Nellcor NPB-40 Service Manual page 42

Technical Supplement
S-10
S3.2.4.1 Power Entry Circuitry
Self-resetting fuse R22 protects the NPB-40 and will open when current in
excess of 0.75 Amps is drawn by the unit. R22 will close when the condition
causing excessive current has been eliminated. CR7 provides reverse current
protection and limits negative voltages (batteries reversed) to safe levels. In
either of these conditions, CR7 will conduct and cause fuse R22 to open. CR5
protects against large voltage transients caused by ESD, EMI, etc. and will
pass these undesirable transients to ground.
S3.2.4.2 Power Supply Feedback Circuitry
The power supply feedback circuit consists of Q1 and its associated
components. When batteries are installed and the NPB-40 is turned off, the
battery voltage (VBAT) is applied through resistor R2 at the collector of Q1
providing a logic 1 level voltage to the D input of flip-flop U1B in the power
switch circuit. When the NPB-40 is turned off, VCC is at 0 volts and Q1 is
turned off. When the NPB-40 is turned on (See "Power Switch Circuitry"), the
VCC potential is applied to the base of Q1, which turns it on. The voltage at
the D input of U1B then drops to a low voltage to provide a logic 0.
S3.2.4.3 Power Switch Circuitry
The power control circuit consists of U1B and its associated components. U1B
is a D flip/flop with asynchronous preset and clear; only the clear is used.
Battery voltage is applied to U1B whenever batteries are installed. The NPB-
40 is turned on and off by pressing the On/Off key on the front panel keypad.
The BTN PWR line is normally pulled up to the battery voltage by R3. When
battery voltage is at an acceptable level and the On/Off key on the front panel
keypad is pressed, the BTN PWR line is grounded and the resultant high-
going pulse from U2A toggles the CLK input of U1B and the high D input
logic level is transferred to the Q output. The logic state of the D input is
controlled by the power supply feedback circuitry (See "Power Supply
Feedback Circuitry"). When the NPB-40 is off the D input of U1B is high.
Pressing the front panel On/Off key sets the Q output and the PWR ON
signal high. This enables the power supply circuits, which then generate the
operating voltages for the NPB-40. The Q-not output of U1B is applied to U4
analog to digital converter input P0.6 through a battery voltage sensing
circuit consisting of U2B, R14, R17, and C14. When the battery voltage drops
below 3.6 Vdc, as measured by U4, the PWR DOWN line at U4 output HSO3
goes high and turns on Q2, which clears flip-flop U1B and the PWR ON
signal at the Q output goes low and turns off the power supply circuits.
When the NPB-40 is turned on, the D input of U1B is low (See "Power Supply
Feedback Circuitry"). When On/Off key on the front panel keypad is pressed,
the BTN PWR line is grounded and the resultant high going pulse from U2A
toggles the CLK input of U1B and the low D input logic level is transferred to
the Q output. This sets the Q output and the PWR ON signal low, which
turns off the power supply circuits.