My key takeaway from A33Zab's schematic is that there is no direct feedback from any of the probes controlling the power level. Temperature control is reactive rather than proactive. If there were temperature feedback sensors in the probe, it would be possible to more finely control the probe power input. That is where the next big area for improvement in Pitot tube design/performance lies.
All the PHC does is supply 115 V to the Flight or Ground circuits of the Probe heater . There is no High or Low setting as some may understand the concept.
Last edited by Machinbird; 3rd Mar 2012 at 23:04.