WO1996002029A1

WO1996002029A1 - Trim heater controller

Info

Publication number: WO1996002029A1
Application number: PCT/US1995/008177
Authority: WO
Inventors: William A. Harris; Arthur N. J. Pearman
Original assignee: Honeywell Inc.
Priority date: 1994-07-07
Filing date: 1995-06-30
Publication date: 1996-01-25

Abstract

A high resolution, low noise trim heater controller. A rate of change of heater temperature sensor is incorporated in the controller to anticipate the heater thermal constant or inertia for better trimming the control of the desired or target temperature. Also, a reference integrator is used to account for actual heat input level and the quantity provided. The trim controller switches the heater on or off at the cross-over of a power cycle, for minimizing noise and maintaining power line quality. The minimum duration of the 'on' or 'off' time of a heater is one power cycle and the maximum is any needed number of cycles. The minimum duration permits very high resolution in maintaining the target temperature. The trim heater control system may be used with power sources having any number of phases.

Description

TRIM HEATER CONTROLLER

BACKGROUND OF THE INVENTION The invention pertains to temperature control of a contained or flowing medium, such as a air in a room or liquid moving in a pipe. Particularly, the invention pertains to heater controllers and more particularly to trim heater controllers for refined control of a temperature.

Past temperature controllers and heater controllers have not been able to provide the extremely accurate temperature control required for spaces used for advanced state- of-the-art processes. Problems of heater thermal constants and the accounting for already generated but not yet measured heat exist in the related art controllers. Further problems are maintaining power line quality and minimizing electromagnetic noise caused by temperature controller systems.

SUMMARY OF THE INVENTION

The present invention is a high resolution, low noise trim heater controller. A rate of change of heater temperature sensor is incorporated in the controller to anticipate the heater thermal constant or inertia for better trimming the control of the desired or target temperature. Also, a reference integrator is used to account for actual heat input level and the quantity provided. There is an analog subtraction involving the desired heat input versus the actual heat input. The trim controller switches the heater on or off only during a cross-over of a power cycle. The minimum duration of the "on" or "off* time of a heater is a full 360 degree power cycle. The maximum time is any needed number of power cycles. The minimum time that the heater can be on or off permits high resolution in maintaining a target temperature by the temperature controller with which the trim heater controller is functioning. Because the heater is on or off for only an integral number, from one on up, of full 360 degree power cycles, the quality of the power line or source is not affected in that a DC level is not created with respect to the power line or source and there is no distortion of the power waveform. The trim heater control system may be for use with power sources having any number of phases or operating at any frequency. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an overall block diagram of a precision temperature control system having three stages of heater and temperature control.

Figure 2 shows the third stage of heater and temperature control which includes a trim heater controller.

Figure 3 is a block diagram of the trim heater controller.

Figure 3a is a diagram of various signals in the trim heater controller.

Figures 4, 5 and 6 reveal a circuit schematic of the trim heater controller

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a temperature control scheme for controlling the temperature of a gas 14, such as air, or a liquid flowing through a duct 12 or pipe 12. One application that the system is used for is controlling the temperature of a room or space wherein the temperature needs to be controlled very tightly within a very small variation of the desired temperature. Air 14 comes into duct 12 from the environment having a delta temperature or temperature variation between plus and minus ten degrees Centigrade (C.) of a desired or target temperature, e.g., 25 degrees C. In duct 12, air 14 enters first stage 16 of temperature control wherein the temperature variation is reduced down to plus or minus one degree C. of the target temperature. Air 14 is heated as necessary by heater 22 which is controlled by controller 24 which responds to sensor 26. Sensor 26 senses the temperature of air 14 such that if air 14 is cooler than the target temperature, heater 22 is turned on to warm up air 14. If air 14 is not heated by heater 22, then air 14 will stay at essentially the same temperature but cool down slightly due to a cooler environment. After air 14 is stabilized at plus or minus one degree C. of the target temperature, air 14 enters stage 18 of temperature control. In this stage 18, the temperature of air 14 is brought within plus or minus 0.1 degree C. of the target temperature. Air 14 is heated as necessary by heater 28 as controlled by temperature controller 32 as guided by a signal from sensor 34. The principle of operation of stage 18 is similar to that of stage 16. Air 14 leaving stage 18 is within plus or minus 0.1 degree C. of the target temperature. Air 14 then enters temperature control stage 20. If air 14 is not heated by heater 36, then it stays at essentially the same temperature but may cool down slightly. Sensor 38 senses the temperature of air 14 in stage 20. Temperature controller 40, as a result of a signal from sensor 38 indicating the temperature of air 14, turns on heater 36 as needed to keep the temperature of air 14 within plus or minus 0.01 degree C. of the target temperature. The invention pertains to stage 20 of the temperature controlling, but also could be applied to stages 16 and 18.

Figure 2 shows further details of stage 20 of the temperature controlling of figure 1. Temperature controller 40 has an industrial temperature controller 42, HONEYWELL model UDC-5000, a transmitter 46 connected to controller 42, a thermistor temperature sensor 48 connected to controller 42, heater control and power supply 50 connected to controller 42 and to heater 36, a heater rate sensor 52 connected to heater control and power supply 50, and a precision temperature sensor 38 connected to transmitter 46. The next devices are mixers 54 in duct 12, which ensure that air 14 from stage 20 is homogeneous in temperature.

Figure 3 is a diagram of trim heater controller 10. Thermistor 52 is proximate to heater 36 and nearly instantaneously senses the heat and the rate of change of heat from heater 36. The resistance of thermistor 52 decreases with an increase of temperature and increases with a decrease of temperature. One terminal of thermistor 52 is connected to a +15 volts DC and the other terminal of thermistor 52 is connected to an input 57 of preamplifier 56 and to a terminal of resistor 58 at node point 60. The other terminal of resistor 58 is connected to ground. Thermistor 52 and resistor 58 form a voltage divider between the +15 volts and ground at node or point 60. The voltage at node 60 is indicated as V^Q ⁼ (15 volts) x [R58 ÷ (R5 + R52)]- R58 is ^tne resistance of resistor 58, and R52 is the resistance of thermistor 52. Resistor 58 is a scaling resistor for setting the voltage at point 60 within operable range for preamplifier 56. The voltage at point 60 is for the purposes of indicating heater temperature rate of change, which is dependent upon thermal heat constant of heater 36. If the temperature proximate to the heater 36 is increasing, the voltage at point 60 increases, and if the temperature proximate to heater 36 decreases then the voltage at point 60 decreases. Preamplifier 56 buffers the voltage signal at point 60 and inverts it at output 62. The voltage at output 62 decreases when the temperature proximate to heater 36 is increasing and vice versa. Signal line 62 goes to capacitor 64. Any change of voltage signal in line 62 indicates a heater rate change of temperature and 95/08177

any rate change involving a change of voltage is capacitively coupled through to node or point 66 at an input of comparator 68. Also, at node 66 and input 68 is a voltage signal at a point between 0 and + 10 volts DC which is the control signal indicating the temperature from controller 42 of figure 2. Controller 42 constitutes the outside loop controller which provides the basic temperature control signal determined by thermistor sensor 48 and precision temperature sensor 38. Control signal controller 42 and signal from capacitor 64 are summed at node 66 at the input of comparator 68. Heater 36 may be an open coil duct 1.5 kilowatt single phase heater (type QUA) from Industrial Engineering and Equipment Co. at 425 Hanley Industrial Court, St. Louis, MO 63144. Thermistor 52 is a FENWALL type 112-104KAJ-B01.

The signal from thermistor sensor 52 compensates for the lag of control signal 71 from controller 42 of the outside temperature control loop of stage 20. For instance, if the heat rate is positive, that is, the temperature of heater 36 is increasing, and voltage 62 at capacitor 64 is decreasing and thus when summed with temperature control voltage 71 from controller 42, it reduces the magnitude of heat increase signal

