US 3742188 A
A dummy load includes a resistive sodium nitrite resistive load directed by insulating tubes to flow in the load. The cooling circuit includes a primary circuit having inlet and outlet thermistors, and a secondary cooling circuit coupled thereto by a liquid to liquid heat exchanger. In the control circuit, the thermistors are serially connected in a bridge circuit, unbalances in the bridge being detected to directly control a motor for controlling a valve in series in the secondary loop. A pair of slower moving valves in series and shunt respectively in the secondary loop are controlled upon the movement of the directly controlled valve to a limit position.
Description (OCR text may contain errors)
United States atent [1 1 Sundbye June 26, 1973 1 1 DUMMY LOAD SYSTEM 3,597,708 8/1971 Perreault 333/22 R  Inventor: Earl W. Sundbye, Garland, Tex. Primary Examiner c. L Albritton [73-] Assignee: Continental Electronics & Attorney-Albert C. Nolte, Jr. et a1.
Manufacturing Company, Dallas, Tex.  ABSTRACT  Filed: Apr. 24, 1972 A dummy load includes a resistive sodium nitrite resistive load directed by insulating tubes to flow in the [21 1 Appl' 247062 load. The cooling circuit includes a primary circuit having inlet and outlet thermistors, and a secondary cool-  U.S. Cl 219/323, 165/108, 165/164, ing circuit coupled thereto by a liquid to liquid heat ex- 219/325, 219/330, 338/56 changer. In the control circuit, the thermistors are seri-  Int. Cl. F24h 1/20 ally connected in a bridge circuit, unbalances in the  Field of Search 219/323, 325, 330, bridge being detected to directly control a motor for 219/314, 316; 338/56; 165/108, 164, 180, controlling a valve in series in the secondary loop. A
181; 333/22 F, 22 R pair of slower'moving valves in series and shunt respectively in the secondary loop are controlled upon the  References Cited movement of the directly controlled valve to a limit po- UNITED STATES PATENTS Smon- 3,660,784 5/1972 Scharfman 333/22 F 15 Claims, 5 Drawing Figures RESERVOIR T HERMIS TOR HEAT (5/ /5O EXCHANGER O 45 I 1 L 1 Sq, r i i M PATENIEUJUII 2 6 I975 SHEEI'HIF4 mm x E/QUI 53mm mwkwi ZoFmO V GE m: g m: w I m DUMMY LOAD SYSTEM This invention relates to arrangements for the dissipation of radio frequency power, and is more particularly directed to an improved dummy load of the type adapted to be connected to an RF transmission line for dissipation of power, and a system for maintaining the temperature of a liquid resistive coolant in the dummy load and determining the dissipated power. Such maintenance of temperature is necessary in order to maintain desired VSWR characteristics, in view of the variation of resistance of the resistive coolant with temperature.
In the past, dummy load devices have been provided of the type in which the central conductor terminates in a broad band resistive element comprised of a pair of coaxial insulating tubes through which a suitable material such as sodium nitrite is adapted to flow. Such arrangements are provided with outer shields connected to the transition lines surrounding the resistive element, the end of the shield tapering inwardly toward the end of the dummy load and terminating with an RF short to the resistive element.
In the cooling circuit for such a dummy load device, the sodium nitrite has been circulated in a loop which may consist of a reservoir of sodium nitrite, and a heat exchanger. Thermistors are individually coupled to the input and output lines of the dummy load device to provide an indication of the temperature of sodium nitrite, and these thermistors were connected to an automatic control system for maintaining the temperature of the coolant at a mean given value, for example, by controlling the flow of fluid by means of a motor driven valve at the input of the heat exchanger from a source of raw water.
- It is an object of the present invention to provide improvements in such dummy load systems, by maintain-- ing improved control over the temperatue of the liquid coolant over wider ranges of power inputs, as well as to simplify the control system, and improve the VSWR characteristics, thereby providing a system having broad band characteristics over an extremely large frequency and power range.
According to the invention, a dummy load for absorbing RF power received by way of a transmission line, is comprised of a resistive central conductor surrounded by a conductive sheath. The central resistive element is comprised of a pair of coaxial insulating tubes arranged so that a flow of coolant, such as sodium nitrite, may be passed in one direction through the inner insulating tube, around the ends of the inner tube at the junction of the resistive element with a transition piece to the central conductor of a transmission line, and thence in the opposite direction between the outer and inner tubes. The conductor transition piece is shaped to maintain nominal impedance and to have anti-corona and high frequency characteristics. The outer sheath has a smooth transition to the outer conductor of the transmission line, and follows a tractorial curve throughout the resistive portion of the dummy load, thereby tapering inwardly to the resistive element at the end of the dummy load to provide an RF short at its termination. The curves of the outer sheath are shaped to maintain the nominal impedance in the transition to the standard transmission line. With this arrangement, the dummy load has characteristics that are extremely flat up to at least 100 megacycles, and the of the liquid coolant. The valves are motorcontrolled,
two of which are in series in the secondary circuit, and the third of which is in parallel in the secondary loop circuit. A pair of thermistors are coupled to the primary loop, at the input and output of the dummy load, and these thermistors are connected in series in one arm of a bridge circuit. A control circuit, responsive to unbalance of the bridge circuit, directly controls the motor coupled to one of the series valves in the secondary loop. The other two motors are controlled by means of back contacts on limit switches of the first motor valve arrangement, and extend the range of control over the liquid flow in the secondary loop, the series valve being controlled in the same direction as the primarily controlled motor, with the shunt valve being controlled in the opposite direction. The coupling between the output of the bridge circuit and the control for the motors may be in the form of an optical meter relay.
Since the thermistors coupled to the primary loop are in series in the bridge circuit, the control system serves to control the temperature of the fluid at the mean. value of temperature as indicated by thermistors. This arrangement provides a simple and inexpensive control over the temperature.
In addition, the primary loop may include heating means, forexample in a reservoir, responsive to a thermal switch, for preheating of the liquid coolant, in order to obviate the necessity for warmup periods for the system, which previously had necessitated low power gradual warmup prior to use of the dummy load as desired.
The invention will now be disclosed in greater detail with reference to the accompanying drawings, in which? FIG. 1 is a simplified schematic diagram of a system for controlling the temperature of a dummy load according to one embodiment of the invention, and illustrating a typical dummy load that may be employed in combination therewith partially in cross section;
FIG. 2 is a simplified circuit diagram of a circuit that may be employed for controlling the system of FIG. 1;
FIG. 3 is a schematic diagram of a more complete system for controlling the temperature of a dummy load, according to the invention, this embodiment illustrating the system for controlling the temperature of a pair of dummy load devices;
FIG. 4 is a simplified circuit diagram of a modification of a portion of the circuit of FIG. 2, the arrangement of FIG. 4 being particularly adapted for incorporation in the system of FIG. 3; and
FIG. 5 is a simplified circuit diagram of a portion of a control circuit which may be incorporated in the systems of FIGS. 1 and 3.
Referring now to FIG. 1, therein is illustrated a dummy load 10 adapted to absorb the radio frequency power from a transmission line 1 1. The central conductor 12 of the transmission line is connected to the resistive portion 13 of the dummy load by way of a conductive transition member 14 having a smooth transition. The resistive portion of the dummy load is comprised of an outer insulating tube 15 sealingly joined at one end to the end of the transition piece 14 and extending axially through the dummy load and terminating in an outlet 16. An inner insulating tube 17, also for example of glass of plastic, is provided within the outer tube 15 and extending coaxially therewith. One end of the inner tube 17 is spaced from the transition piece 114, and the other end extends to an inlet 18 for providing fluid coolant to the structure. The tubes are arranged to that the liquid coolant flows from the inlet 18 upwardly through the tube 17, and thence downwardly between the inner and outer tubes to the outlet 16. The end of the transition piece 14 may be shaped to provide a smooth flow for the fluid at the upper end of the inner tube 17.
