Battery Charger
This circuit is an AC-powered current source designed for
recharging batteries. It can crank out as much as 1 amp and can be
modified to go even higher by choosing different devices for Q1.
Since this circuit uses AC line voltages and currents, please
exercise extreme caution during assembly, turn-on, and test.
NiCAD batteries have a capacity specification called
milliamp-hours. This value called "C" is a measure of how much
total current they can provide in one hour. Milliamp-hours is
another way to express the energy contained in the battery. To
recharge a NiCAD battery conservatively, it is common practice to
pump a current of 0.1 C into the anode or positive terminal for
about 12 hours.. Therefore, if you had a D-size NiCAD with a
capacity of 4000mAh, you would want to charge it at 400mA for about
12 hours. Another advantage of this charging technique is that it
is gentle on batteries and doesn't cause them to lose capacity as
quickly as the fast charge techniques.
The output current of this device is controlled by
the summation of the bandgap reference diode and the base-emitter
junction of the PNP transistor. The PNP transistor provides
negative feedback to the gate of the MOSFET. As noted in the
schematic, the batteries being charged can have a total of 12V
which is equivalent to about 8 NiCAD's in series. The output
current is determined by the value of R1 which is determined
by:
R1=3.2Volts/Iout
The power dissipation of R1 will equal:
Pr1=3.2Volts*Iout
Be sure to provide pleanty of heatsink for Q1 and
choose an appropriately sized resistor for R1. The following table
summarizes some of the resistor current combinations that are
possible:
Iout |
Resistor Value |
Resistor Power |
100mA |
33 ohms |
1 watt |
500mA |
6.2 ohms |
2 watt |
1Amp |
3.3 ohms |
5 watt |
The power dissipation of Q1 as a function of Output
current and load voltage can be shown as: