Novametrix Medical Systems 515 Service Manual page 11

Service Manual: Model 515 Theory of Operation:Power Supply Be
When the battery runs low a warning message is displayed which instruct;
reconnect the mains supply in order to recharge the battery. If the battery i
charges to a predetermined level (=8 VDC) the unit will automatically turn
self off to avoid damage to the lead acid cell.
1.2.1 Transformer and rectifier
When the unit is powered by AC (mains) the secondary voltage from
power transformer is rectified by D13 and filtered by Cl6. Transistor Q4 is
ased on, this powers IC4, a power switching regulator. This regulator is se:
13.8 volts DC by VR2, it not only charges the battery while under AC (mai
operation but also supplies power to the VBACK regulator. If the AC (mai
is removed then the internal lead acid battery powers the monitor. With
mains power diode D6 is forward biased, this enables the battery to power
monitor. When AC is present D6 is reverse biased due to the fact that
voltage on its cathode (approx. 22 VDC) is more positive than the voltage
its anode (approx. 12 VDC). Therefore the battery receives only a charge w
age from IC4 via D4, FI, and R14. Fuse FI and resistor R14 are to protect
battery from excessive charge currents. Diode D4 isolates IC4 and ICS wl
under battery power.
1.2.2 Backup regulator
Backup memory which is located on the digital board, and the control (
cuitry which turns the unit on, requires power even when the unit is off and
connected to the AC mains. The backup regulator IC2 will have either 4
VDC (battery power) or +13.8 VDC (AC mains power) at pin 8, this is tl
regulated to +5 VDC by adjustment of VRl and appears at pins 1 and 2 of I(
This voltage powers ICl and IC3 which control turning the unit on and off, ii
also run to the 2283 digital board via P401 pin 14 as VBACK which pow
the backup memory.
1.2.3 Control circuitry
The operation of turning the unit on and off is done by ICl and IC3, tht
ICs are constantly powered by either the battery or by mains regardless
whether the unit is on or off. ICl is a dual D flip-flop that is configured so tl
it's Q1 output (pinl) will toggle with each CLK (pin 3) pulse received. 1
CLK input to ICl is driven by IC3 pin 3 which monitors the front panel POWI
button (P401 pin 12). With each depression of the front panel POWER butt
pin 3 of IC3 will pulse high, therefore pin 1 of ICl will toggle from its previc
state.
When the unit is off pins 12 and 13 of IC3 are high, therefore pin 11 is lo
this keeps Q1 off which in turn biases Q2 off. With Q2 off IC6 receives no po
er so the DC-DC converter is inoperative. By pressing the POWER button f
1 of ICl will toggle low, this brings IC3 pin 11 high, Q1 is turned on biasing (