Temperature controller saves energy
Edited by Brad Thompson
Tito Smailagich, ENIC, Belgrade, Yugoslavia -- EDN, February 3, 2005
Given the high cost of electrical power, replacing a conventional on/off temperature control with a proportional controller can often save energy and money. Figure 1 shows a low-cost, high-efficiency, time-proportional temperature controller for a residential water heater. An Analog Devices ADT14, IC1, serves multiple functions as a temperature sensor, quad-setpoint, programmable analog temperature monitor and controller. Resistors R1, R2, R3, R4, and R5 adjust desired temperature at setpoints SETP1, SETP2, SETP3, and SETP4, which IC1 compares with the actual temperature from its internal sensor. The ADT14's active-low open-collector outputs drive Input Port A of IC2, an 8-bit Motorola/Freescale 68HC908QT4 microcontroller that provides 4 kbytes of flash memory, 128 bytes of RAM, and an on-chip clock oscillator.
Click here for a zip file that contains, Listing 1, which in turn contains commented assembly-language software. When you load it into the microcontroller's flash memory, the software provides the time-proportional control algorithm. When IC1's OUT1, OUT2, OUT3, and OUT4 outputs are inactive, IC2 switches its output PTA4 to a totally on-state, 100% duty cycle for maximum heating. Also in the zip file, Listing 2 contains an assembled version of the software, and Listing 3 presents the hex code for programming IC2.
When IC1's OUT1 output is active, IC2 produces a 75%-duty-cycle output on PTA4. In similar fashion, when IC1's OUT2 output goes active, IC2 produces a 50%-duty-cycle output on PTA4, and when IC1's OUT3 output goes active, IC2 produces a 25%-duty-cycle output on PTA4. When IC1's OUT4 output goes active, IC1 disables the output on PTA4 to produce a totally off state (0% duty cycle). Table 1 summarizes the relationship of IC2's inputs and output duty cycle.
To minimize component count, IC2's internal oscillator generates a 12.8-MHz clock that divides to produce a sample pulse whose basic width is 0.1 sec for each 1% of output on-time. One cycle of output comprises 100 samples for a total duration of 10 sec. Thus, for a 25% duty cycle, IC2's output PTA4 generates a 2.5-sec on interval followed by a 7.5-sec off interval. One section of an open-collector hex inverter, IC3A, a 74LS06, drives optocoupler IC4, an MOC3043, which features an internal zero-crossing circuit and pilot triac. Power triac Q1, a TIC263M rated for 600V and 25A, controls application of power to the water heater's 2-kW resistive heating element. For best results, place IC1 in close thermal contact with the water heater's inner tank.
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