You can effectively and safely charge a supercapacitor to its rated voltage with a basic supercapacitor charger circuit.
Batteries cannot be quickly charged or drained, but supercapacitors can.
But compared to batteries, they also have a far lower energy density.
The output voltage to the supercapacitor in this kind of circuit is controlled by a voltage regulator chip, such the LM317.
After comparing the supercapacitors voltage to a reference voltage, the voltage regulator modifies its output voltage.
Circuit Working:
Parts List:
| Component Type | Description | Quantity |
|---|---|---|
| Resistors | 1k, 1/4 watt | 3 |
| 10k, 1/4 watt | 3 | |
| 2.2k, 1/4 watt | 1 | |
| 1.5k, 1/4 watt | 1 | |
| 3.3k, 1/4 watt | 1 | |
| Capacitors | Ceramic 1nF | 1 |
| Supercapacitor 5.5V 1F | 1 | |
| Semiconductors | IC LM317 | 1 |
| IC LM311 | 1 | |
| MOSFET IRFZ44 | 1 | |
| Transistor BC557 | 1 | |
| LEDs | Red 5mm, 20mA | 1 |
| Green 5mm, 20mA | 1 |
In this circuit power comes from a 12V DC wall adaptor.
We use an LM317 voltage regulator to precisely control the input 12V to 5.5V so that the supercapacitor may be charged safely and effectively.
A MOSFET, which functions as an electrically controlled switch then delivers this regulated 5.5V to the capacitor.
Being a clever gatekeeper, the MOSFET functions with a voltage divider circuit consisting of resistors R2 and R3, the voltage on pin 3 of the LM311 comparator must be lower than the preset value 4.86V in order for current to flow and charge the capacitor.
To create the 4.86V reference, this voltage divider connects to a 12V source.
The LM311 comparator constantly watches this reference voltage as the capacitor charges and its voltage rises.
The LM311s output pin 7 swings low, shutting off the MOSFET and stopping the charging process to avoid overcharging when the capacitor voltage rises over 4.86V.
The voltage control of this circuit does not directly require an operational amplifier (op amp).
There is another purpose for the BC557 PNP transistor.
When the charge is done it functions as an indication by turning on an LED.
When the MOSFET is off (capacitor voltage over 4.8V) the LED illuminates visually indicating that the capacitor is completely charged.
Formula:
LM317 Voltage Regulator Output Formula:
Vout = 1.25 * (1 + R2 / R1)
Here,
- Vout: The output voltage of the LM317 regulator is 5.5V.
- R1: The value of the first resistor in the feedback loop e.g. 1k.
- R2: The value of the second resistor in the feedback loop e.g. 3.3k.
You may regulate the output voltage of the LM317 regulator within its designated range by varying the values of R1 and R2.
The resistor values of 1k and 3.3k are utilized in this particular circuit to provide an output voltage that is nearly 5.5V, which is needed to charge the supercapacitor.
Note:
This formula may not apply to other regulator types, it is exclusive to the LM317 voltage regulator.
How to Build:
To build a Simple Supercapacitor Charger Circuit you need to follow the below mentioned connections steps:
- Assemble all the components as shown in diagram above.
- Connect pin 1 of IC1 to capacitor C1 through resistors R1 and R2, connect pin 2 of IC1 to drain of MOSFET Q1, connect pin 3 of IC1 to positive supply of +12V.
- Connect pin 1 and pin 4 of IC2 to ground .
- Connect pin 2 of IC2 between resistor R3 and resistor R4.
- Connect pin 3 of IC2 as a reference voltage.
- Connect pin 5 and pin 6 of IC2.
- Connect pin 7 of IC2 to gate of MOSFET Q1.
- Connect pin 8 of IC2 to positive supply.
- Connect Q1 MOSFET gate pin to pin 7 of IC2, connect drain pin to Vout pin of IC1 and connect source of MOSFET to ground through supercapacitor.
- Connect transistor Q2 collector to pin 8 of IC2, connect base of transistor Q2 to pin 8 of IC2 through resistors R5 and R7, connect emitter of transistor Q2 to ground through resistor R6 and green LED1.
- Connect LED2 and resistor R8 from positive supply to ground.
- Connect resistor R9 from gate pin of MOSFET Q1 to ground.
- Connect capacitor C1 from Vout pin of IC1 to ground.
Conclusion:
To conclude, this circuit offers a regulated and secure way to charge a supercapacitor.
The charging voltage is kept within the capacitors safe operating range by the LM317 voltage regulator.
As a clever gatekeeper the MOSFET only permits current to pass when the capacitor needs to be charged.
In order to prevent overcharging, the LM311 comparator continuously checks the capacitor voltage and stops the charging process when it exceeds a certain threshold.
An LED is illuminated by the BC557 transistor which acts as a visual indicator when the capacitor is completely charged.