CURRENT LIMITER RESPONSIVE TO CURRENT FLOW AND TEMPERATURE RISE

摘要

1467023 Automatic control of current and temperature TELEDYNE Inc 15 Jan 1974 01778/74 Heading G3R A current limiting circuit includes a main series transistor controlled by a driver transistor the base of which is connected to a constant current source. When the current in the main transistor reaches a predetermined value, a shunt circuit diverts part of the current from the constant current source away from the driver transistor so as to reduce the conductivity of the main transistors and prevent the current exceeding the predetermined value. As shown, a power transistor 16 and current sensing resistor 18 are connected between input and output terminals 12, 14. A driver transistor 20 is biased to saturation by a constant current source comprising a transistor 22, diodes 24, 26 and resistor 28. If the current in resistor 18 approaches the limit value, a shunt transistor 30 begins to conduct, diverting part of the biasing current away from the base of driver transistor 20. The impedance of power transistor 16 is thereby increased to limit the current, and power dissipation increases the temperature of the transistor. This temperature is sensed by a normally non- conductive transistor 32 which turns on when a predetermined temperature is reached, biasing on a second shunt transistor 48 which, by virtue of a positive feedback path, rapidly saturates and turns transistor 16 completely off. If a switch 52 is set to a contact 58, a latching transistor 50 holds the circuit off until the switch is moved to a contact 60, whereupon conduction restarts if the temperature of transistors 16, 32 is below a second predetermined value. The switch may be normally set to contact 60 to provide an automatic restart. If switch 52 is set to contact 56, transistor 22 is turned off and the whole circuit is disabled. A diode 66 protects the circuit against reverse voltages; a Zener diode 68 protects against voltage spikes; and a light emitting diode 70 visually indicates limiting or fault conditions.