It, Part 2 By Figure 2.
Resistance changes with temperature for molybdenum disilicide and tungsten heaters.
In this conclusion to my two-part series on how a heater's construction materials affects how you control it, I'll look at molybdenum disilicide and tungsten heaters. Figure 2 shows how resistance varies for these units. In the face of low element resistance, you need current-limiting to protect wiring and transformer windings and to avoid nuisance fuse blowing and breaker tripping. At all values of resistance, you have to limit the power to a level that keeps the elements' watt density below the manufacturer's recommended watts per square inch. The temperature controller puts a control signal representing demanded power into the SCR, which delivers power in proportion, regardless of variations in line voltage and load resistance.