If you need an automatic nimh battery charger circuit that automatic cuts off. The circuit uses IC-AN4558 or LM1458 OP-AMP, to detect a battery voltage. And, it will cut off fast the battery when the power is full. It is a simple project, and low cost to save your budget.
How does it work?
We are reviewing and understanding a ni-cad battery. Someone may not know it. They have body same a general battery. But they are different. When the battery runs out. We can rechargeable into them again.
Figure 1 NIMH NICD Battery Charger circuit with automatic cuts off
And incidentally, the general battery of 1.5 VDC per one unit.
but the nicad battery that has just voltage of 1.2 VDC only.
In the circuit diagram, as Figure 1 this circuit, we use a DC power supply 6 volts to 9 VDC. Secondly, they are applied to input point diode-D1 and capacitor-C1, which is filtered current to smooth up.
Then, IC1 is an OP-Amp is connected to a voltage comparator circuit.
When we apply the battery to charging. During the first, the voltage across both battery less than 2.5 VDC, Which a positive will connects with pin 2 of IC1 so cause the voltage at pin 2 less than pin 3 of IC1. The voltage at pin 3 of IC1 will be adjusted at 2.6-2.8 VDC by a potentiometer-VR1. The output voltage at pin 1 of IC1 will have voltage is positive, LED2 glow and transistor -Q1 on can conduct current to the battery.
While the battery is charged, LED1 will do not grow. The circuit will still this states until the fully charged.
When the battery is charged fully then the voltage at pin 2 of IC1 will have value more than the voltage at pin 3 of IC1, the output voltage at pin 1 will be 0 VDC cause LED2 go out, but LED1 will glow instead. The transistor-Q1 will stop working because current that bias transistor-Q1 will flow through D2. This project will stop working.
On this automatic NiMH battery charger circuit,we can select current charging are 15 mA and 50 mA by selector switch S2.
How to build them.
In Figure 2 is an actual-size of Single-sided Copper PCB layout.
Figure 3 is the components layout and wiring.
Remember we should look at the correct polarity parts, because this circuit may not works well such as electrolytic capacitors, LED, Diode, Zener etc.
The setting and application.
After that, we assemble all parts successfully. Then we adjust the voltage at pin 3 of IC1 by variable resistor-VR1. Which we use digital volt meter measure by positive lead at pin 3 of IC1 and negative lead to ground. We need to have voltage about 2.6-2.8 VDC which more than the voltage of nicad battery 1.2V + 1.2V = 2.4V.
The application is easy just we take DC adapter at more current than 300 mA up. And the voltage of 6-9 VDC to the input source. .we can select current 2 level is 15 mA and 50 mA. By it is set with resistors R1 and R2 and use switch S2 is a selector.
Calculate the time it takes to charge it with the formula.
Time (hour) = The capacity of the battery (mA) / The charging currents (mA)
If select current 50 mA will charge the nicad battery AA size which has a capacity of 500 mA, we need to take charge equal to 10 hr. But actually, we do not have to worry about it. When fully Charger will stop charging automatically. And LED1 indicates that the battery is fully charged will light up to us as well.
The component list
Resistors ¼ W +5%
ZD1, ZD2—1N4731A___4.3V 0.5W
S1, S2—SPDT- (Single-pole, Double-throw) switch.
IC1—AN4558 or LM1458
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