FAIL-SAFE CIRCUIT FOR ELECTRONIC CIRCUIT IN AUTOMOTIVE VEHICLE

Abstract: A fail-safe system for automotive electronic circuitry (4) blows a main power supply fuse (3) forcibly when the battery (1) voltage exceeds acceptable levels. The fuse (3) is con­ nected between the battery (1) and a voltage regulator (5). The battery (1) voltage is monitored by comparison with a constant reference voltage derived from the regulated volt­ age. When the battery (1) voltage exceeds the reference voltage, a normally-open shorting circuit (7) is closed. The shorting circuit is connected between the fuse and ground in parallel with the voltage regulator (5) and has effectively zero impedance. Thus, when closed, the shorting circuit (7) draws a large current from the battery (1) causing the fuse (3) blow. The operating status of the fuse (3) may be indicated by a monitor lamp (9) connected in parallel with the voltage regulator (5). The monitor lamp (9) may be either illumi­ nated or turned off when the fuse (3) blows, provided it exhibits the opposite state while the fuse (3) is intact.

Description: FAIL-SAFE CIRCUIT FOR ELECTRONIC CIRCUIT IN AUTOMOTIVE VEHICLE BACKGROUND OF THE INVENTION The present invention relates generally to a fail-safe circuit for protecting electronic circuitry in an automotive vehicle, such as an electronic engine control system or an anti-skid brake control system from excessively high power supply voltage. More particularly, the invention relates to a fail-safe system which blows a fuse in the power supply circuit for the automotive electronic circuitry to prevent the electronic circuitry from being damaged by abnormally high power supply voltage. In recent years, various electronic circuits have been employed in automotive vehicles for controlling vehicle devices such as the engine, the brake system and the suspension system ' and for providing various information such as driving distance, driving time, remaining fuel supply, vehicle range with the remaining fuel, etc. Such electronic circuits receive power supply from a vehicle battery. If the regulator in an alternator is damaged or when two batteries are connected in series for better cold start characteristics in cold areas, it is possible for the output voltage of the battery to become abnormally high, even far above its rated voltage. If such abnormally high voltage is applied to electronic circuit for a long period of time, IC's in the circuits may be damaged. In some cases, the fuse in the power supply circuit will not blow even when the power supply voltage is abnormally high. Some electronic circuits include fail-safe circuitry responsive to abnormal battery voltage. However, such fail-safe systems are mainly directed protecting systems against weak battery voltage. Therefore, protection for electronic circuitry against abnormally high power supply voltage is not yet available. SUMMARY OF THE INVENTION Therefore, it is an object of the invention to provide a fail-safe circuit which protects an electronic circuit against abnormally high power supply voltage. Another and more specific object of the invention is to provide a fail-safe circuit which ensures protection of the electronic circuit by forcibly blowing a fuse when abnormally high power supply voltage is detected. In order to accomplish the above-mentioned and other object, a fail-safe circuit, according to the present invention, comprises a circuit for detecting abnormally high voltage and a shorting circuit normally held open. The shorting circuit is closed by the abnormally high voltage detecting circuit when the latter circuit detects a high voltage. When closed, the shorting circuit connects the battery to ground through a fuse in the power supply circuit in order to blow the fuse. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the present invention, which, however, should not be taken to limit the invention to the specific embodiments but are for explanation and understanding only: In the drawings: Fig. 1 is a circuit diagram of the first embodiment of a fail-safe circuit according to the present invention; and Fig. 2 is a circuit diagram of the second embodiment of a fail-safe circuit according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, particularly to Fig. 1, the first embodiment of a fail-safe circuit 4 is generally connected between a vehicle battery 1 serving as a power source and an electronic circuit 20. The battery 1 is connected to the fail-safe circuit 4 by way of an ignition switch 2 and a fuse 3. The fail-safe circuit 4 includes a three-terminal voltage regulator 5 which regulates the battery voltage VB to a constant voltage Vcc and distributes the resulting voltage as the power supply voltage. The electronic circuit 20 receives the constant voltage Vcc from the regulator 5. An operational amplifier 6 is included in the fail-safe circuit 4. The negative input terminal of the operational amplifier 6 receives the constant voltage Vcc divided by a voltage divider comprising resistors R1 and R2. This divided voltage at the negative input terminal serves as a reference voltage to be comparted with the input voltage of the operational amplifier 6. The input voltage applied to the positive input terminal of the operational amplifier 6 is the battery voltage VB divided by a voltage divider comprising resistors R3 and R4. The fail-safe circuit 4 also includes a switching transistor 8. The base electrode of the transistor 8 is connected to the battery to receive the battery voltage VB. The collector electrode of the transistor is connected to a monitor lamp 9 which is in turn, connected to the battery. The transistor thus conducts power through the monitor lamp 9 as long as the battery voltage is applied to its base electrode. The output terminal of the operational amplifier 6 is connected to the gate of a thyristor 7. The anode of the thyristor 7 is connected to the battery and the cathode thereof is grounded. The circuit connecting the battery to ground through the thyristor 7 serves as a shorting circuit which closes during failsafe operation. In this arrangement, the resistance of the resistor R4 is selected to be smaller than that of the resistor R3 so that the voltage