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SI-2 OPTION CARD CIRCUITRY DESCRIPTION (S790-6)
The SI-2 option card filters and regulates external power for the SI-2 main board. Unregulated power enters
the option card at header J2. Various protection elements are built into the option card that improve reliability
and functionality of the SI-2 main board. The circuit is broken into blocks as follows.
Input Protection
Automatic Resetable fuse F1 provides over-current protection. Diode D1 provides reverse voltage protection
while D4 (sidactor), VR1 (metal oxid varistor or MOV), and D5 (transient voltage suppressor) protect against
over-voltage surges and mementary transients. C1, C2, and L1 are configured as low-pass filters designed to
remove high frequency components from the power line.
Voltage Monitoring
U1 is configured as a low voltage detector, where R1 and R2 provide the trip point for enabling the main
supply regulator. Capacitor C12 allows a time delay between when power is applied and when power is
available to the SI-2 main board. Capacitor C3 provides filtering for the voltage trip point. Transistor Q1 and
resistors R3 and R6 provide a logic inverting output for the main switching regulator. Capacitors C5 and C6
provide additional redundant power supply filtering.
Main Power
U2, a LM2595 switching adjustable voltage regulator from National Seminconductor, provides main power for
the SI-2 main board. Pin 2 of U2 is the main power input, while pin 5 controls the output state of the regulator.
Schottky diode D6 with L2 provide the main circuit components for the switching power supply. Resistor R4,
R5 set the output voltage at around 7.2 volts DC.
Post Filtering and Fusing
Capacitors C7, C8, and inductor L3 filter the regulated output voltage. Capacitors C9 and C10 provide
additional high frequency filtering. Fuse F2 is a 0.75 amp fast blow (non-resetable) fuse for additional output
current safety. Additionally, D2 and D3 offer over-voltage protection for the RS232 input and output lines.
Theory
U1 senses the input unregulated voltage Vin and performs the following action:
(1) If Vin is below approximately 11 volts DC, the output to the switching regulator will be at logic high
(switching regulator off).
(2) If Vin is above approximately 11 volts DC the output to the switching regulator will be at logic low (switch-
ing regulator on).
The delay start-up capacitor C12 provides insures that the power reaching the SI-2 option card is stabilized
before power switches on to the SI-2 main board.
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