The outer conductor 20 of the transmission line is connected to the outer conductive shield v21 of the dummy load. The outer shield extends downwardly from the outer conductor 20 with a smooth transition to an enlarged diameter region at the upper end of the resistive portion of the dummy load, and thence tapers downwardly and inwardly to the outer surface of the outer tube 15 at the bottom of the device. The curve of the outer shell 21 is preferably in the form of a tractrix in the portion thereof surrounding the resistive portion of the dummy load. The upper portion of the outer shield 21 is shaped to maintain the nominal impedance in the transition region to the transmission line, and the inner transition member 14 is also shaped to maintain the nominal impedance, as well'as to provide the desired anti-corona and high frequency characteristics.
The cooling system in the arrangement of FIG. 1 includes a primary loop adapted to circulate a suitable resistive coolant such as sodium nitrite through the tube 15 and 17 of the dummy load, and a secondary loop 31 coupled to the loop 30 by way of a liquid to liquid heat exchanger 32 for circulating a coolant such as water to remove heat from the primary loop.
As illustrated in FIG. 1, theprimary loop 30 includes a liquid reservoir 35. A suitable pump 36 draws sodium nitrite solution from the reservoir and passes it through the heat exhanger 32, and thence into the inlet 18 of the dummy load. The outlet 16 of the dummy load is connected to return the fluid to the reservoir 35.
A thermistor 37 is coupled to the fluid path at the inlet 18 of the dummy load, and a second thermistor 38 is coupled to the fluid path at the outlet 16 of the dummy load. The thermistors 37 and 38 are employed to control the temperature of the fluid in a manner that will be discussed in more detail in the following paragraphs. The system also includes a temperature responsive switch 40 of conventional nature arranged in thermal contact with the liquid in the reservoir 35, the switch 40 being serially connected with a suitable electric heater 41 disposed in a position to heat the liquid in the reservoir 35 by means of a suitable source 42, as will be discussed in more detail in the following paragraphs.
The secondary cooling loop 31 is adapted to circulate a suitable coolant, such as water, from an external heat exhanger 45 through the heat exhanger 32 by way of motor controlled valves 46 and 47, these valves being controlled by suitable means from the shafts of motors 48 and 49 respectively. An additional motor controlled valve 50, controlled by motor 51, is connected between the inlet and outlet of the heat exchanger 45, and, if necessary, pumping means such as pump 52 may be provided to circulate fluid in the secondary loop 31. The source of fluid and pumping arrangement for the secondary loop as illustrated in FIG. 1 is exemplary only, and it will be apparent that other techniques may be employed for circulating this fluid in the secondary loop.
Referring now to FIG. 2, a control circuit for the system of FIG. -1 is comprised of a bridge circuit 60, the resistive elements of the thermistors 37 and 38 of FIG. 1 being serially connected in one arm of this bridge circuit. Suitable resistance elements 61, 62 and 63 are provided in other arms of the bridge circuit, preferably at least one of these other elements being variable to permit adjustment and calibration of the bridge circuit. A suitable voltage source 64 is connected to the source terminals of the bridge circuit, for example by way of a variable calibration resistor 65, and the output diagonals of the bridge circuit are connected to the inputof a bridge detector circuit 70. The bridge detector detects unbalances in the bridge circuit resulting from variation in the resistance of the thermistors, and actuates a relay control circuit 71 for controlling the valve control motors 48, 49 and 51. The bridge detector cir cuit and relay control circuit 71 may be of any conventional nature, and in one suitable embodiment of these devices an optical meter relay has been employed for the control circuit. FIG. 2 illustrates a switch 72 in the relay control circuit 71 to show the operational function of the control circuit. Thus, the switch serves to connect an input power lead 73 selectivelyv to output control leads 74 or 75 if the bridge is unbalanced, the connection being determined by the sense of the unbalance. As noted above, this representation of the control of the circuit 71 is functional only, and it is apparent that any conventional technique for achieving this function, in dependence upon the bridge unbalance, may be suitably employed in the circuit of FIG. 2.
Still referring to FIG. 2, the circuit further includes a manual-automatic switch 80, having contacts 81, 82, 83, 84, and 86. This switch is shown in the automatic position, in which the valve control motors 48, 49 and 51 are automatically controlled in response to the variation of resistance of the thermistors 37 and 38 in a detennined manner. In the manual position of the switch 80, the valve control motors 48, 49 and 51 may be selectively manually controlled by means of switches 87, 88 and 89 respectively, as will be explained in more detail in the following paragraphs.
The valve control motor 48 is a reversible motor of conventional type, such as a split phase motor for operation from single phase current as illustrated. Suitable limit switches 90 and 91, operable at the respective opposite extremities of the desired control range of the valve 46, serve to connect the opposite ends of the windings of the motor 48 to the leads 74 and 75, as illustrated in the figure, when the switches 90 and 91 are off their limits. The common lead 92 is connected to a reference potential. Similarly, the valve control motor 49 is coupled to limit switches 93 and 94 operable at opposite extremities of the control range of the valve 47, and valve control motor 51 is coupled to limit switches 95 and 96 operable at opposite extremities of the range of the valve 50. The common leads of the motors 49 and 51 are also connected to reference potential.
The normally not connected contacts of limit switch 90 are connected by way of switch 84 and limit switch 93 to one lead of the motor 49, and by way of contacts 83 and limit switch 96 to one lead of the motor 51. Similarly, the normally non-connected contacts of limit switch 91 are connected by way of switch 86 and limit switch 94 to the other operating lead of motor 49, and by way of contacts 85 and limit switch 95 to the other operating lead of the motor 51.
Power for operating the motors is derived from terminals 98 at a suitable source of electric power, and applied to the arm of the switch 82, and thence, in the automatic" position, to the lead 73 so that the control circuit 71 determines the operation of the motors. In
the manual position of the switch 82, the power source is connected to the arms of the switches 87, 88 and 89. The switches 87, 88 and 89 are of the type having a central unconnected position, so that they may be manually controlled to apply power to either one of the fixed contacts. The fixed contacts of the switch 87 are connected to the leads 74 and 75 for operation of the motor 48. Similarly, the fixed contacts of switch 88 are connected to opposite leads from the motor 49, and the fixed contacts of switch 89 are connected to opposite leads of the motor 51, for selective operation of these motors.
The manual-automatic switch 80 of FIG. 2 also includes contacts 81 having the fixed contact in the automatic position connected to the lead 74, and the fixed contact in the manual position connected to the source terminal 98. The arm of the switch 81 is connected by way of thermally operated switch 40 to a relay coil 99 having contacts 100 connected to apply power from a suitable source 101 to the resistive heater 41. As seen in FIG. 1, the thermal switch 40 and the heater 41 are disposed on or in the reservoir 35.
In operation of the arrangement of FIG. 2, it is apparent that, in the automatic position of the switch 80, the relay control circuit 71 controls the motor 48, and hence the valve 46, and that the motors 49 and 51 will not be energized as long as the limit switches 90 and 91 of the motor 48 have not been operated. As seen in FIG. 1, the valve 46 is in series in the secondary loop 31, so that the relay control circuit 71 directly controls this valve by way of the motor 48, in response to variation in resistance of the thermistors 37 and 38. For example, if the mean temperature of the dummy load fluid is too low, as indicated by the resistance values of the thermistors, the relay control applies power to the lead 74, and thence by way of limit switch 90 to the motor 48, to operate the valve 46 in the closing direction. Simultaneously, the power is applied by way of the lead 74, contact 81 of switch 80, and thermal switch 40 to energizethe relay 99, thereby applying heat to the fluid in' the reservoir by means of the heater 41. The heater 41 and thermal switch 40 provide means whereby the liquid in the first loop may be heated prior to operation of the dummy load, and thereby obviating the necessity for operating the RF source connected to the dummy load at a lower power during a warmup period, as was the previous practice. The system of FIG. 2 thereby prepares the cooling circuit for proper operation without the warmup period being required. If, during the closing of the valve 46, the limit switch 90 is operated, the limit switch then applies the operating power to windings of the motors 49 and 51 by way of switch contacts 84 and 83 respectively. The motor 49 operates the valve 47 in the same sense as the valve 46, since the valve 47 is also in series in the second loop, while the valve 50 will be operated in the opposite sense, i.e., to open this valve when the temperature is low, since this valve is in parallel in the second loop. The motors 49 and 51 are preferably arranged to operate at a lower speed, for example, at one fourth the speed, with respect to the motor 48, and these valves are arranged to vary the fluid flow in the second loop in predetermined limited amounts, so that the three valves 46, 47 and 50 permit the system to automatically handle widely varying power levels without the necessity of any adjustments. In the above examples, the limit switch 94 of motor 49 limits the control of the valve 47 in the opening direction, while the limit switch 95 of the motor 51 limits the control of the valve 50 in the closing direction.
When the bridge detector 70 indicates that the temperature to be controlled is too high, the relay control energizes the lead 75, to effect the control of the motor 48 in the opposite direction, and in the similar fashion, to control the motors 49 and 51 in the opposite direction when the limit switch 91 is operated. In this case, of course, no additional heat is applied to the fluid in the first loop 30 by the heater 41.
As noted above, the thermistors 37 and 38 are connected in series in one arm of the bridge circuit, and as a result of this connection, the system controls the temperature of the fluid in the first loop so that the mean temperature in the dummy load is maintained substantially constant. Thus, when radio frequency power is applied to the dummy load, the outlet temperature increases, and the cooling system controls the fluid temperature, by controlling the valve, to permit the inlet temperature to drop, thereby keeping the mean temperature at substantially a constant value. For example, except in extremely hot environments, the mean temperature may be set to approximately 70C. By the use of the three motor driven valves in the secondary loop, which controls the secondary loop flow through the liq uid to liquid heat exchanger 32, it has been found that the VSWR of the dummy load may be maintained within 1.2:1. I
FIG. 3 illustrates a more complete system in accordance with the invention, as it may be employed in an actual embodiment for the control of the sodium nitrite coolant of a pair of dummy loads and 111. These dummy load devices may be of the same form as that illustrated in FIG. 1. In the arrangement of FIG. 3, the cooling channels of the dummy load are connected in parallel in the primary cooling circuit 30. The primary circuit, in addition to the elements disclosed in FIG. 1, may also include a series thermistor 112 in the inlet side of the line, a series thermistor unit 1 13 in the outlet side of the system, an under temperature thermal may be removably coupled to the primary loop by means of suitable valves.
The secondary loop of the arrangement of FIG. 3 is also essentially the same as the secondary loop of FIG. 1, with the addition of thermometers 130 and 131 in the two sides of the loop at the heat exchanger 32, as well as pressure gauges 132 and 133 in each side of the line at the input of the loop. A flow meter 134 may be provided in the loop. In addition, a solenoid controlled valve 135, controlled by a solenoid 136, may be provided for bypassing the valves 46 and 47.
FIG. 4 illustrates a variation of a portion of the circuit of FIG. 1 which may be adaptable, for example, for use in the system of FIG. 3. In this arrangemennthe bridge circuit 60 is essentially the same as that of FIG. 2, with the output of the bridge circuit being connected to a watt meter indicator 140 for indicating the power dissipation in the dummy loads. The output of the flow meter 116 of FIG. 3 is connected to a suitable converter 117 for providing a suitable flow indication quantity to the watt meter indicator, and a flow indicator 118 may be provided connected to the converter 1 17 to provide visual indication of the flow. The thermistors 112 and 113 are also connected to the watt meter indicator to provide the necessary indication therein. In the arrangement of FIG. 4, the relay control circuit, which controls the application of power to the leads 74 and 75 from the source lead 73, is comprised of a conventional optical meter relay control circuit 119.
FIG. illustrates an auxiliary circuit which may be employed in combination with the system of FIG. 3. In this arrangement, the flow switch 118 of FIG. 3 is con nected to operate a relay 140, the undertemperature switch 112 is connected to operate a relay 141, the
over-temperature switches 121 and 122 are serially connected to operate a relay 142, and a pump control switch 143 (not shown in FIG. 3) is connected to operate a relay 144. An additional relay 145 is provided having contacts for applying power to the pump 36. In the arrangement of FIG. 5, the relay 144 is a delay 'relay, and the switch 143 is connected to apply power to the pump relay 145 by way of the normally closed contacts of the relay 144. In view of the delay in this relay, the power will be applied to the pump in this manner until the flow switch 118 operates to energize the relay 140, whereupon the pump relay 145 will remain energized by way of normally open contacts 150 of the relay 140. The pump 36 will thereby remain energized as long as the flow switch 118 in the primary loop is the removal of power from the pump 36. Normally closed contacts 151 of relay 140 also energizes a standby" lamp 152. When the flow ofliquid in the primary loop is maintained, and the relay 140 thereby energized, an operate lamp 153 is energized by way of the contacts 151 and normally closed contacts of the relay 141. Over temperatureof the liquid is indicated by a temperature light 154, which is energized by way of normally closed contacts of the relay 141, and normally open contacts of the relay 142. The relay 142 is also provided with contacts 155 which are connected to apply power to the solenoid 136 in the event of over. temperature indicated by the over-temperature switches 121 and 122, and as illustrated in FIG. 3, this results in the rapid opening of the solenoid valve 135 to bypass the valves 46 and 47, and thereby topermit the maximum flow of liquid in the secondary loop 31.
In order to deenergize a transmitter in the event of excessive temperature of the coolant, an interlock circuit for the transmitter may be provided extending from interlock terminals 160 through normally open contacts of relays 140 and 142, normally closed contacts of relay 141, and a manual switch 161.
While the invention has been disclosed with reference to particular embodiments, it will be apparent that many modifications and variations may be made therein without departing from the invention, it is therefore intended in the following claims to cover all such variations and modifications as fall within the true spirit and scope of the invention.
.W hat is claimed is:
l. A dummy load system comprising a primary liquid circulation loop for circulating a resistive liquid coolant, a dummy load having a circulating channel connected in said first loop whereby said liquid in said channel forms a resistive load in said dummy load, a
secondary liquid circulation loop', a liquid to liquid heat exchanger intercoupling said primary and secondary serially connected in one arm of said bridge circuit, and
closed, and failure of the flow or this switch will effect 5 means responsive to the balance of said bridge circuit for controlling said motor controlled valve means whereby the mean temperature of said liquid in said channel is maintained substantially constant.
2. The dummy load system of claim 1 wherein said motor controlled valve means comprises first and second motor controlled valves in' series in said secondary loop, a third motor controlled valve in shunt in said secondary loop, said means responsive to the balance of said bridge circuit being connected to directly control said first motor controlled valve, said first motor controlled valve having limit switch means, and means responsive to the operation of said limit switch means at predetermined positions of said first valve for deenergizing said first valve means and energizing said second valve means to operate in the same respective direction and to energize said third valve means to operate in the opposite relative direction.
3. The dummy load system of claim 2 wherein said second and third valves operate at a predetermined lowerspeed than said first valve when energized.
4. The dummy load system of claim 1 further comprising resistance heater means coupled to heat the liquid in said primary loop, and thermal switch means thermally coupled to said first loop and connected to energize said resistance heater means.
