4: Theory of Operation
4-4
PRC-PS-MS
4.3.3
Ready LED
During the float charge state, pin 10 goes to a high impedance condition with
the voltage level equal to pin 13 (normally about 2.5 Vdc). This causes U2B
to go low. When the U2B output is low (near ground potential), the READY
LED conducts through D3 and R4. The 12V supply provides the voltage for
the CHARGE and READY LEDs.
4.4
DC Power Supply
In addition to charging BB-LA6 batteries, the PRC-PS provides 14 Vdc to the
PRC1099A. The power supply circuitry uses an adjustable 5 Vdc reference
diode D2 to control the output voltage. Series resistors R2 and R3
forward-bias the diode and R4 adjusts the reference voltage which is typically
about 4.6 Vdc. This reference voltage feeds the non-inverting input of
operational amplifier U1B (pin 5).
The output of U1A (pin 1) connects to the U1B inverting input and is typically
at ground potential. Resistors R11 and R12 bias U1B to provide a stage gain
of 3, setting the output to about 14.0 Vdc. This output drives the Darlington
pass transistor array formed by Q1, Q8, Q7, Q6 and Q5. Emitter resistors R33
through R36 ensure each transistor conducts an equal portion of the current.
Series resistor R6 acts as a current sensor for the output current path. When
the voltage dropped across R6 exceeds 0.3 Vdc, the output differential
amplifier U1A begins to rise from its typically low state. Adjustable resistor
R8 provides current limiting by controlling the trigger voltage level required
to cause the output of U1A to start increasing. Since the output of U1A is
connected to the inverting input of U1B, as the output of U1A increases, the
output of U1B decreases. This drives the pass transistor array to decrease the
power supply current. R8 is set so that by the time the total output current
reaches 6A, the power supply output voltage is 0V. Refer to Figure 4.2 below.
Figure 4-2. Power Supply Output Voltage and Current Chart
5
10
15
1
2
3
4
5
6
7
VOLTAGE
CURRENT
