When the battery voltage is low the current flows into the gate terminal of SCR1 through resistor R5 and diode D7. Therefore the
SCR1 will conduct heavily for every half wave cycles and charges the battery with large currents. The voltage of the battery is sensed
(or detected) by the serially connected resistor R3 and the preset resistor PR2. The R3 and PR2 form voltage divider circuit. The PR2,
in this circuit, can be considered as two resistors: PR2 (u) and PR2 (l).
Therefore PR2 = PR2 (u) + PR2 (l)
The total resistance R
T above the center tap of PR2 is
RT = R3 + PR2 (u)
The voltage develops at the center tap of the preset PR2 (V
PR2(l) ) can be calculated using the voltage divider equation:
VPR2(l) = ( PR2 (l) / R
T + PR2 (l) ) V
Battery
VPR2(l) = ( PR2 (l) / R3 + PR2 ) V
Battery
The center tap of the preset resistor PR2 is connected to the cathode of the zener diode ZD1. The zener voltage of ZD1 is 11 V/1W.
The anode of the ZD1 is connected to the gate of SCR2. The SCR2 does not conduct if the voltage V
c
develops at the center tap of the
PR2 is less than the zener voltage.
If the battery voltage becomes high due to the charging current the voltage V c will also high. If Vc
becomes higher than the zener
voltage, the SCR2 will conduct for every half wave cycles. The conduction of SCR2 will start at 90 degree of the positive half cycle.
In the other word, it is the position for the largest charging current for the battery by the SCR1. Therefore large charging current is
shunt to the power resistors R5, R6 and SCR2.
In this condition SCR2 is conduct early than the SCR1. The voltage drop across the anode and cathode terminal of SCR2 will re duce
when the SCR2 is conducts heavily. Therefore the voltage between R5 and R6 becomes low and it makes the diode D7 reverse biased.
This causes the gate current of SCR1 stop. And therefore the conduction of SCR1 is stop. Therefore the charging of the batter y
through SCR1 is stop.
The voltage between the anode and cathode of the SCR2 is approximately zero voltage when the SCR2 is heavily conducted. In this
condition the voltage at the junction of resistors R5 and R6 can be calculated by assuming the peak voltage of the rectifier output to be
about 16 V (V
P = 16 V) as follows:
The voltage at the junction = ( R6 / R5 + R6) V
P
= ( 47 / 47 + 47) 16 V
= 8 V
The voltage is fed to the gate of the SCR1 via diode D7. The voltage is much lower than the cathode voltage of SCR1. Therefor e
SCR1 is not conduct in this condition. The power ratings of the resistors R5 and R6 are 2 W respectively. They are chosen to
withstand the current passing through the heavily conducted SCR2.
Although there is no charging current to the battery through the SCR1 there is another charging current path to the battery. This is the
path formed by the diode D5, the resistor R1 and the preset resistor PR1. This is actually the slow charging current path (or trickle
charging). This prevents the battery from the overcharging.
The amount of slow charging current can be set by adjusting the preset resistor PR1. Similarly the full terminal voltage of the battery
can be set by adjusting the preset resistor PR2. The full terminal voltage for a 12 V battery is about 14.5 V
The port B pin (RB1) is configured as digital output pin. The RB1 normally produces logic ’0’ level while the battery is char ging. The
RB1 will produce logic ’1’ when the full terminal voltage of the battery is get. The RB1 output pin is connected to the b ase of the
transistor TR1 through the current limiting resistor R. The emitter of the transistor is connected to the ground. The collect or is
connected to the positive terminal of the battery through the relay coil. It is shown in Figure 4.4. The transisto r is OFF when the RB1