5. The dummy. load system of claim 4 wherein said means for controlling said motor control valve means comprises means for energizing said resistance heater means in response to the detection of mean liquid coolant tempe'ratures below a predetermined level in said channel coolant. q
6. The dummy load system of claim 1 wherein said motor control valve means comprises a first motor controlled valve serially connected in said secondary loop, said means for controlling said motor control valve means comprising means for directly controlling said first motor control valve, said first motor control valve having limit switch means for inhibiting further energization thereof at,predetermined positions thereof, a
second motor control valve serially connected in said secondary loop and connected for energization in the same respective direction to said first motor control valve means by means of said limit switch means, and a third motor control valve connected in shunt in said secondary loop and connected for energization to said limit switch means upon operation thereof, in the opposite respective direction to said first motor control valve.
7. The dummy load system of claim 6 further comprising resistive heater means coupled to said primary loop, said means for controlling said motor control valve means including means for energizing said heater means in response to the detection of predetermined low temperature of said liquid coolant.
8. The dummy load system of claim 6 further comprising manual switch means for selectively energizing said first, second and third motor control valves.
9. The dummy load system of claim 6 further comprising over-temperature switch means coupled to said primary loop, solenoid control valve means in said secondary loop bypassing said first and second motor control valve, and means for energizing said solenoid control valve means from said over-temperature switch means in response to predetermined temperature of said liquid coolant in said primary loop.
10. The dummy load system of claim 6 wherein said means for controlling said motor control valve means comprises optical meter relay means.
11. The dummy load system of claim 1 wherein said dummy load is comprised of an outer insulating tube coupled to said primary loop, an inner insulating tube coaxial with said outer tube and connected to said primary loop, a conductive transition member sealed to the end of said outer tube for connection to the central conductor of a transmission line, whereby said liquid coolant circulates in one direction in said inner tube and the opposite direction between said outer and inner tubes and around the end of said inner tube between said inner tube and transition member.
12. The dummy load system of claim 11 wherein said dummy load has an outer sheath in the shape of a tractorial curve in the region thereof surrounding said inner and outer tubes, said outer sheath tapering toward said tubes at the bottom of said dummy load and forming thereat a short circuit for RF currents.
13. In a dummy load system of the type including a dummy load having a resistive liquid coolant serving as a load, and a circulating path therein for said coolant, and wherein said system further comprises a circulating loop for circulating said coolant through said path, first and second thermistors thermally coupled to the loop at the inlet and outlet of the path, motor driven valve means for controlling the temperature of said liquid in the circulating loop, and means responsive to the resistance of said thermistors for controlling said motor driven valve means; the improvement wherein said means responsive to the resistance of said thermistors comprises a bridge circuit, said thermistors being connected in series in one arm of said bridge circuit, and means responsive to the balance of said bridge circuit for controlling said motor driven valve means, whereby said valve means controls the temperature of said liquid coolant to maintain the mean temperature thereof in said path substantially constant.
14. The dummy load system of claim 13 further comprising a secondary liquid circulating loop, liquid to liquid heat exchanger means intercoupling said second and first mentioned circulating loops, and wherein said motor driven valve means comprises first valve means serially connected in said secondary loop and connected to be directly driven by said means for controlling said motor driven valve means, limit switch means on said first motor driven valve means, a second motor driven valve means connected serially in said secondary loop for energization in the same respective direction as said first motor driven valve means upon operation of said limit switch means at predetermined positions of said first valve means, and a third motor driven valve means connected in shunt in said secondary loop for energization upon operation of said limit switch means and in the opposite respective direction to said first motor driven valve means.
15. The dummy load system of claim 14 further comprising resistive heater means coupled to said first mentioned loop for heating the liquid therein, and wherein said means for controlling said motor driven valve means comprises means for energization of said resistive heater means in response to the detection of a mean temperature below a given value in said circulating path.