JPH01121767A - Current detecting circuit - Google Patents

Abstract

PURPOSE:To enable a circuit breaker from being tripped even for an overcurrent which exceeds a rated current slightly by generating a DC voltage proportional to a current flowing through a cable run, charging it in a charging means which has a prescribed time constant, and discharging the means by a discharging means which has a constant period and a constant time constant. CONSTITUTION:The overcurrent flowing through the circuit breaker 21 is detected by a current transformer 22, an AC current 11 as its output is inputted to a waveform converting circuit 1 and converted into a DC voltage 12 corre sponding to the effective value or mean value of the current 11, and the DC voltage is sent to a time limit generating circuit 2. The circuit 2 incorporates a capacitor 2b through a resistance 2a and the capacitor is charged with charges inputted according to the prescribed time constant and then discharged for a constant time at a constant period by using a pulse generating circuit 2c through a resistance 2e and a switching transistor 2d. At this time, a terminal voltage 13 developed across the capacitor 2b is inputted to a comparing circuit 3 and compared with a prescribed voltage and a signal 14 is outputted when the prescribed voltage is reached to trip the breaker 21.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a current detection circuit used in circuit breakers and the like.

[Conventional technology] FIG. 3 is a diagram showing a conventional current detection circuit.

In the figure, an overcurrent flowing through a circuit breaker (21) is detected by a converter (22), and the output of the alternating current (11) is input to a waveform converter (1). A waveform conversion circuit (1) outputs a DC voltage (12) corresponding to the effective value or average value of this AC current (11), and its output is a time generator circuit composed of a resistor (2a) and a capacitor (2b). Number of occurrences of 0 time period inputted to (2) m (
2) has a time constant due to the resistor (2a) and capacitor (2b), so it has a time limit depending on the magnitude of the DC voltage (12) (the larger the DC voltage (12), the shorter the time limit). As a result, the terminal voltage (13) of the capacitor (2b) increases. This terminal voltage (13) is input to a comparator circuit (3), where it is compared with a predetermined voltage (ie, output generation level). As a result, the terminal voltage (13
) has reached a predetermined voltage, outputs a signal (14),
Trip II circuit breaker (21).

[Problems to be solved by the invention] In the conventional current detection circuit as described above, the time limit generation circuit (
2) consists of a resistor (2a) and a capacitor (2b), so an overcurrent that slightly exceeds the constant current of the circuit breaker (21) is flowing, and the circuit breaker (21)
) requires a long time period to trip,
In order to obtain a long time until the terminal voltage (13) of the capacitor (2b) reaches a predetermined voltage, the resistance value of the resistor (2a) and the capacitance 1 of the capacitor (2b) must be respectively very large. There is. Therefore, there was a problem in that standard parts could not be used and special parts, which were relatively expensive, had to be used. Furthermore, when the capacitance of the capacitor (2b) increases, leakage current increases, and there is a problem in that the discharging current becomes larger than the charging current.

This invention was made to solve the above problems, and uses a standard component configuration to trip the circuit breaker 5 (21) in response to an overcurrent that slightly exceeds the rated current. It is an object of the present invention to provide a current detection circuit that can have a long time period up to .

[Means for Solving the Problems] The current detection circuit according to the present invention receives a current proportional to the current flowing in an electric circuit, and outputs a DC voltage corresponding to the effective value or average value of this current using a waveform conversion means. , this output is stored as an electric charge with a predetermined time constant by a charging means, this electric charge is discharged with a predetermined time constant at a certain period by a dissipation means, and the value of this electric charge is set to a predetermined value by a comparison means. A signal is output when a predetermined value is reached.

[Operation] In the present invention, the discharging means discharges the electric charge stored by the charging means at a fixed period for a fixed period of time, thereby creating a charging process and a discharging process.

[Embodiment] Fig. 1 is a circuit diagram showing an embodiment of the present invention. In Fig. 0, an overcurrent flowing through a circuit breaker (21) is detected by a current transformer (22), and its output is AC current (
11) is input to the waveform conversion circuit (1).

The waveform conversion circuit (1) outputs a DC voltage (12) corresponding to the effective value or average value of this AC current (11), and the output is input to the time generator circuit (2) and is applied via a resistor (2a). and connect the capacitor (2b) to 1. Charging with a time constant determined by the resistor (2a) and the capacitor (2b). Furthermore, a discharge circuit is provided in parallel with this capacitor (2b). In other words, the pulse generation circuit (
2C) generates a pulse with a constant period and a constant pulse width to switch the switching transistor (2d) at a constant period, so the charge in the capacitor (2b) is transferred to the resistor (
The terminal voltage (13) of the capacitor (2b) is input to the comparator circuit (3), which discharges for a certain period of time with a time constant determined by the resistance value of the capacitor (2e) and the capacitance of the capacitor (2b). and is compared with a predetermined voltage (ie, output generation level) in the comparison circuit (3). the result,
When the terminal voltage (13) reaches a predetermined voltage, the signal (1
4) to trip the circuit breaker. Figure 2 is a graph showing the charging characteristics of the capacitor (2b) when a constant overcurrent is flowing through the circuit breaker (21, Figure 1), that is, the charging level (terminal voltage) against the charging time. be. However, this graph shows only the portion of the charging characteristic from condenser to no (2b) that has initial linearity. The charging characteristic (15) according to this embodiment is the charging process (1
5a) and the discharging process (15b) are repeated for a certain period of time until the output generation level E is reached at time t1. There is no discharge circuit, the same capacitor (2b) and resistor (2a
), it can be seen that the time limit for reaching the output generation level E is extended from to to tl when compared with the charging characteristic (16) in the same overcurrent state using a conventional current detection circuit configured as shown in FIG. Also, the larger the overcurrent flowing through the circuit breaker (21° Fig. 1), the more the charging process (15a)
The slope of the charging process (15a) becomes smaller as the overcurrent becomes larger, and conversely, the smaller the overcurrent becomes. Therefore, a short time limit can be accurately assigned to a large overcurrent, and a long time limit can be accurately assigned to a small overcurrent that slightly exceeds the rated current.

In this way, by discharging the capacitor (2b) at a certain period and for a certain period of time, it is possible to operate from a short period of time to an extremely long period of time without increasing the resistance value of the resistor (2a) or the capacitance of the capacitor (2b). can be obtained accurately and easily. Therefore, the resistor (2a) and the capacitor (
2b) does not require the use of special items and can be constructed from standard parts.

In the above embodiment, the case where the present invention is applied to a current detection circuit of a circuit breaker has been described, but it goes without saying that the present invention can also be applied to a delay circuit such as a timer.

[Effects of the Invention] As described above, according to the present invention, since a circuit is provided in parallel with the capacitor of the time limit generation circuit to discharge a discharge for a fixed period of time at a fixed period, it is possible to accurately and inexpensively control a time limit from a short time limit to an extremely long time limit. This has the effect of providing a current detection circuit that can be provided to

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a graph showing the operation of the time generator circuit shown in FIG. 1, and FIG. 3 is a circuit diagram showing a conventional current detector. In the figure, (1) is a waveform conversion circuit, (2) is a time generator circuit, (2a) is a resistor, (2b) is a capacitor, (2C
) is a pulse generation circuit, (2e) is a resistor, (2d) is a switching transistor, (3) is a comparison circuit, (11) is an alternating current, (12) is a direct current voltage, (13) is a terminal voltage,
(14) is a signal. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] (1) A waveform conversion means that receives a current proportional to the current flowing in the electric circuit and outputs a DC voltage corresponding to the effective value or average value of this current, and stores the output of the waveform conversion means as an electric charge with a predetermined time constant. charging means, and charging the electric charge stored in the charging means at a certain period for a certain period of time.
A current detection circuit comprising a discharging means for discharging with a time constant, and a comparing means for comparing the value of the charge stored in the charging means with a predetermined value and outputting a signal when the predetermined value is reached.