JPH04242032A - Power supply cut detection circuit - Google Patents

Abstract

PURPOSE:To surely perform power supply cut detection relating to a power supply cut detection circuit. CONSTITUTION:A power supply cut detection circuit is composed in such a way as having a current control means 3 to be connected in series to a relay winding 1 and a voltage detection means 4 for detecting power supply voltage, as aforesaid in series connection and aforesaid voltage detection means 4 being mutually connected in parallel to aforesaid power supply voltage, and as aforesaid current control means 3 being turned to an OFF state so as to reduce the current flowing through aforesaid relay wiring 1 at least to be lower than the aforesaid first prescribed value when voltage detected by said voltage detection means 4 becomes lower than a second prescribed value.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power failure detection circuit. Generally, when the power of the device is cut off, for example, when the power switch is turned off or the power supply circuit is broken, a lamp or alarm output (ALM) is required to indicate the power cut, and a circuit to detect the power cut is required. . If the device specifications for the power failure detection circuit require relay contact output,
A relay circuit is used for the power failure detection circuit.

0002

2. Description of the Related Art FIG. 5 shows an example of a power failure detection circuit using a conventional relay circuit. In Figure 5,
30 is a power supply circuit, 31 is a power switch, 32 is a power failure detection circuit, 33 is a relay winding, 34 is a diode, 35 is a relay contact, and 35 is an operating circuit that receives power from the power supply circuit 30. .

The relay contact 35 is connected to a power-off alarm, and when the relay contact 35 is in an ON state, the power-off alarm is activated. When the power switch 31 of the power supply circuit 30 is ON and the power supply circuit 30 is operating normally, the output voltage of the power supply circuit 30 is
3, and in response, the relay contact 35 turns OFF.
in a state. Power switch 31 of power supply circuit 30 is OFF
Or, when the power supply circuit 30 is not operating normally, the output voltage of the power supply circuit 30 applied to the relay winding 33 decreases and falls below a predetermined open circuit voltage, and accordingly, the relay contact 35 turns ON, and a power-off alarm is activated.

0004

[Problems to be Solved by the Invention] However, the open circuit voltage of the above relay is usually 5 to 10% of the rated voltage (0.25 to 0.5 V when the rated voltage is 5 V).
The voltage is quite low. Therefore, since the open circuit voltage of the relay is low, there is a problem in that the relay does not turn off when the capacity of the power supply is large or current flows in from other operating circuits, making it impossible to detect a power outage.

[0005] Generally, circuits other than a specific power supply circuit, such as a communication interface circuit, operate within a device even when the power supply circuit is not operating, and current flows into them from peripheral circuits. Even when the power is turned off, a voltage higher than the open circuit voltage of the relay is applied to the relay winding, and the relay does not turn off, causing a problem in that the power cannot be detected.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a power-off detection circuit that can reliably detect a power-off.

[0007]

[Means for Solving the Problems] FIG. 1 is a diagram showing the basic configuration of the present invention. In FIG. 1, 1 is a relay winding, 2 is a relay contact, 3 is a current control means, and 4 is a voltage detection means. The relay winding 1 is connected to a power supply to be detected for disconnection, and the output voltage of the power supply is applied thereto.

[0008] The relay contact 2 takes either an open or closed state depending on whether the current flowing through the relay winding 1 falls below a first predetermined value or not. Current control means 3 is connected in series to the relay winding 1. The voltage detection means 4 detects the power supply voltage. The above series connection and the voltage detection means 4 may be connected in parallel with each other to the power supply voltage.

[0009] When the voltage detected by the voltage detection means 4 falls below a second predetermined value, the current control means 3 is turned off and the current flowing through the relay winding 1 is controlled to at least the second predetermined value. below a predetermined value of 1.

0010

[Operation] According to the power-off detection circuit of the present invention, the voltage detected by the voltage detection means 4 when the power is cut off is the second voltage.
is below the first predetermined value, the current control means 3 turns off and reduces the current flowing through the relay winding 1 to at least below the first predetermined value. As a result, the open/closed state of the relay contact 2 changes, so by appropriately setting the second predetermined value in the voltage detection means 4, a power outage can be reliably detected.

[0011]

Embodiment FIG. 2 is a diagram showing the configuration of a first embodiment of the present invention. The embodiment shown in FIG. 2 shows the configuration of a power-off detection circuit for a circuit that operates with a 5V power supply. In Figure 2,
11 is a relay winding, 12 is a diode, and 13 is a logic inverter circuit.

The logic inverter circuit 13 has an inverted output characteristic as shown in FIG. 3, has an open collector output, and is commercially available as an IC. When the 5V power supply is normal, the input of IC13 is +5V (H level)
Therefore, the open collector output becomes L level,
The relay is in the ON state. When the 5V power supply drops, as shown in FIG. 3, when the power supply voltage reaches around 2.3V, the open collector output becomes H level and the relay becomes OFF.

FIG. 4 is a diagram showing the configuration of a second embodiment of the present invention. In FIG. 4, 21 is a relay winding, 22 and 23 are diodes, 24 is a switching transistor, 25 and 26 are resistors, and 27 is a relay contact. One end of the relay winding 21 is connected to the power supply voltage VCC, the other end is connected to the collector terminal of the switching transistor 24, and the base terminal of the switching transistor 24 is connected to the power supply voltage VCC.
A voltage VB obtained by dividing power supply voltage VCC by resistors 25 and 26 is applied. The collector terminal of the switching transistor 24 and the ground are connected by diodes 22 and 23 for preventing reverse bias of the transistor. As a relay, power supply voltage VCC
A rated VCC-2VD lower by 2VD, which is the voltage drop of diodes 22 and 23, is used.

The power supply voltage VCCD at which the switching transistor 24 is turned off is VCCD when the breakdown voltage of the base-emitter voltage is VBES and the resistance values of the resistors 25 and 26 are R1 and R2, respectively.
= (R1+R2)VBES/R2, and the power supply voltage V
When CC falls below VCCD, switching transistor 24 is turned off and the current flowing through relay winding 21 is cut off. Therefore, the relay contacts operate reliably when the power is cut off.

[0015]

As described above, according to the power failure detection circuit of the present invention, power failure detection can be performed reliably.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of the present invention.

FIG. 2 is a diagram showing the configuration of a first embodiment of the present invention.

FIG. 3 is a diagram showing the operating characteristics of the IC of FIG. 2;

FIG. 4 is a diagram showing the configuration of a second embodiment of the present invention.

FIG. 5 is a diagram showing an example of a power-off detection circuit using a conventional relay circuit.

[Explanation of symbols]

1...Relay winding 2...Relay contact 3...Current control means 4...Voltage detection means 11...Relay winding 12...Diode 13...Logic inverter circuit 21...Relay winding 22, 23...diode 24...Switching transistor 25, 26...Resistance 27...Relay contact

Claims (3)

[Claims] 1. A relay winding (1) connectable to a power supply to be detected for disconnection, to which an output voltage of the power supply is applied;
A relay contact (2) that takes either an open or closed state depending on whether or not the current flowing through the relay winding (1) falls below a first predetermined value;
1); and a voltage detection means (4) for detecting the power supply voltage.
The series connection and the voltage detection means (4) may be connected to the power supply voltage in parallel with each other, and when the voltage detected in the voltage detection means (4) falls below a second predetermined value, the The current control means (3) is in an OFF state, and the current flowing through the relay winding (1) is controlled to at least
A power-off detection circuit characterized in that the power is decreased below the first predetermined value. 2. The voltage detection means (4) and the current control means (3) have open collector terminals, and are controlled by a logic circuit (13) that connects the open collector terminals to the relay winding (1). A power-off detection circuit according to claim 1, wherein the power-off detection circuit is implemented. 3. The voltage detection means (4) comprises voltage distribution resistors (25, 26) connected to the power supply voltage and distributes the power supply voltage at a predetermined distribution ratio; ) is connected between the transistor (24) whose base voltage is the distributed voltage, whose collector terminal is connected to the relay winding (1), and whose emitter terminal is grounded, and between the collector terminal and ground. , and a diode (22, 23) for applying a bias voltage to the collector terminal.