System for Indicating Interruption in Operation of Automotive Lamps

1. Field of the Invention

The present invention relates to a indicating system for automotive lamps for informing a driver of the inoperativeness of one or more lamps.

2. Description of the Prior Art

In a conventional indicating system of this kind, for example as exemplified in the U.S. Pat. No. 3,868,628, which granted on Feb. 25, 1975 to Yasuhiko Sakurai, a current winding is connected between a current source and a plurality of automotive lamps and a voltage winding is connected between the current source and ground for generating an opposite magnetomotive force to that of the current winding. A lamp interruption is detected in such a manner that a relay is energized to light an indicating lamp when the net magnetomotive force of the two windings exceeds a predetermined value.

However, the conventional system just described sometimes fails to indicate lamp interruption when all of the lamps are inoperative, since the magnetomotive force of the voltage winding in itself is still large enough to maintain the relay at its normal condition.

It is, therefore, an object of the present invention to provide a system for automotive lamps which indicates lamp interruption even when all of the lamps are inoperative.

It is another object of the present invention to provide an indicating system which can operate over a wide range of voltage variations of the current source.

These and other objects of the present invention will become apparent by reference to the following description, taken in connection with the accompanying drawings.

FIG. 1 shows an electric wiring diagram of an indicating system according to the present invention, and

FIG. 2 is a diagram showing voltage-current characteristics of lamps.

In FIG. 1, a lamp switch 2 is connected to a battery 1 mounted in an automotive vehicle, and also to a plurality of automotive lamps 3, 4, 5 and 6 through a current winding 7. A series circuit comprising a voltage winding 8, a resistor 9 and a zener diode 12 (connected at its cathode to the resistor 9 and at its anode to the ground) is connected in parallel with winding 7 and lamps 3-6. The voltage winding generates a magnetomotive force opposite to that of the current winding 7 when the windings are energized. A reed switch 10 disposed adjacent to the two windings, closes its contacts when the net magnetomotive force of the current winding 7 and the voltage winding 8 exceeds a predetermined value. The reed switch 10 is connected across the battery 1 through a resistor 21. A junction between the resistor 21 and the reed switch 10 is connected to a base of a switching transistor 23 having a collector-emitter path connected across the battery through indicating means such as lamp 22.

To facilitate the understanding of the invention, FIG. 2 is presented. However, it should be understood that the particular plot illustrated in FIG. 2 is achieved with an exemplary set of circuit parameters. The invention itself can be carried out with a wide variety of such parameters, and therefore, it must be appreciated that the specific example in FIG. 2 is merely a representative one for illustrative purposes.

Referring more specifically to FIG. 2, the plot has an abscissa the voltage V of a battery corresponding to battery 1 in FIG. 1. The ordinates AT and -AT of the plot represent the net magnetomotive force (in ampere-turns) developed by current flow through the current winding 7 and/or the voltage winding 8 illustrated in FIG. 1. Solid line A represents the net magnetomotive force achieved with all of the lamps 3-6 in FIG. 1 operating as voltage V varies, while line B indicates the net magnetomotive force with one of the lamps being inoperative. The solid line C represents the net magnetomotive force developed by voltage winding 8 when all of the lamps 3-6 are inoperative and when winding 8 is connected in series with a zener diode 12. The parameters of the reed switch 10 dictate the amount of magnetomotive force required for it to switch from one state to another, and in the circuit operation plotted in FIG. 2, these values are represented by ATR and -ATR. The value ATV is merely an indication of the net magnetomotive force developed by the voltage winding 8 with lamps 3-6 inoperative and with voltage V equal to 19.5 volts. The dotted line which parallels line C represents what the net magnetomotive force would be with lamps 3-6 inoperative and the zener diode 12 eliminated, and the angle .theta. represents the rate of increase of the magnetomotive force produced by winding 8 under these conditions. Obviously, this angle .theta. is a function of the parameters of the circuitry involved.

