Ohmeda 3000 - Functional Description; Power Supply Board

1/Functional Description
A. Power Supply Board
This is a functional description for the Infant
Warmer System Power Supply Board Part No. 0631-
5032-700. Réfer to Schematic No. 0676-0327-000 for
a detailed circuit diagram.
The power supply board contains circuitry for the
control and monitoring of voltage devices. The
board also provides power to the control board and
the display board. Also found on the board is a line
voltage sensing circuit that provides an indication of
line voltage to the microcontroller.
. The control circuits for each line voltage device on
the power supply board are functionally identical
with a logic “High” signal from the control board
switching "On” the desired device. This is per-
formed with an opto-isolator triac driver so low/line
voltage circuits can interact but remain electrically
isolated (2500 volt dielectric).
The heater is controlled from the supply board with
an opto-isolator triac driver and triac. There is also a
relay contact connected in series with the neutral to
the heater triac. This is used to switch “Off" the
heater if the heater triac fails or there is a failure on
the control board.
The regulator circuits provide a +5 Vdc supply to
the display board and +5 Vdc, and +9 Vdc,
supplies to the control board. A ni-cad battery
supplies the § Vdc supply and a de-rated 9 Vdc sup-
ply for standby power, in the case of a power loss.
Standby power of 9 volts is used to activate the
transducer alarm, while the 5 volt supply provides
power to the microcontroller and associated IC's for
memory retention purposes.
5 Volt LEDS
A nominal 8 Vac is input to the power supply board
at J11 pins 3 and 4. The line frequency is also con-
nected to the control board via J12 pin 2. The
bridge rectifier CR2 and capacitor C11 provide a fil-
tered unregulated 8 Vdc to the relay, opto-isolators,
and the regulator VR12. The 8 Vdc unregulated sup-
ply can be measured at TP-1. The unregulated sup-
ply must be a minimum of 7.32 volts for proper op-
eration of the relay circuit.
The output of regulator VR2 is nominally +5 Vde
and supplies power to drive the LED displays on the
display board. The output is measurable at J12 pin
12 (TP-10). When the supply voltage is within 10%
of nominal, the output voltage should be between
4.8 and 5.2 volts dc with a maximum load of 500
ma. The maximum allowable ripple voltage is 150
millivolts.
Line Voltage Sensing
A voltage of approximately 11 Vac from the trans-
former secondary is input to the board at J11 pins 1
and 2. Bridge rectifier CR1 and capacitor C12 pro-
vide a full wave, filtered voltage of approximately 12
Vdc. Resistor R3 is preset to produce an output of
approximately 0.6 volts at J12 pin 11 (TP-11) when
the line voltage is at the nominal value for the unit.
The analog voltage signal at J12 Pin 11 connects to
the control board and is fed into the A/D Converter,
ADC 3711 (U6), via the multiplexer, MC14051B (U-
13). The digital output of the A/D converter is input
to the microcontroller where the software then ad-
justs the power to the heater to compensate for var-
iations in line voltage.
9 Volt Standby
The output of regulator VR3 is adjusted by R4 to
provide 9.0 + 0.2 volts (TP12). This voltage is used
for charging the NI-CAD battery, and supplying the
input voltage to the +5 standby regulator.
5 Volt Standby
When line voltage is available, current flows from
the output of VR3 and through CR5 to provide 9.0
+ 0.2 volts to the input of VR4, and to J12 pin 3
(TP-12). In tum, regulator VR4 outputs a voltage of
5.0 + 0.2 volts to J12 pin 14 (TP-9) with a maximum
ripple voltage of 150 millivolts.
If power loss occurs with the unit switched ON, the
7.2 volt NI-CAD battery maintains a de-rated output
voltage of approximately 6.5 volts to pin 3 of J12
(TP-12). It also provides input to VR4. The output of
VR4 only regulates to approximately 5.0 volts as the
input voltage drops below 7.0 volts.
Observation Light, Alarm Lights, Motor Control
The control circuits for the observation light, and
the alarm light are similar. The alarm light is a resis-
tive load and does not have a snubber circuit in
parallel with the load. The snubber circuit on the
observation light supply protects the triac from
large voltage spikes characteristic of inductive
loads. If the control lines are logic “Low", less than
0.45 volts, this keeps the FET, triac driver, and triac
switched “Off”. The triac acts as a switch to the
line voltage circuit, removing voltage from the load.
When a device should be switched “On”, a logic
“High” of 2.4 volts minimum is output to the corres-
ponding FET. The FET switches “On” causing the
LED ot the isolator-driver to switch on. The isolator/
driver output drives sufficient current to the triac
gate, switching the triac "On" allowing the selected
device to switch “On”.
Heater Status and Control
The heater circuitry consists of a monitoring circuit,
a controller for the heater, and à relay to switch
“Off" the heater in the event of a triac or system
failure.
The full wave bridge rectifier CR6 takes a sample
(through R 13) of the ac signal supplied to the hea-
ter and provides rectified dc to the opto-isolator U3.
If the heater is “On” the de output switches “On”
the LED in the opto-isolator, except at voltage levels
below the forward bias voltage. When the LED is
“On” the transistor goes into saturation causing the
output at J12 pin 1 to go “Low” (about 0.3 volts).
When the heater is “Off” the de signal is in the re-
gion of zero potential and there is insufficient for-
ward bias voltage for the LED. This switches “Off”
the transistor allowing capacitor C10 to charge and
causes J12 pin 1 to go high (5 volts), When the hea-
ter is switched “On" the LED switches the transis-
tor “On” again, and the capacitor discharges. The
low output shows small glitches caused by the
charge/discharge of the capacitor at every half
cycle. The glitches are acceptable provided they do
not exceed the trigger voltage of 1.4 volts for the
74LS132 on the Control Board.
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