applied to the positive input terminal of the operational amplifier will never be excessively high. In -operation, closing the ignition switch 2 initiates power supply. The battery voltage is applied to the base electrode of the transistor 8 to render it conductive. As a result, the monitor lamp 9 turns on. Therefore, as long as the battery in connected to the fail-safe circuit 4 through the fuse 3 and the ignition switch 2 is closed, the monitor lamp 9 is lit. At the same time, the battery voltage is applied to the positive input terminal of the operational amplifier 6 after being divided by the dividing resistors R3 and R4. The negative input terminal receives the reference voltage Vs which is the regulated voltage Vcc from the regulator 5 divided by th dividing resistors R1 and R2. The reference voltage Vs is selected to be greater than the divided battery voltage at the positive input terminal when the battery voltage falls within a normal range. As a result, the output at the output terminal of the operational amplifier 6 normally remains low. Therefore, the thyristor 7 is held nonconductive so that the battery is not shorted to ground therethrough. When an abnormally high battery voltage is applied to the positive input terminal of the operational amplifier, the output level of the operational amplifier goes high. This renders the thyristor conduct, whereby the battery is connected directly to ground. As a result, a large current flows from the battery to ground through the fuse 3. This current will be sufficient to blow the fuse 3. Thus, power supply to the electronic circuit 20 through the fail-safe circuit 4 is interrupted. At this time, the monitor lamp turns off due to the loss of power supply thereto. Therefore, the driver can recognize when the fuse 3 is blown by observing the monitor lamp. As set forth above, the shown embodiment ensures interruption of the power supply to the electronic circuit when an abnormally high voltage of power supply is detected. This protects the electronic circuit from damage due to abnormally high voltages. Fig. 2 is the second embodiment of the failsafe circuit according to the present invention. In this embodiment, the arrangement for detecting abnormally high voltages and shorting the battery to ground is substantially the same as in the first embodiment. The characteristic trait of this embodiment is that the monitor lamp 9 is illuminated only after the fuse 3 has blown. Therefore, in this embodiment, the monitor lamp 9 words as a power-interruption alarm. This operation can be done by using transistors 10 and 11 in place of the single transistor 8. The base electrode of the transistor 11 is connected to the regulator 5 to receive the regulated voltage through a resistor R7. The collector electrode of the transistor 11 is connected to the base electrode of transistor 10 and to the battery through the monitor lamp 9, and resistors R5 and R6. The collector electrode of the transistor 10 is connected to the battery by way of the monitor lamp 9 and resistor R5. As will be appreciated from Fig. 2, the monitor lamp 9 is connected to the battery 1 via two separate lead wires 9a and 9b. The lead wire 9a is connected to the power supply line la upstream of the fuse 3. On the other hand, the lead wire 9b is connected to the power supply line la downstream of the fuse 3. It should be noted that the resistance of the resistor R6 is substantially higher than that of the resistor R5. As long as the power supply voltage remains in its normal range, the transistor 11 remains conductive due to the high-level input from the regulator 5 at its base electrode. As long as the transistor 11 is conductive, the potential at the base electrode of the transistor 10 remains low, rendering the transistor 10 nonconductive. Thus, the monitor lamp 9 also remains off. When the fuse 3 blows, the potential at the base electrode of the transistor 11 goes low due to failure of power supply from the battery to the regulator 5. As a result, the potential at the base electrode of the transistor 10 goes high, closing the transistor on and illuminating monitor lamp 9. With the arrangement set forth above, power supply break-down can be recognized by observing the monitor lamp. As will be clear to those skilled in the art, the comparison between the battery voltage and the regulated reference voltage also serves to monitor voltage regulator performance. That is, the regulated voltage Vcc should drop, the operational amplifier output will go high just as it does if the battery voltage should abruptly increase. As set forth above, the fail-safe circuit according to the present invention satisfactorily protects the electronic circuit from abnormally high battery voltages which may otherwise cause errors or, in the worst case, damage the IC's in the electronic circuit. Therefore, the invention fulfills all of the objects and advantages sought therefor.

Claims: WHAT IS CLAIMED IS: 1. A fail-safe circuit for an electronic circuit comprising: a power source generating a current at a variable voltage; a voltage regulator for adjusting the power source voltage to a constant power supply voltage and outputting same to said electronic circuit; a fuse inserted between the power source and the voltage regulator and rated to blow when subjected to the unimpeded power source current; a normally-open shorting circuit' connecting said power source directly to ground at a low impedance through said fuse when closed; and a detector circuit comparing the power source voltage to a reference voltage derived from the regulated voltage to detect when the power source voltage exceeds the reference voltage, said detector circuit being closing said shorting circuit when the power source voltage exceeds the reference voltage. 2. The fail-safe circuit as set forth in claim 1, which further comprises a monitor lamp which remains on as long as the fuse is intact and is turned off when the fuse blows. 3. The fail-safe circuit as set forth in claim 1, which further comprises a monitor lamp illuminated after said fuse blows.

Inventors: MATSUDA, TOSHIRO
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1984
Main classification: H02H9
IPC codes: H02H9, H02H7