US 3639805 A
A vehicle-headlamp-energizing circuit for headlamps connected in series. Each headlamp has a breakdown diode connected in parallel, and each breakdown diode has a breakdown voltage slightly larger than the normal potential drop across its parallel connected lamp so as to conduct current around the lamp when it fails and maintain conduction through the remaining lamps.
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Description (OCR text may contain errors)
United States Patent Muench et al.
154] SERIES LAMP SAFEGUARD CIRCUIT  Inventors: Nils L. Muench, Bloomfield Hills; Charles B. Murphy, Fraser, both of Mich.
General Motors Corporation, Detroit, Mich.
 Filed: May 25,1970
 Appl.No.: 40,184
 U.S.Cl. ..3l5/l22,3l5/82,315/l85 R  Int. Cl. ..H05b 37/00  Field oiSearch ..315/74,75,82,83,122,123,
 References Cited UNITED STATES PATENTS 1 Feb. 1,1972
FOREIGN PATENTS OR APPLICATIONS 427,021 6/1967 Switzerland ..3 15/122 Primary Examiner-Raymond F. l-lossfeld Attorney-J. L. Carpenter and Paul Fitzpatrick  ABSTRACT A vehicle-headlamp-energizing circuit for headlamps connected in series. Each headlamp has a breakdown diode connected in parallel, and each breakdown diode has a breakdown voltage slightly larger than the normal potential drop across its parallel connected lamp so as to conduct current around the lamp when it fails and maintain conduction through the remaining lamps.
7 4 Claims, 1 Drawing Figure s i Q SERIES LAMP SAFEGUARD CIRCUIT The conventional automotive vehicle headlamp system has headlamps connected in parallel across a battery. This is necessary so that if one lamp malfunctions and does not conduct current the other lamp will not be affected.
However, there are often advantages to be gained by connecting headlamps in series. Without changing the battery voltage, for example, two headlamps of one-half rated battery voltage can be used. These lower voltage lamps are more rugged and function longer, on the average, than higher voltage lamps. Or, if the battery voltage itself were doubled to reduce current loads in the rest of the vehicles electrical system, the two conventional voltage headlamps could be used in series rather than double voltage lamps in parallel.
The main problem with series connected lamps is, as said,
' that if one lamp stops conducting because of a broken or oxidized filament the other is prevented from conducting by the break in the circuit. We will disclose, however, a simple, inexpensive and reliable circuit which contains lamps connected in series but which keeps the unaffected lamp shining should the other burn out or break.
The FIGURE is a circuit diagram of a preferred embodiment of our invention.
Our lamp circuit contains headlamps l0, 12 connected in series with headlamp switch 14 across battery 16. Connected across each headlamp is a breakdown or zener diode 20, 22. Each breakdown diode has an intrinsic breakdown voltage associated with it such that the breakdown diode presents an extremely large resistance to voltages below this breakdown voltage and an extremely small resistance to voltages greater than the breakdown voltage. The value of the breakdown voltage of each breakdown diode in this circuit is slightly larger than the normal operating voltage of the headlamp across which it is connected. For instance, if the battery potential is 12 volts and each lamp normally drops 6 volts the breakdown voltage of each diode would be approximately 7 volts. Thus, neither of the breakdown diodes 20, 22 would normally conduct more than negligible current.
Suppose, however, that lamp l burns out or breaks so that the circuit opens. At once the voltage across the open circuit attempts to rise to battery potential. However, that voltage is also the voltage across breakdown diode and when it reaches 7 volts diode 20 begins to conduct. Since the breakdown diode 20 can conduct large amounts of current at approximately 7 volts the potential across the other headlamp 12 will not fall below 5 volts. Headlamp 12 will dim slightly but will remain shining. In the same way breakdown diode 22 will conduct to keep headlamp l0 shining should headlamp l2 fail.
The circuit described is the simplest embodiment of our invention, but many variations are possible. Any number of lamps of the proper voltage ratings, each with accompanying diode could be added in series. Lamps of different voltage ratings could be combined, provided the breakdown voltage of each diode was just larger than the operating voltage of the lamp across which it is connected.
Obviously, the same type of circuit connection may be used with lamps other than headlamps; for example, tail lamps, instrument panel lights, or side marker lamps on trucks and trailers.
1. An electriclamp circuit comprising, in combination, a source of current at substantially constant potential, a plural number of electric lamps connected in series across the source, and a plural number of zener breakdown diodes, one of which is connected in parallel with each lamp, each breakdown diode having a breakdown voltage slightly greater than the voltage drop in normal operation of its parallel lamp, so that each breakdown diode is substantially nonconducting but will conduct upon failure of its parallel lamp and maintain the energizing circuit to the remaining lamp or lamps.
2. A vehicle headlamp energizing circuit comprising a plurality of headlamps connected in series with a battery, each lamp having a normal potential drop across it while operating,
each lamp havin a zener breakdown diode connected in parallel with it, e breakdown diode having a breakdown voltage slightly greater than the normal potential drop of its parallel headlamp so as to conduct current around the headlamp only when the headlamp fails.
3. A circuit as defined in claim 2 in which only two headlamps are connected in series with the battery in the said circuit.
4. A circuit as defined in claim 3 in which the battery is a power source of substantially X volts and the breakdown diodes have a breakdown voltage slightly greater than one-half X volts whereby failure of a headlamp causes the diode in parallel therewith to conduct so as to effect a voltage drop of slightly less than one-half X volts across the headlamp which remains energized.