In operation, when the lamp switch 2 is closed, electric current flows from the battery 1 through the lamp switch 2, the current winding 7, and the plurality of automotive lamps 3, 4, 5 and 6, to thereby cause them to light. At the same time, the current winding 7 generates a magnetomotive force and the voltage winding 8 also generates a magnetomotive force opposite to that of the current winding 7. The combined (net) magnetomotive force of the above two windings 7 and 8 acts on the reed switch 10.

When all of the lamps 3, 4, 5 and 6 are normally lit by the closure of the lamp switch 2, the reed switch 10 is energized to close its contacts since the net magnetomotive force of the two windings 7 and 8 acting thereon is sufficiently large as will be described with reference to solid line A in FIG. 2.

As stated previously, in FIG. 2, the solid line A indicates a characteristic curve of the net magnetomotive force (AT) in ampere-turns for variations in the battery voltage V when all of the lamps 3 through 6 are normally operated. It should be noted that the net magnetomotive force designated by the line A exceeds the predetermined value (indicated by ATR), above which the reed switch 10 is closed, whenever the voltage of battery 1 is above 6.0 volts. Under these conditions, the indicating means 22 can not be energized.

However, when one of the automotive lamps 3 through 6 is rendered inoperative, the net magnetomotive force is decreased below the predetermined value ATR as indicated by a solid line B, whereby the reed switch 10 is opened. When the reed switch 10 is opened, the transistor 23 is supplied with base current through the resistor 21, resulting in transistor conduction so that the indicating lamp 22 is lit to inform a driver in a motor vehicle of inoperativeness of the lamp.

As noted from FIG. 2, the more the battery voltage increases, the larger the net magnetomotive force becomes. When the battery voltage exceeds 19.5 volts, the reed switch 10 is no longer kept opened when one of the lamps is inoperative since the net magnetomotive force is higher than ATR when the battery voltage is above 19.5 volts. Therefore, the inoperativeness of one lamp can be appropriately detected in the voltage range from 6.0 volts to 19.5 volts.

On the other hand, when all of the automotive lamps 3 through 6 are inoperative, no current flows through the current winding 7. Under these circumstances, only the magnetomotive force generated at the voltage winding 8 acts on the reed switch 10.

If a zener diode 12 is not provided, the voltage winding 8 would generate the magnetomotive force as indicated by a dotted line in FIG. 2, whereby the reed switch 10 would be closed when the battery voltage is higher than 18.0 volts. Thus, above this level no indication by lamp 22 would occur even though all of the lamps are inoperative.

If the angle .theta. of the dotted line were to be reduced to overcome the above defect, the magnetomotive force generated at the voltage winding would become smaller than ATR above the battery voltage of 19.5 volts, so that interruption of all of the lamps would be detected. However, the decrease of the angle .theta. would cause an increase of the gradient of the net magnetomotive force when one or more lamps are operative, which would result in the detecting range for one-lamp inoperativeness becoming smaller. Accordingly, it is apparent that the larger the angle .theta. becomes, the wider the detecting range for one-lamp inoperativeness extends.

Based on the this fact, in the present invention the zener diode 12 is provided in series with the voltage winding 8 as shown in FIG. 1, the diode being brought into conduction when the voltage drop thereacross becomes higher than 6.0 volts.

Provision of the zener diode 12 makes it possible for the angle .theta. of the line obtained by the voltage winding to be maintained as shown by solid line C in FIG. 2, which limits the magnetomotive force to the level ATV which is less than ATR when the battery voltage is around 19.5 volts. Therefore, the inoperability of all of the lamps can be detected in the wide range of battery voltage variations from 6.0 volts to 19.5 volts while allowing the inoperability of one lamp to be detected in this voltage variation range.

In the present invention, a series connection of a plurality of diodes with polarities opposite to the zener diode 12 can serve as a replacement for the zener diode 12.