JUMO 709061 TYA 201 - Single-Phase Power Controller Operating Manual - Manual (Page 80)

6 Special device functions
78 2012-12-01/00561071 [SCR Power Controller TYA201]
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 (U
2
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
2
· R.
When the current falls below the preset limit value, the automatic current limit-
ing is no longer effective, and the power controller operates with the subordi-
nate U
2
control, i.e. if the resistance continues to increase at a constant volt-
age level, the power fed to the heating elements is automatically reduced
P
Load
= .
(6) P
Load
input signal of power controller (0 to 20 mA) ∼
U
Load
2
R
---------------