NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, West Virginia Electronics Division Internal Report No 71

Transcription

1 NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, West Virginia Electronics Division Internal Report No 71 THE FRONTEND BOX TEMPERATURE CONTROLLER FOR THE 36-FOOT TELESCOPE K. H. Wesseling JUNE 1968 NUMBER OF COPIES: 100

2 THE FRONTEND BOX TEMPERATURE CONTROLLER FOR THE 3(3-FOOT TELESCOPE K. H. Wesseling Introduction: Weight and space limitations at the focus of the 36-foot radio telescope in Tucson, Arizona do not allow the use of the standard NRAO frontend box temperature control system. For this reason a new system was designed. It holds the frontend box temperature to within one degree Centigrade of the set-temperature. Better performance is expected after minor modifications are made. The air temperature cycles over O. 5 'V p-p due to thermal lag in the system, with a period of the order of 10 sec. Block Diagram: 1. Cooled air from an air condition unit mounted on one of the lower telescope platforms is blown to the frontend box via an air duct along one of the feed support legs. A light-weight reheater is located near the frontend box. The reheat elements form part of a servo loop which regulates the reheater exhaust air temperature. The reheat unit consists of a number of ceramic cone heater elements with a total power rating of 5 kw. AC power to the reheat element is controlled with TRIAC s. 2. A thermometer measures the frontend box temperature. It consists of a YSI thermistor on an electronic readout circuit. The temperature can be read on a front panel meter. 3. Due to the high electric power involved, a number of safety precautions are taken. a) No AC power is directed to the reheat elements if the AC power to the blower fan is disconnected.

3 - 2 b) The system does not operate when the AC power to the cooling unit is disconnected. A thermistor mounted close to the cone heater elements senses an overheat condition when either the heaters become very hot or the cool input air flow is interrupted. It shuts off the 208 V AC power to the reheater and gives an audible and visible warning signal. d) When the frontend box temperature is higher than 31 'V, an overheat condition is also created, with the same effects. Circuit Diarams: The circuit diagrams should speak for themselves and do not contain any novelties. The TRIAC power amplifier circuit was taken from a GE application note. It has three positions: off, half power, or full power. The TRIAC' s switch during the zero crossings of the AC voltage cycle. Isolation between the 208 V AC system, which is not grounded, and the rest of the system is obtained with the input control relay. The reed relay contacts switch only 1 ma at 1 V, resistive. A Raytheon Raysistor could have been used also in this function, but was not available. An improvement in the system can be made, by locating the servo thermistor at a point where the reheated air is more thoroughly mixed, and at a point closer to the frontend box.

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10 Tech.nipower M A ± 15v power supply to servo overheat rela coil not shown Mallory Sonalert 16K SC628 N. P A 1 May, S2 S3 13 Deutsch PN " AC POWER" vao from blower 115vac from blower corn. 14 "COOLING" lamp on compressor relay ac corn. > blower relay corn. blower relay 24vac reheat relay reheat relay corn. Amphenol "REHEAT POWER" Il = NE-51 "ON" 12= 1820 "COOLING" 13 = 1820 "REHEAT" I = 1820 "OVERHEAT WARNI FRONT END BOX TEMPERATURE CONTROLLER SWITCHING CIRCUIT

11 Deutsch OSN -059 to "AC POWER" _ ALC_ COE ERFAS Qii _ 1-24 vac 56 va 1 1 oran e 5 yellow 4c no v IL PIA 18 violet 19 white 9C no.20 no t OUTLET BOX ac outlets I 11.5v lw I g n I 4 c no.16 I Barrier strip in conduit box on telescope pedestal.111. Amphenol A B <D w D > E bk t t E <F <H gn J bk Amphenol to re-heat box on south feed support leg c > MINIBOX IN REAR OF RACK A/C EVAPORATOR + BLOWER 208v LP1A 6 shielded twisted pair no. 22 Amphenol to "REHEAT POWER" bk I shielded twisted air no Bendix QWLD I S to re-heat box on A B south feed support leg 4c no. 8 14,P0A, 1 May, 1968 FRONT-END BOX TEMPERATURE CONTROLLER INTERCONNECTING CABLING NRAO 36! TELESCOPE