PID control??

[jlaw]

i am confused by the JLD612's claim to be a PID controller. my understanding of PID control was that the difference between the set-point and the current state is measured along with the historical trend of this error to calculate a new input to the system. therefore, i would imagine that as the temperature difference increases, the power to the heater would increase, and has the temperature gets closer to the setpoint, power to the heater would decrease.

with this relay-based controller, it seems power is either on 100% or of completely. how is this pid control?

[richiem]

The heater is always on 100% when it is on -- but the average power delivered is proportional to how much of the time it is on versus how much of the time it is off. The P in PID is Proportional, so in the P control element, the duty cycle (ratio of time on to time off) is changed to manage total power, with the duty cycle getting lower (time on much smaller than time off) as the set value is approached.

For example, when my hot tub is heating cold water, the heater is on all the time for many hours, but when it is in equilibrium, the heater is on for about 1 second in every 10 (the SSR makes such a short on time possible; you wouldn't do this with a mechanical relay) which neatly compensates for the heat loss through the walls of the tub to ambient air temp. So the on time gets shorter and shorter as equilibrium is approached. That's proportional control in action.