The post discusses a simple NiCd charger circuit with an automatic overcharge protection and a constant current charging.
When it comes to correctly charging a Nickel-Cadmium cell, it is strictly recommended that the charging process is halted or cut off as soon as it reaches the full charge level. Not following this may adversely affect the working life of the cell, reducing its backup efficiency significantly.
The simple Ni-Cad charger circuit presented below effectively tackles the overcharging criterion by including facilities like a constant current charging as well as cutting off the supply when the cell terminal reaches the full charge value.
Main Features and Advantages
- Automatic cut off at full charge level
- Constant current throughout the charging.
- LED indication for full charge cut off.
- Allows the user to add more stages for charging up to 10 NiCd cells simultaneously.
How it Works
The simple configuration detailed here is designed to charge a single 500 mAh 'AA' cell with the recommended charge rate of close to 50 mA, nonetheless it could conveniently be customized cheaply to charge several cells together by repeating the area shown in dotted lines.
Supply voltage for the circuit is acquired from a transformer, bridge rectifier and 5 V IC regulator.
The cell is charged with a T1 transistor which is configured like a constant current source.
T1 on the other hand is controlled by a voltage comparator using a TTL Schmitt trigger N1. During the time the cell charges the terminal voltage of the cell is held at around 1.25 V.
This level appears to be lower than the positive trigger threshold of N1, which keeps the output of N1 high, and the output of N2 becomes low, enabling T1 to get the base bias voltage through the potential divider R4/R5.
As long as the Ni-Cd cell gets charged the LED D1 remains illuminated. As soon as the cell gets close to the full charge status its terminal voltage climbs to approximately 1.45 V. Due to this, the positive trigger threshold of N1 rises causing the output of N2 to go high.
This situation instantly turns off T1. The cell now stops charging and also the LED D1 is shut off.
Since the positive activation limit of N1 is approximately 1.7 V and it is controlled by a specific tolerance, R3 and P1 are incorporated to alter it to 1.45 V. The negative trigger limit of the Schmitt trigger is around 0.9 V, which happens to be lower than the terminal voltage of even a completely discharged cell.
This implies that connecting a discharged cell in circuit will never trigger the charging to initiate automatically. For this reason a start button S1 is included which, when pressed, takes the input of NI low.
To charge more number of cells the portion of the circuit revealed in the dotted box may be repeated separately, one for each battery.
This ensures that, regardless of the discharge levels of the cells, each one of them is individually charged to the correct level.
PCB Design and Component Overlay
In the PCB design below two stages are duplicated for enabling two Nicad cells to be charged simultaneously from a single board set up.