Improvements in or relating to temperature-dependent transistor circuit arrangements

摘要

880,492. Automatic temperature-control systems. FERNSEH G.m.b.H. April 10, 1958 [April 10, 1957], No. 11379/58. Class 38 (4). [Also in Groups XXXVIII and XL (c)] In a retroactive temperature-controlling circuit in which an output current is developed in the collector-emitter circuit of a transistor as the result of an input potential difference applied between points in the emitter-base circuit of the transistor which are connected across a diagonal of a temperature-sensitive bridge, overloading of the transistor is prevented by providing two alternative paths constituted by a resistor and a rectifier respectively in the emitter-base circuit of the transistor, the rectifier being so poled and biased that it is rendered non-conductive when the input potential difference exceeds a predetermined value whereby for small inputs the resistor is by-passed by the rectifier while for larger inputs the resistor is effective to reduce the gain of the circuit. In the circuit shown for the regulation of the temperature within the housing 15 of a television camera, a thermistor 16 is arranged within a bridge circuit, the other circuit elements of which are arranged within a separate housing 19. The bridge is energized from the terminals 1, 2 connected across one diagonal of the bridge and the base-emitter path of transistor 20 together with resistor 21 and rectifier 27 are connected across the other diagonal. The resistor 21 and rectifier 27 are connected to opposite ends of a resistor 24 whereby the rectifier 27 is normally maintained conductive. The collector-emitter circuit includes a relay winding 43; when the out-of-balance voltage falls due to a rise in temperature within the casing 15 causing the resistance of thermistor 16 to fall, the normally closed contacts of the relay close thereby energizing a cooling device 18, while when the out-of-balance voltage rises due to a fall in temperature the relay is actuated and heating device 17 is energized. If the out-ofbalance voltage becomes sufficiently large the rectifier 27 is cut off, thus increasing the effective base resistance of the transistor and hence maintaining the base current within the limits of safety. A further high-resistance circuit energized from the potential difference developed across the resistor 42 and including a current meter in series with the thermistor 16 is provided to give a temperature indication. Resistor 25 is placed close to the transistor 20 and chosen to have such a resistance and temperature coefficient of resistance as to compensate for temperature effects in the transistor in the manner described in Specification 852,746.