74 to comparator 68. Reference integrator 70 outputs a signal 72 to comparator 68, which is compared to control signal 74 as modified by the heat rate signal, from node 66. Signal 72 indicates what amount of heat that heater 36 has been delivering or providing to air 14 in duct 12 at stage 20, during a previous number of cycles of the AC power supply line 80 from a power source via isolation transformer 81. Signal 72 from reference integrator 70, indicating what has been delivered at the moment, is compared with signal 74 indicating what is being asked for in terms of heat or what heater 36 should be delivering. If signal 74 has a voltage level higher than that of signal 72, then output signal 76 of comparator 68 is a logic high that indicates that more heat is needed, and if signal 72 has the higher voltage, then a logic low signal 76 indicates that no more heat is needed. Depending on signal 76, high or low, respectively, heater 36 is to be on or off, during the next complete cycle of AC power line 80. The magnitude of signal 72 is proportional to the amount of heat that heater 36 has been outputting over previous cycles. A complete cycle, as opposed to one- half of a cycle, for when the heater is on or off, at a minimum, is used to keep the DC level of AC line 80 voltage at zero and to enhance the quality of supply AC line 80 voltage. Figure 3a shows the relationship of signals 72, 76, 80 and 82. In Figure 3, zero-crossing detector 78 detects the zero positive crossing of the AC line 80 voltage which may be at 60 Hz, 400 Hz or any other frequency. At the zero positive crossing, detector 78 outputs a positive pulse 82 or a logic high signal 82 for about 10 to 30 microseconds. During the period of high signal 82 and while signal 76 is at a high, logic 84 , which outputs a signal 86 that is high which turns the respective SCR 88 via its driver 88 resulting in heater 36 being energized and providing heat for at least one full cycle, or a multiple of only full cycles, as provided by logic 84, and continuing so long as signal 76 is at a logic high. When signal 76 is high, it indicates that heater 36 should be turned on and signal 82 indicates the start of the next cycle. SCR 88 will fire on the leading edge of signal 82. If signal 76 is a low, then signal 82 cannot go through logic 84 on to driver and SCR 88 via signal 86. When heater 36 is off, it is off for at least one full cycle or a multiple of full cycles.

Bias power supply 90 provides the ± 15 volts DC for the electronics and heater of controller 10. Signal 77 provides the input to reference integrator 70. The duty cycle or time at + 15 VDC of signal 77 is proportional to the on-time of heater 36.

Integrating this on-time signal 77 gives a voltage 72 proportional to the power delivered by heater 36. The voltage 72 is input to circuit 68 and compared to the desired power level represented by input signal 71, to determine if the next full power cycle should be applied to heater 36. Signal 77 from logic 84 goes to a first input of reference integrator 70 at a first terminal of resistor 99 which has a value of 100 kilohms. The second terminal of resistor 99 is connected to a positive terminal of capacitor 92 which has a value of 10 microfarads, and to a first terminal of resistor 94 which has a value of 196 kilohms. The second terminal of resistor 94 and the negative terminal of capacitor 92 are connected to a first terminal of resistor 95 which has a value of 100 ohms. The second terminal of resistor 95 is connected to ground or a reference terminal. Signal 76 from comparator 68 goes to a second input of integrator 70 at a first terminal of resistor 97 which has a value of 100 kilohms. The second terminal of resistor 97 is connected to the negative terminal of capacitor 92 and the second terminal of resistor 94. The first terminal of resistor 94 conveys output signal 72 of integrator 70 through resistor 113 to the non inverting input of comparator 68. Figure 3 shows trim heater controller 10 for single phase power line 80. Trim heater controller 10 may be modified for three phase power or for power having any other number of phases. Modifications for a multi-phase power application include adding a gate circuit 96 (in Figure 4) of logic 84, a driver and SCR 88 and a heater 36, connected in a similar fashion for each additional phase of power 80.

Figures 4-6 reveal circuit details of trim heater controller 10. These figures show extended circuitry for a three-phase trim heater controller 10. The circuitry and logic are of expended detail and somewhat different format to accommodate readily available hardware. In comparison, Figure 3 is simplified for functionally illustrative purposes.

Figure 4 shows the detail circuitry of preamplifier 56, comparator 68, reference integrator 70 and logic 84 for a three-phase powered configuration of the invention. The phases of power are referred to as phase one (φl), phase two (φ2) and phase three (φ3). Heater rate sensor 52 is a thermistor having a resistance of about 100 kilohms at 25 degrees C. One terminal of sensor 52 is connected to +15 volts DC through resistor 101 which has a value of 49.9 kilohms. The other terminal of sensor 52 is connected through resistor 102 to the inverting input of amplifier 103. Resistor 102 has a value of 49.9 kilohms. Amplifier 103 has a non-inverting input connected to ground or a reference terminal through resistor 104 which has a value of 100 kilohms. Amplifier 103 is a type TL082. The output of amplifier 103 is connected through resistor 105 to the inverting input. Resistor 105 is one megohm. Connected across resistor 105 is capacitor 106 which has a value of 0.01 microfarad. The inverting input of amplifier 103 is connected through resistor 107 to -15 volts DC. Resistor 107 has a value of 196 kilohms. Amplifier 103 and its associated circuitry constitute preamplifier 56. The output of amplifier 103 is connected to a first terminal of potentiometer 108. The second terminal of potentiometer 108 is connected to ground and the third terminal is a tap on the resistor which is of a variable adjustment, which is connected to capacitor 64 which has a value of one microfarad. The maximum resistance of potentiometer 108 is 10 kilohms. Resistor 108 is the scaling adjustment for trim heater control 10. Capacitor 64 is connected to resistor 109 which has a value of 100 kilohms. The other side of resistor 109 is connected to node 66. A temperature control signal which is a voltage from zero to a positive ten volts DC is a signal 71 which goes through resistor 110 to node 66. The signals from preamplifier 56 and controller 42 (of Figure 2) are summed at node 66. A signal 72 from reference integrator 70 is connected through resistor 111 to be compared to signal 74. Resistor 111 has a value of 10 kilohms. Signal 74 is input to the inverting input of amplifier 112. Signal 72 is input to the non inverting input of amplifier 112. The output of amplifier 112 is connected through resistor 114 to the inverting input of amplifier 112. The value of resistor 114 is ten megohms. The output of amplifier 112 is connected to one terminal of capacitor 115 and the other terminal of capacitor 115 is connected to one terminal of resistor 116 and the other terminal of resistor 116 is connected to the inverting input of amplifier 112. Capacitor 115 has a value of 100 picofarads and value of resistor 116 is 100 kilohms. Amplifier 112 and its associated circuitry constitute comparator 68. The output of amplifier 112 is connected through resistor 117 to the D input of latch 118. The value of resistor 117 is 4.7 kilohms. Latch 118 is a type 4013. The output of amplifier 112 is connected through resistor 120 to a first input of N AND gate 121. Resistor 120 has a value of 4.7 kilohms. The second input of N AND gate 121 has a φ ϊ P CR signal which is an inverse positive zero-cross reference signal for phase one from zero-crossing detector 78 of phase one. The φ T P CR signal goes through an inverter 130 to the clock input of latch 118. The Q output of latch 118 is connected to a first input of NAND gate 122. Signal φ NCR is input to second input of NAND gate 122. Signal φ ϊ NCR is an inverse negative zero-cross reference signal for phase one from zero-crossing detector 78. The outputs of NAND gates 121 and 122 are input into NOR gate 123. This gate is of type 4001. Output signal TTΪ of NOR gate 123 is input into driver and SCR 88 for phase one. NAND gates 121 and 122 and NOR gate 123 constitute gate portion 96 of logic 84. Two gates 123 may be paralleled for a more robust output drive. The Q output of latch 118 is inverted by buffer circuits 124 and 126 for input to circuits 164 of phase 2 and phase 3, respectively. Circuit 164 (type MOC3043) will only turn on at zero crossing if the respective control inputs TT 2 and TT 3 are low. Thus, if phase 1 heater 36 is turned on at the positive zero crossing of the power cycle of phase 1, (line 80 of Figure 3), then phase 2 and phase 3 will also be turned on at both positive and negative zero crossings as long as the Q output of latch 118 is high. The Q output of 118 is brought low at the first positive zero crossing of phase 1 (line 80 of Figure 3), and the comparator 68 output 76 (of Figure 3) is high.

Figure 5 shows bias power of supply 90 which is an ACOPIAN device. The input winding of transformer 140 is connected to 115 volt RMS terminals L and N, L being a line terminal and N being the ground or neutral terminal. The output winding of transformer 140 is connected to filter and regulator electronics 141. Filter and regulator electronics 141 has an output of +15 and -15 volts DC relative to its ground terminal.

Transformer 142 of zero-crossing detector 78 for phase one has a primary winding connected to phase one lines φl-Ll and φl-L2. One output of the secondary winding of transformer 142 is connected to resistor 143 which has a value of 10 kilohms and a rating of 2 watts. The other output of the secondary winding of transformer 122 is connected to the anode of diode 144. Resistor 143 is connected to the cathode of diode 144 and the anode of diode 145. The cathode of diode 145 is connected to ground. The cathode of diode 144 is connected to capacitor 146 which has a value of 0.047 microfarad. The other side of capacitor 146 is connected to the inverting input of amplifier 147. Amplifier 147 is of type TL082. Resistor 148 is connected between the inverting input of amplifier 147 and ground. Resistor 148 has a value of 4.7 kilohms. Resistor 149 is connected between the non-inverting input of amplifier 147 and ground. Resistor 149 has a value of 4.7 kilohms. A resistor 150 is connected between the output and the non-inverting input of amplifier 147. Resistor 150 has a value of 470 kilohms. A 10 picofarad capacitor 151 is connected across resistor 150. Resistor 152 is connected between -15 volts DC and the non-inverting input of amplifier 147. Resistor 152 has a value of 220 kilohms. A resistor 153 is connected between the output of amplifier 147 and the input of an inverter 154. The value of resistor 153 is 4.7 kilohms. Inverter 154 is of type 4049. The output of inverter 154 is signal φ ΪNCR . The cathode of diode 144 is connected to capacitor 155 which has a value of 0.047 microfarad. The other side of capacitor 155 is connected to the non-inverting input of amplifier 156. Amplifier 156 is of type TL082. A resistor 157 is connected between ground and the inverting input of amplifier 156. Resistor 157 has a value of 4.7 kilohms. A resistor 158 is connected between the output and the non-inverting input of amplifier 156. The value of resistor 158 is 470 kilohms. A capacitor 159 having a value of 10 picofarads is connected across resistor 158. A resistor 160 is connected between a -15 volts DC and the non- inverting input of amplifier 156. Resistor 160 has a value of 220 kilohms. Resistor 161, having a value of 4.7 kilohms, is connected between ground and the non- inverting input of amplifier 156. A resistor 162, having a value of 4.7 kilohms, is connected between the output of amplifier 156 and the input of inverter 163. Inverter 163 is of type 4049. The output of inverter 163 is a signal φ ϊ P^~CR . The circuit details of zero-crossing detector 78 for phase two is the same as those of zero-crossing detector 78 for phase one. Likewise, the circuit details of zero-crossing detector 78 for phase three are the same as those of zero-crossing detector 78 of phase one. The inputs to zero-crossing detector 78 for phase two are φ2-Ll and φ2-L2. The outputs of zero-crossing detector 78 for phase two are φ 2 NCR and φ ^~2 P~ CR . The inputs for zero-crossing detector 78 for phase three are φ3-Ll and φ3-L2. The outputs of zero- crossing detector 78 for phase three are signals φ 3^~ NCR and φ 3^~ P CR . In Figure 6, driver electronics 164 has an input signal T T ϊ at terminal 2.

Terminal 1 is connected to one end of resistor 165 having a value of 2.2 kilohms. The end of resistor 165 is connected to +15 volts DC. Terminals 3 and 5 of driver 164 are not connected. Terminal 6 of driver 164 is connected to one end of resistor 166 and terminal 4 is connected to the gate of SCR 167, a type MAC 320-6. Resistor 166, having a value of 300 ohms, is connected to a first MT terminal of SCR 167 and to a terminal of heater 36 for phase one. The other terminal of heater 36 for phase one is connected to signal φl-Ll. The second terminal MT of SCR 167 is connected to a line φl-L2. The electronics of driver and SCR 88 for phase two and phase three are the same as the driver and SCR 88 electronics for phase one. Signal T T 2 is the input for driver and SCR 88 for phase two. One output of driver and SCR 88 for phase two is connected to heater 36 for phase two and the other terminal of heater 36 for phase two is connected to line φ2-Ll. The other output of driver and SCR 88 for phase two is connected to line φ2-L2. Signal TT 3^" is input to driver and SCR 88 for phase three. One output of driver and SCR 88 for a phase three is connected to a terminal of heater 36 for phase three and the other terminal of this heater is connected to φ3-Ll . The other output of driver and SCR 88 for a phase three is connected to line φ3-L2. Driver electronics 164 typically is a type MOC 3043 but alternatively may be MOC 3063 or MOC 3083. SCR 167 typically is a type MAC 320-6 or may be an MAC 320-8 or MAC 320-10 (20 amp. rating). A second choice of SCR 167 is a type MAC 15-6, -8 or -10 (15 amp. rating).

Trim heater controller 10 may be connected to a single-phase 120 volt RMS power supply, or a 208 volt RMS 3-phase wye (Y)-connected or a 208 volt RMS 3- phase delta (Δ)-connected power supply. Of course, the above-described circuit may be modified to handle any number of phases of various connection configurations at any particular voltages. For 120 volt RMS single-phase connections, φl-Ll is connected to the 120 volt line and φl-L2 is connected to the 120 volt neutral termination. φ2-Ll, φ2-L2, φ3-Ll and φ3-L2 are not connected in the single phase system.

For a connection of system 10 to a 3-phase wye (Y) connected supply at 208 volts RMS, φl-Ll is connected to the A line, φl-L2 is connected to the neutral terminal, φ2-Ll is connected to the B line, φ2-L2 is connected to the neutral terminal, φ3-Ll is connected to the C line and φ3-L2 is connected to the neutral terminal.

For connecting system 10 to a 3-phase delta (Δ) connected source of 208 volts RMS, φl-Ll is connected to the A line, φl-L2 is connected to the B line, φ2-Ll is connected to the B line, φ2-L2 is connected to the C line, φ3-Ll is connected to the C line and φ3-L2 is connected to A line. The above-noted power connections are shown in the termination table below.

Supply Volts => 120 VRMS 208 VRMS 208 VRMS

Input Lines (1 Phase) 3 phase - Y 3 phase - Δ

φl-Ll 120 V. Line A Line A Line φl-L2 120 V. Neut. Neutral B Line

φ2-Ll (N.C.) B Line B Line φ2-L2 (N.C.) Neutral C Line

φ3-Ll (N.C.) C Line C Line φ3-L2 (N.C.) Neutral A Line

Claims

THE CLAIMS

1. A trim heater controller, connected to a temperature controller, for maintaining a precision temperature level, comprising: a heater; a switch connected to said heater; a temperature sensor proximate to said heater; a comparator connected to the temperature controller, said temperature sensor, and to said switch;

2. An N-phase powered trim heater controller, where N is a positive integer greater than zero, connected to a temperature controller, for maintaining a precision temperature level, comprising:

N heaters; N switches individually connected to said N heaters; a temperature sensor proximate to said heaters; a comparator connected to the temperature controller, said temperature sensor, and to said N switches; a reference integrator connected to said comparator; and N zero-crossing detectors individually connected to said N switches.

3. A trim heater controller comprising: a heater having first and second terminals; a switch having a first terminal connected to the second terminal of said heater and having a second [and third terminals]terminal; a temperature sensor, proximate to said heater, having; a comparator having a first terminal connected to the terminal of said temperature sensor and having second and third terminals; a reference integrator having a first terminal connected to the second terminal of said comparator, a second terminal connected to the third terminal of said comparator and having a third terminal; and a zero-crossing detector having a first terminal connected to the second terminal of said switch and to the third terminal of said reference integrator, and having a second terminal connected to the first terminal of said heater.

