Thermistor shunt for series wired light string

• US 6,323,597 B1
• Filed: 07/10/2000
• Issued: 11/27/2001
• Est. Priority Date: 05/15/2000
• Status: Expired due to Term

• Current Assignee: JLJ, Inc.
• Original Assignee: JLJ, