Fresenius Medical Care 4008 E Technical Manual page 458

Heater relay control and monitoring
The heater relay is controlled by CPU 2. Relay RL2 is switched on by an L level at X639A, pin 17.
For reasons of safety, the voltage supply for the relay has been looped via the sensor cable
(HEAT_REL_24V), so that it is impossible to operate the heater relay with the sensor cable pulled
off.
The voltage applied across the coil of the heater relay is monitored by comparator IC 1, pins 2 and
3. Resistors R64 to R67 are laid out such that the condition of the relay is recognized as being cut
off (IC 1, pin 1, at L level) only with a voltage lower than 3 V being applied across RL2. The output
signal of the comparator can be read in by CPU 2 via X639A, pin 12. In addition, this signal also
acts upon the heater control (see below).
Battery charging circuit
Emergency operation of the machine is guaranteed by three series lead-acid storage batteries of
6 V each. An integrated module IC 10 of type UC2906 is used to charge the batteries. The
connected 24-V voltage (24V_SW) is used to provide for the supply voltage required for charging.
The charging procedure is accomplished in three stages.
Up to a battery voltage of 15 V, the minimum charging current amounts to 14 mA (in this charging
state, the charging current decreases with increasing battery voltage).
After a voltage of 15 V has been reached, the batteries are charged with a constant current. The
charging current amounts to approx. 250 mA. This state is maintained until the battery voltage
has risen to 22.4 V.
Then the charging current starts dropping. As soon as the current has fallen below 14 mA, IC 10
switches over to the float state, maintaining the battery voltage at approx. 21 V.
As soon as current consumption causes the battery voltage to drop, the above procedure starts
again.
8-161
Fresenius Medical Care 4008 4/09.03 (TM)