4. A trim heater controller for trim controlling a temperature of a medium controlled by a temperature controller which indicates a target temperature of that medium is to be at, comprising: heating means for heating the medium; temperature change rate sensor means for indicating the temperature change rate of said heating means; reference integration means for indicating the amount of heating that the medium has been provided by said heating means; and comparing means, having an input connected to said temperature change rate sensor means, the temperature controller and said reference integration means, and an output connected to said heating means for comparing the sum of the temperature change rate of said heater and the target temperature of the medium with the amount of heating that the medium has recently been provided by said heating means, wherein the combination of the temperature rate of said heater and the target temperature of the medium is greater or less than the amount of heating that the medium has recently been provided by the said heating means, then said heating means is turned off or on, respectively.

5. A trim heater controller for a temperature controller of a temperature of a medium in an environment, the temperature controller having a heater, comprising: a comparator having a first input connected to the temperature controller for receiving a temperature control signal indicating a temperature to be attained the medium, having second and third inputs and an output; a heater temperature rate sensor for outputting to the second input of said comparator, a signal indicating a rate of change of temperature of the heater; a zero-crossing detector having an input connected to an alternating current power source for the heater and having an output for providing a signal indicating an occurrence of each full cycle of the alternating current power source; a reference integrator, connected to the output of said comparator and to the output of said zero-crossing detector, for providing to the third input of said comparator a signal indicating the amount of heat provided by the heater during past full cycles of the alternating current power source; and a logic switch having a first input connected to said zero-crossing detector for receiving the signal indicating an occurrence of each full cycle of the alternating current power source, a second input connected to the output of said comparator for receiving a signal indicating whether or not the heater is to be connected to the alternating current power source or is not to be connected to the alternating power source, the signal from said comparator being a result of comparing the signal indicating a rate of change of temperature of the heater, the signal indicating the temperature to be attained in the medium and the signal indicating the amount of heat provided by the heater during past full cycles of the power source, and an output for connecting the power source to the heater for a duration of at least one or more full cycles or disconnecting the heater for a duration of at least one or more full cycles.

6. A heater controller, for more closely controlling temperature for a temperature controller, comprising: a heater having an input; • a thermistor having an output; a preamplifier having an input connected to the output of said thermistor and having an output; a capacitor having a first electrode connected to the output of said preamplifier and having a second electrode; a comparator having a first input connected to the second electrode of said capacitor and to an output of the temperature controller, and having a second input and an output; a reference integrator having a first input connected to the output of said comparator, having an output connected to the second input of said comparator, and having a second input; a zero-crossing detector having an output connected to the second input of said reference integrator, and having an input; a logic circuit having an input connected to the output of said comparator, second input connected to the output of said zero-crossing detector, and having an output; and a switch having a first input connected to the output of said logic circuit and having an output connected to the input of said heater; and wherein: said thermistor is proximate to said heater and has an output signal indicative of heater temperature; the output signal of said thermistor goes to, via said preamplifier, the first electrode of said capacitor; a rate of change of heater temperature signal appears on the second electrode of said capacitor; a target temperature signal indicating the target temperature set by the temperature controller is summed with the rate of change of heater temperature signal at the input of said comparator; the input of said zero-crossing detector receives an AC cycle power supply line signal, and the cross-over at zero amplitude of each cycle of the AC power line signal is indicated by a cross-over signal at the output of said cross-over detector; a comparison signal at the output of said comparator, and goes to an input of a latch of said logic circuit, and a comparison signal of positive polarity causes the latch to be in a positive mode and the comparison signal of negative polarity causes the latch to be in a negative mode, the negative and positive modes indicated by a latch signal; the latch signal is integrated to indicate a heat output level; an analog signal from the output of said reference integrator, of an increasing or decreasing amplitude, goes to the second input of said comparator and is compared to the rate of change of heater temperature signal and the target temperature signal; and a sum of the rate of change of heater temperature signal, having a minus signal indicating an increasing rate of change of heater temperature and a positive signal indicating a decreasing rate of change of heater temperature, and the target temperature signal, indicating what temperature to be maintained, is compared with the analog signal to indicate the amount of heat delivered in previous AC power supply line cycles, and wherein a resulting positive output of said comparator turns on said heater for at least one AC power line cycle and a resulting negative output of said comparator turns off said heater for at least one AC power line cycle.