JPH10164748A - Overvoltage protecting circuit for current transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress an overvoltage generated in the secondary winding of a current transformer by connecting a transistor to the output side of a rectifier circuit which rectifies the secondary voltage generated in the secondary winding of the current transformer, and connecting a zener diode between the collector and the base of this transistor. SOLUTION: A voltage generated in the secondary winding 3 of a current transformer 1 is rectified by a rectifier circuit 11. Here, a transistor 17 is in an off-state, when a current flowing in the primary winding of the current transformer 1 is small, or the impedance of a load 5 is small. And the voltage of the zener diode 18 becomes constant, and time transistor 17 moves into an active state, when the primary current of the current transformer 1 becomes large, or the impedance of the load 5 is large. At this time, the secondary voltage V1 of the current transformer 1 is clamped at V1 +Vbe +2Vt (where Vz is the zener voltage of the zener diode 18, Vbe is the voltage between the base and the emitter of the transistor 17, and Vf is the forward voltage of the diode of the rectifier circuit 11) to protect the current transformer 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current transformer overvoltage protection circuit for protecting an overvoltage generated in a secondary winding of a current transformer.

[0002]

2. Description of the Related Art If the load impedance on the secondary side increases, the current transformer becomes excessively high and may exceed the capacity of the current transformer and be damaged. For this reason, the load impedance has been reduced and a protection circuit has been provided. FIG. 2 shows an example of this type of protection circuit.

In FIG. 1, reference numeral 1 denotes a current transformer, and a primary current I1 flows through a primary winding 2. A load 5 is connected to the secondary winding 3 of the current transformer 1, and is used as a display of a primary current and as a current detection signal of an automatic control system. Reference numeral 21 denotes an overvoltage protection circuit for a secondary voltage of the current transformer 1, and diodes 22a, 22b...
a, 22b... 22n in the opposite direction to diodes 23a, 23
b ... 23n are provided.

When the load 5 is released or flows through the primary winding 2 but becomes excessive, the secondary voltage of the secondary winding 3 of the current transformer 1 becomes n forward voltages of the diode. It is clamped to the voltage to protect the current transformer 1 and prevent the current detection signal of the automatic control system from becoming an excessive signal and being input to the automatic control system.

[0005]

By the way, in the above protection circuit, the diode needs to be twice the number obtained by dividing the clamp voltage to be set by the forward voltage per diode, and the number of parts of the protection circuit is small. There is a problem that it will increase.

[0006]

According to the present invention, there is provided a rectifier circuit for rectifying a secondary voltage generated in a load connected to a secondary winding of a current transformer. It comprises a transistor provided on the output side and a Zener diode provided between the collector and the base of the transistor.

If the current flowing through the current transformer is a small current, the secondary voltage generated at the load is low, and the Zener diode is off. If the current flowing through the current transformer is large,
The secondary voltage generated in the load increases, the Zener diode exhibits a constant voltage characteristic, activates the transistor, and the output of the rectifier circuit becomes a constant voltage. Thereby, the secondary voltage of the current transformer is clamped to the voltage controlled by the Zener diode, and the overvoltage of the current transformer is suppressed.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an overvoltage protection circuit for a current transformer according to the present invention will be described with reference to FIG.
In the figure, 1 is a current transformer, 2 is a primary winding of the current transformer 1, 3 is a secondary winding, and 5 is a load, which is the same as a conventional one. Reference numeral 11 denotes a rectifier circuit configured by a bridge circuit of diodes 12 to 15 for rectifying an AC input. The collector and emitter of the transistor 17 are connected to the output of the rectifier circuit 11, a zener diode 18 for setting a clamp voltage is connected between the collector and the base of the transistor 17, and the base and the emitter of the transistor 17 are connected. Is connected to a resistor 19 higher than the impedance of the load 5.

When a current flows through the primary winding 2 of the current transformer 1, a current flows from the secondary winding 3 of the current transformer 1 to the load 5.
The voltage generated in the load 5 is rectified by the rectifier circuit 11.

At this time, when the torsion flowing through the primary winding of the current transformer 1 is small or the impedance of the load 5 is small, the Zener diode 18 does not reach a constant voltage, and the transistor 17 is turned off. ing. And one of the current transformers 1
When the current flowing in the next winding is large or the impedance of the load 5 is large, the Zener diode 18 has a constant voltage, the base current flows through the transistor 17, and the transistor 17 shifts to the active state.

When the transistor 17 enters an active state, the voltage between the collector and the emitter of the transistor 17, that is, the output V2 of the rectifier circuit 11 is Vz + Vbe (where Vz is the Zener voltage of the Zener diode 18, and Vbe is the voltage between the base and the emitter of the transistor 17). Voltage). Therefore, assuming that the forward voltage of the diode of the rectifier circuit 11 is Vf,
The secondary voltage V1 of the secondary winding 3 of the current transformer 1 is Vz + Vbe
It is clamped at + 2Vf.

[0012]

According to the overvoltage protection circuit of the current transformer of the present invention,
Even if the primary current of the current transformer becomes an overcurrent and the load impedance becomes large, the secondary voltage V1 of the current transformer becomes the Zener voltage of the Zener diode, the voltage between the base and the emitter of the transistor, and the diode in order. The current transformer is protected by being clamped by the directional voltage. Even if the output of the current transformer is used for the current detector of the automatic control system, no excessive signal is input to the automatic control system.

[Brief description of the drawings]

FIG. 1 is a connection diagram of an embodiment of an overvoltage protection circuit for a current transformer according to the present invention.

FIG. 2 is a connection diagram of a conventional current transformer overvoltage protection circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Current transformer 2 Primary winding 3 Secondary winding 5 Load 11 Rectifier circuit 12-15 Diode 17 Transistor 18 Zener diode 19 Resistance

Claims (1)

[Claims] 1. A rectifier circuit for rectifying a secondary voltage generated in a load connected to a secondary winding of a current transformer, a transistor provided between outputs of the rectifier circuit, and a collector and a base of the transistor. The overvoltage protection circuit of the current transformer composed of the Zener diode provided in the power supply.