JUMO 709061/8-01-032 Operating Manual page 88

6 Special device functions
86
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P Eingangssignal des Leistungsstellers (0 ... 20 mA)
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Heating elements that have a positive temperature coefficient (TC) (i.e. where
the electrical resistance increases with increasing temperature) are usual-
ly driven by a power controller that incorporates a subordinate control loop (U
control) (Figure 1).
These are resistive materials such as
- Kanthal Super
- Tungsten
- Molybdenum
- Platinum
- Quartz radiators
Their cold resistance is substantially lower than their resistance when hot (by a
factor of 6 to 16). These heating elements are usually run at temperatures
above 1000 °C.
Figure 1: Heating element with a positive TC
Power controllers need current limiting for the starting phase. The constant
current and the increasing resistance mean that, initially, the power in the heat-
ing element increases in proportion to R, since the power P = I
When the current falls below the preset limit value, current limiting is no longer
effective, and the power unit operates with the underlying V
resistance continues to increase, the power fed to the heater elements falls,
since the voltage is held constant:
2
U
P = automatically becomes smaller.
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R
V1.04/EN/00561071 [Thyristor Power Switch TYA 201]]
(6)
2
2
· R.
2
control, i.e. if the