LINEAR_TRUE_MEAN_SQUARE_TEMPERATURE_CONTROLLER

Published:2009/7/14 4:14:00 Author:May | From:SeekIC

The power-control circuit shown outputs true-mean-square power in direct proportion to a 4- to 20-mA current-loop control input. The delivered power is tightly regulated, thus canceling line-voltage variations. In addition, it is proportioned on a fast 8.33-ms timebase, which avoids thermal ripple. The circuit operates as follows: 4- to 20-mA control inputs are converted by A3 and Q5 to negative 20 to 100μA (I1). I5 (fixed at±20 μA) zero-corrects I1, and the 0- to 80-μA difference is applied to the summing point of the A1 integrator. A4 compares the accumulated integral to a 2.5-V reference tapped from the ADJUST terminal of VR1 and, when the integral rises above that, it turns on the 3043 triac trigger optocouplen Zero-cross switching of the Q8 triac minimizes generated noise. Ac half-cycles through Q8 heat RI and push load-monitor current I2 through LED E2. To balance the resulting Q7 photocurrent, A2 produces I3, which causes matching conduction in the E1/Q6 optocoupler. Close tracking between sections of the 2501-2 dual optocoupler assures good proportionalitybetween I2 and I3 Because I3 also biases series-connected Q2 and Q3, voltage applied to the base of Q4 will be 2[X log (Y×I2)+Z], where X and Z are constants common to all five transistors in the 3046 monolithic array, and Y is set by the Fullscale Cal pot. As a result, antilog transistor Q4's emitter current is closely given by (Y ×12)2/I6. When integrated by A1, it gives an accurate prediction of the true-mean-square power dissipated in R1. The resulting feed-back loop acts to adjust Q8's duty cycle to regulate R1 power, allowing the temperature controller to accurately and linearly track the 4- to 20-mA control input signal. Operating power for the circuit is developed from the 4- to 20-rnA loop current, eliminating any need for another power source.



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