JP2000244292A - Failsafe circuit of load driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the failsafe circuit of a load driving device by which a load is not driven despite of the output of a driving signal which is outputted in accordance with the program operation if a microcomputer when a first switching element is short-circuited. SOLUTION: The failsafe circuit of a load driving device has a configuration such that a watch dog timer 3 is connected to the output terminal 1a of a pulse signal outputted in accordance with the program operation of the microcomputer, the load 11 is connected to the output terminal 3a of the watch dog timer through a first switching element 9, the driving power source 15 of the load 11 is connected to the load 11 through a second switching element 14 and the second switching element 13 is changed-over by the driving signal 1b outputted in accordance with the program operation of the microcomputer. In the circuit, a logical circuit 21 for deciding the short circuit of the first switching element 9 is connected between the output terminal 3a of the timer 3 and the output terminal 9a of the element 9 and the circuit is provided with a means 26 by which the supply of the driving power source to the load 11 is cut-off when the short-circuit of the first switching element 9 is decided by the logical circuit 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fail-safe circuit of a load driving device using a watch dog timer.

[0002]

2. Description of the Related Art In general, as shown in FIG. 3, a watch dog timer 53 is connected to an output terminal 51a of a pulse signal output according to a program operation of a microcomputer 51, and the output of the watch dog timer 53 is output. A relay (load) 57 is connected to the terminal 53a via a first transistor (switching element) 55, and a second transistor (switching element) 5 is connected to the relay 57.
9, a driving power supply 61 for the relay 57 is connected via the switching circuit 9, and the second switching element 59 is switched through a third transistor 63 operated by a driving signal from an output terminal 51b of the microcomputer 51. Devices are known.

In this type, when a pulse signal is output from the output terminal 51a, the potential of the output terminal 53a of the watch dog timer 53 increases, and the first switching element 55 becomes conductive. When the drive signal is output from the output terminal 51b, the second switching element 59 is turned on via the third transistor 63, and the drive power supply 61 supplies power to the relay (load) 57.

[0004]

However, in the conventional configuration, when the first switching element 55 is short-circuited, the drive signal is output from the output terminal 51b even if the pulse signal is not output from the output terminal 51a. If
There is a problem that the second switching element 59 conducts through the third switching element 63 and is supplied with power from the drive power supply 61 to the relay 57, so that the relay 57 operates.

[0005] Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, and when a first switching element is short-circuited, a drive signal output according to a program operation of a microcomputer is output. An object of the present invention is to provide a fail-safe circuit of a load driving device in which a load is not driven.

[0006]

According to the first aspect of the present invention,
A watch dog timer is connected to the output terminal of the pulse signal output according to the program operation of the microcomputer, and a load is connected to the output terminal of the watch dog timer via the first switching element. In a fail-safe circuit of a load drive device configured to connect a drive power supply of the load via a second switching element and switch the second switching element with a drive signal output according to a program operation of the microcomputer, A logic circuit for determining whether the first switching element is short-circuited is connected between the output terminal of the watch dog timer and the output terminal of the first switching element. Means for shutting off the supply of drive power to the load when determined. It is an butterfly.

According to the first aspect of the present invention, the short circuit of the first switching element is determined by the logic circuit. When the short circuit is determined, the supply of the driving power to the load is cut off even if the driving signal is output. Therefore, a malfunction of the load is prevented.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, reference numeral 1 denotes a microcomputer (hereinafter referred to as a microcomputer). A watch dog timer 3 is connected to an output terminal 1a of a pulse signal output according to the program operation of the microcomputer 1. The watch dog timer 3 includes a first capacitor 4, a first diode 5, a second capacitor 6, and a second diode 7.

When a pulse signal is output through the output terminal 1a of the microcomputer 1, the watch dog timer 3 supplies a current to the DC element capacitor 4, the first diode 5, and the charging capacitor 6 in this order, and charges the current. The electric charge is stored in the capacitor 6, thereby increasing the potential Vb of the output terminal 3 a of the watch dog timer 3,
When a signal other than a pulse signal is output through the output terminal 1a of the microcomputer 1, a current flows in the order of the DC element capacitor 4 and the second diode 7, and the watch dog
The potential Vb of the output terminal 3a of the timer 3 is kept low.

A resistor 8 and a first transistor (switching element) 9 are sequentially connected to the output terminal 3a of the watch dog timer 3, and a relay (load) is connected to the output terminal 9a of the first transistor 9. 11 are connected.

The relay 11 is connected to, for example, a combustion system of a gas-fired heater, and when the relay 11 operates, a combustion operation is performed.

A second transistor (switching element) 13 is connected to the relay 11, and a driving power supply 15 for the relay 11 is connected to the transistor 13. The third transistor 17 is connected to the second transistor 13, and the output terminal 1 b of the microcomputer 1 is connected to the third transistor 17.

A logic circuit 21 for determining whether or not the first transistor 9 is short-circuited is connected between the output terminal 3a of the watch dog timer 3 and the output terminal 9a of the first transistor 9. I have.

