Fresenius Medical Care 4008 E Technical Manual page 459

Heater control
The temperature sensor is applied to X639A, pin 21. Together with R33 and R10, it constitutes a
voltage divider. The rated-value specification for heater control is superimposed over this voltage
divider via X639A, 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
X639A, 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 X639D) via R51. The external
main triac accepts the load current of the heater rod. The triac is fired with each zero crossover of
the 230-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 X639A, 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 X639A, pin 20, causes R34 and R10 to be connected in parallel,
resulting in a rated temperature value of approx. 84 °C.
8-162
Fresenius Medical Care 4008 4/09.03 (TM)