Electrical System
Chapter 3: Theory of Operation
3 - 4 Affinity® Four Birthing Bed Service Manual (195826 REV 2)
Voltage Regulators
U2 is a National Semi LM2595 adjustable switching regulator, set for a +12.14 V DC +/- 4% output. U5 is
a fixed 5 V regulator with an output of +5.0 V DC +/- 0.2 V. These two regulators are only on when AC
power is available or during battery wakeup mode. U40 is a low current 5 volt fixed regulator for the
battery wake up circuit. The output is 5.0 V+/- 0.2 V DC and the typical current draw is 30mA with a
maximum of 50mAdc. This regulator is operational when AC power is available and as long as the
battery is greater than approximately 13 V DC. U37 is a 3.3 V DC fixed regulator. Output is 3.3 V DC +/-
2%. U38 is a 1.8 V DC fixed regulator. Output is 1.8 V DC +/- 4%
Battery Charger
U4 is a unique regulator designed for battery charging. The battery charge voltage is 27.25 V+/- 1.4 V;
0.55 A max current draw. The current is limited to 0.55 A by the 1 ohm resistor, R126. This is a voltage
mode battery charger. When the battery is low, it charges at the maximum current possible until the
voltage approaches 27.25 V. Then the current starts to decrease and will eventually float at 27.25 V DC
+/- 1.4 V.
Battery Charge Indicator
The battery charge indicator LEDs on the siderails will only show the charge status while on AC power or
in battery wake up mode. The battery indicator has 3 modes: off, flashing, and on. The LEDs should be
off if the battery voltage is less than 22v, flashing between 22v and 24 volts, and on if greater than 24
volts. The battery circuit is simple and should not be used during bed articulation. The battery charge
status should be reasonable when the bed is idle (no bed articulation) and the bed is on AC power or in
battery wake up mode.
Night Light
The night light is always on while AC power is on or in battery wake up mode. Power to the night light is
12 V from the switching regulator
Siderail Interface Circuit
The siderail interface has 4 functions: siderail LED connection/driver, part of the wake up circuit, power
to the siderails, and SPI interface.
The siderail control and LED functions of both rails are controlled from connectors P11 and P13 on the
control board. Each rail is provided connections or a driver to control the LEDs on the siderail boards.
The wake up function consists of a logic high signal from P11-5 or P13-5. The signal from each rail is
filtered (for ESD) and de-bounced and then “or”-ed for the battery wake up signal via U21, U22, and U19.
This circuit is powered by the VCC5BAT power rail.
The SPI (serial peripheral interface) is the communication network between the siderail user controls
and the CPLD, U9, on the control board. The CPLD is the master and the caregiver siderail boards are the
slaves. The CPLD receives inputs (MISO) only, which are the user control switch data. The CPLD supplies
a common chip select (shift/load signal) and the clock signal. These 3 SPI interface circuits, clock, MISO,
shift/load, are designed with two different power supply sides. The CPLD side uses the VCC5 power rail
for supply power while the siderail side uses the VCC5BAT power rail. The two sides must be isolated so
that when in the sleep mode, battery power does not leak into the unpowered logic power, VCC5. This
back feeding could power up the logic side and draw unnecessary power from the battery.
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