The logic circuit 21 includes a NAND circuit 23. One input terminal 23a of the NAND circuit 23 is connected to the output terminal 9a of the first transistor 9 via a first inversion circuit 24, and the other input terminal is connected to the other input terminal. The terminal 23b is connected to the output terminal 3a of the watch dog timer 3 via the second inverting circuit 25. The output terminal 23c of the NAND circuit 23 is connected to the control terminal 17a of the third transistor 17 via the third diode 26.

Next, the operation of this embodiment will be described.

Normally, when a pulse signal is output from the output terminal 1a of the microcomputer 1, the potential of the output terminal 3a of the watch dog timer 3 rises as described above,
When the drive signal is output from the output terminal 1b while the first transistor 9 is conducting, the third transistor 17
, The second transistor 13 conducts, and the driving power supply 15
Is supplied to the relay 11 and the relay 11 operates.

In short, the relay 11 operates only when an appropriate signal is output from both the output terminal 3a and the output terminal 3b of the microcomputer 1.

However, in this configuration, when the first transistor 9 is short-circuited, the relay 11 is only output from the output terminal 1b and the relay 11 is output even if the pulse signal is not output from the output terminal 1a of the microcomputer 1. Will work.

In order to solve this, in this embodiment,
When the first transistor 9 is short-circuited, the logic circuit 21 operates as follows.

FIG. 2 shows the control of the logic circuit 21. In FIG. 2, the output terminal 3a of the watch dog timer 3
And the output terminal 9a of the first transistor 9
And the output terminal 23c of the NAND circuit 23
Is represented by a high potential H or a low potential L, respectively.

In the logic circuit 21, for example, when the potential Vb is H and the potential Vc is L, the inversion circuits 24 and 25 respectively invert the potential, and the potential L and the potential H are input to the NAND circuit 23. H (potential Va) is output from the output terminal 23c of the circuit 23. When the potential Va is H,
The third diode 26 is non-conductive. Example, potential Vb
Is low and the potential Vc is low, the inversion circuits 24 and 25 respectively invert the potential, and the potential H and the potential H are changed to the NAND circuit 23.
, And L (potential Va) is output from the output terminal 23c of the NAND circuit 23. When the potential Va is L, the third diode 26 conducts, and the drive signal from the output terminal 1b of the microcomputer 1 is grounded through the NAND circuit 23. The third diode 26 constitutes means for interrupting the supply of drive power to the relay (load).

In FIG. 2, the example is a normal operation.

On the other hand, an example is an abnormal operation. That is, in the example, the potential Vc indicates L even though the potential Vb is L, and in the example, the potential Vc indicates H even though the potential Vb is H. In the example, it is a short-circuit failure of the first transistor 9, and in the example, it is a disconnection failure of the first transistor 9.

In the conventional example, in this case, the relay 11
Does not malfunction, but there are few problems, but in the case of the example, the problem is large because the relay 11 malfunctions.

In this embodiment, in the case of the example, as described above, the third diode 26 conducts and the microcomputer 1
Since the drive signal from the output terminal 1b is grounded through the NAND circuit 23, the third transistor 17 and the second transistor 13 do not operate, and the drive power is supplied to the relay (load) 11. Since there is no,
This relay 11 does not malfunction.

Although the present invention has been described based on one embodiment, it is apparent that the present invention is not limited to this. In the above embodiment, the load is the relay 11, but is not limited to this, and may be various loads.

[0028]

According to the first aspect of the present invention, when the short circuit of the first switching element is determined in the logic circuit, the supply of the drive power to the load is cut off even if the drive signal is output. Therefore, a malfunction of the load is prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a fail-safe circuit according to the present invention.

FIG. 2 is a diagram for explaining control of a logic circuit.

FIG. 3 is a circuit diagram showing a conventional load driving device.

[Explanation of symbols]

 Reference Signs List 1 microcomputer 1a, 1b output terminal 3 watch dog timer 9 first transistor (switching element) 11 relay (load) 13 second transistor (switching element) 15 drive power supply 17 third transistor 21 logic circuit 23 NAND circuit 24, 25 Inverting circuit 26 Third diode

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J032 AA11 AB02 AC18 5J055 AX21 AX31 AX38 BX16 CX21 DX04 DX44 DX55 EY10 EY12 EY17 EZ00 EZ07 EZ25 EZ39 EZ42 GX01

Claims (1)

[Claims] An output terminal of a pulse signal output according to a program operation of a microcomputer has a watch terminal.
Connecting a dog timer, connecting a load to an output terminal of the watch dog timer via a first switching element, connecting a driving power supply of the load to the load via a second switching element, In a fail-safe circuit of a load driving device configured to switch the second switching element by a driving signal output according to a program operation of a microcomputer, the output terminal of the watch dog timer and the first switching element Means for connecting a logic circuit for determining short-circuiting of the first switching element between the output terminals, and for interrupting supply of drive power to the load when the short-circuiting of the first switching element is determined by the logic circuit; A fail-safe circuit for a load driving device, comprising: