time. This charging configuration can also be known as Pulse Time Modulation.
How to Set Up
You just need to have a decent digital voltmeter and a configurable power supply to tune the controller circuit. Adjust the supply to 14.9 V which is 14.3 V representing the battery setting plus the 0.6 V approximate value across the Schottky diode. Next, adjust the trimpot until you see the LED becomes completely dark. This is the switch point and then you will notice the LED will start to flicker. Sometimes, you might need to adjust a couple of times because the closer you get to the comparator to switch at precisely 14.3 V, the more accurate the charging operation will be. After that, detach the power supply from the charge controller because you need to connect the solar panel now.
The 14.3 V setting applied to this 5 amp solar controller charger circuit should be working for most sealed and submerged-cell lead-acid batteries. However, it is fundamental that you check and verify the value of the producer. Choose the solar panel in a way that its amps versatility is operable within the safe charging threshold of the battery you desire to use.