SU1365225A1 - Arrangement for temperature protection of electric installation - Google Patents

Abstract

The invention relates to the field of electrical engineering and is intended to disconnect the load when the temperature exceeds a predetermined value. The purpose of the invention is to increase the reliability and simplify the device. When the thermistor 7 is warmed to a temperature exceeding the alarm value, the voltage drop across resistor 8 exceeds the voltage unlocked by the threshold element. Transistors 11, 12 open and shunt the series-connected Zener diodes 16 and 17 and the control circuit of the triac 1. Therefore, the triac 1 closes and the load 2 is disconnected. To re-enable the electric heater (load), it is necessary to de-energize the circuit of the device for removing the voltage from transistors 11 and 12, and then reapply the supply voltage to terminals 5 and 6. 1 Il. about

Description

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The invention relates to electrical engineering and is intended to disconnect the load of, for example, an electric heating element when it exceeds a predetermined temperature in the control zone.

The aim of the invention is to increase the reliability of operation and simplify the device.

The drawing shows a diagram of the proposed device.

The circuit contains a triac 1, through which the load 2, made in the form of an electric heating element with output pins 3 and 4, is connected to terminals 5 and 6 connected to the AC network; a first voltage divider made up by a thermistor 7 and a resistor 8; the second voltage divider formed by resistors 9 and 10, transistors 11 and 12, forming a threshold element, first capacitor 13, diode bridge 14, zener diodes 15-17, filter capacitor 18, resistor 19, capacitors 20 and 21.

The device works as follows.

When supply voltage is applied through a series circuit formed by a capacitor 13, opposite zener diodes 16 and 17, and a control electrode-cathode junction 1, an alternating current passes through the power circuit and load 2, for example, a heating element. Up to an amplitude value of the voltage drop at the Zener diodes 16 and 17 through the rectified bridge 14, the filter capacitor 18 is charged. If the temperature in the installation area of the thermistor is below the specified overheating temperature of the protected electrical equipment, the resistance of thermistor 7 is high and, therefore, the voltage falling on the resistor 8 is less than the voltage unlocking the threshold element in transistors 1 and 10.

The breakdown voltage of Zener diode 15 is set below the stabilization voltage of Zener diodes 16 and 17, but higher than the difference in voltage across the resistor 9, therefore the current does not pass through Zener diode 15 and transistors 11 and 12 either. Therefore, all the current limited by the capacitor 13 passes through the zener diodes 16 and 17 and

triac control circuit 1. Due to this, triac 1 is disconnected at the beginning of each half-period of the mains voltage and the entire mains voltage is in the load. Since the systor control current is limited by the capacitor 13, it is ahead of the anode current of the triac, which makes it possible to reliably unlock the triac in the control circuit of a lower value of the Active current than a constant or active current. This makes it possible to reduce the power consumed by the proposed circuit. When the thermistor 7 reaches the temperature exceeding the alarm value, the voltage drop across the resistor 8 will exceed the voltage unlocked by the threshold element. Transistors 11 and 12 will open. Through the zener diode 15, the resistor 19 and the diode bridge 14, these transistors shunt the series-connected zener diodes 16 and 17 and the control circuit of the systistor 1. Since the voltage

incident on the Zener diode 15 and open transistors 11 and 12, less than the voltage of the breakdown of the Zener diodes 16 and 17, then all the current flowing through the capacitor 13 now passes through the shunting zener diodes 16 and 17 and the simistor control circuit formed by the diode bridge 14, the zener diode 15 and transistors I1 and 12. Therefore, triac 1 is closed, and. load 2 is shut off. Due to the DC current flowing through the circuit composed by the Zener diode 15, the resistor 19 and the transistors 11 and 12, under the action of the voltage on the filter capacitor I8, the transistors are held open and after the temperature in the thermistor 7 decreases, ensuring that the electric heater 2 is turned off Therefore, to re-enable the electric heater, it is necessary to de-energize the device circuit in order to remove voltage from transistors 11 and 12, and then reapply the supply voltage to terminals 5 and 6. Capacitors 20 and 21, oedin guides emitters of transistors 11 and 12 with their bases provide relay switching transistors and antijamming circuit.

Claims (1)

The invention The device for thermal protection of electrical installations, containing a power source, including a current-limiting capacitor, an output bridge and a filter capacitor, are connected to the output of the power source two voltage dividers, the first of which is formed by a resistor and a thermistor, and the second is by two resistors , the threshold element made on two transistors, the collector of the first and the base of the second transistor are connected to the midpoint of the second divider and the output of the first capacitor, and the base n the first-, and. the collector of the second transistor is connected through the second capacitor to the emitter of the first transistor and the negative pole of the power source, the executive body connected consistently, in the power supply circuit of an electrical installation, it is distinguished by the fact that, in order to increase the reliability of operation and simplify the device, three Zener diodes are introduced into it, and the actuator is designed as a triac, the control electrode of which is connected through two the opposite side of the zener diode to the junction point of the rectifying bridge and the current-limiting capacitor of the said power source, the emitter of the second transistor is connected to the midpoint of the first divider, the second lead of the first capacitor and through the series-connected resistor and the third stabilitron to the power supply pole.