Temperature Limiting; Figure 1-3 Airbath Oven Heater Control Loop - Siemens Maxum II PD PA AP Service Manual

Oven Types
1.1 Isothermal Air Bath Oven Heater System (Single and Split)
1.1.2

Temperature Limiting

The temperature-control path of the oven is controlled via an RTD sensor, set-point resistor,
associated circuitry and a solid-state relay. The temperature of the air itself is monitored via
two independent control paths which ensure that all temperatures remain below possible
ignition limits.
The limits set by the set point resistors determine the "T-rating" of the entire analyzer in any
given situation. The sensors are located inside the heater assembly near where the air exits
the heater.
The two control paths operate two relays that control the power to the heater. One relay
maintains the set point temperature and temperature limit of the heater. The second relay is
connected in series with the first relay and is used for overtemperature shutdown.
The first control path has a "temperature limit" (TL) sensor that detects whether the air has
reached a certain limit based on the applicable T-rating. If this temperature limit is reached as
indicated by this sensor, then the power to the heater is cut until the temperature falls below
the limit point.
If a component the TL control path should fail, the overtemperature shutdown (OTS) feature
is activated. The "Overtemperature" (OT) sensor, with its separate control circuit and relay,
activates at a slightly higher temperature than the TL sensor. If this happens, the power to the
oven heater is interrupted. Because this sensor should not activate unless there is a
mechanical failure of the TL control path, the power to the heater cannot be reactivated without
powering down and restarting the analyzer.
Because the solid-state relays controlled by these two circuits are in series, power is removed
from the heater if either one opens.
Oven
setpoint
in
software
OT = Overtemperature shutdown
TL = Temperature limit
PWM = Pulse-width modulation
Figure 1-3
10
PECM
OT
Compare
Setpoint
Resistor
TL
Compare
Setpoint
Resistor
Temp cntl
I 2 C
PWM
Temp
limit
Shutdown
Air Pressure
Airbath Oven Heater Control Loop
Overtemperature Shutdown
Analog
Analog
Analog
Airflow
loss
Shutdown
Switch
Airbath Oven
Overtemp RTD
Temp Limit RTD
Temp Control RTD
AC
Line
Maxum II Valves and Oven Components
Service Manual, 10/2018, A5E42019844001
Heater