Fresenius Medical Care 4008 E Technical Manual page 486

After 15 V has been reached, the system switches over to charging with a constant current. Here,
the charging current is approx. 240 mA. This level is maintained as long as necessary for the
battery voltage to rise to 22.4 V.
Subsequently, the charging current starts to decrease. As soon as the current falls below 14 mA,
IC10 switches to the float state and maintains the battery voltage at approx. 21 V.
If the battery voltage is decreased because power is consumed, the above procedure is restarted.
Heater relay control and monitoring
The heater relay is controlled by CPU 2. Relay RL2 is switched on by an L level at X647A, 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 X647A, pin 12. In addition, this signal also
acts upon the heater control.
Heater control
The temperature sensor is applied to X647A, pin 2. Together with R33 and R10, it constitutes a
voltage divider. The rated-value specification for heater control is superimposed over this voltage
divider via X647A, pin 18, and resistors R32 and R34.
The resultant voltage is directed to the operational amplifier IC 2, pin 6 (LM358), which is
connected as PID regulator. The regulator is compensated via a voltage, which is applied to
X647A, pin 24, and is converted to the requisite range of voltage via IC 1, pin 6. This voltage
cannot be influenced by means of the hardware (software compensation!). The output voltage of
the PID regulator is superimposed over a delta signal, which is generated by IC 3, and is then
delivered to comparator IC 2, pin 3. This comparator activates transistor stages T2 and T12. The
L level applied to IC 11, pin 2, causes the triac coupler IL410 to ignite its internal triac, thus
delivering the gate trigger voltage for the external main triac (1, 2, 3 X744D) via R51. The external
main triac accepts the load current of the heater rod. The triac is fired with each zero crossover of
the 220-V supply voltage only. To this end, a zero voltage recognition is integrated in the coupler
IC 11. As a result, any disturbances, which are caused by the steep build-up of current under load
in case of phase-shift control, are avoided.
The heater control can be inhibited by an H level at X647A, pin 19. As already mentioned above,
the heater relay monitoring unit also acts upon the control. Via diodes D9 and D10 as well as the
RC element R72, R71, C42, the heater relay IC 1, pin 1, if de-energized (IC 1, pin 1, at H level),
causes the heater control to be inhibited. The heater control is released only after the heater relay
(IC 1, pin 1, turns to L level) has been activated and C42 has been unloaded via R71. This
measure is intended to protect the triac coupler IC 11 and to treat the relay contacts of the heater
relay gently.
For hot rinsing, an H level at X647A, pin 20, causes R34 and R10 to be connected in parallel,
resulting in a rated temperature value of approx. 84 °C.
8-189
Fresenius Medical Care 4008 4/09.03 (TM)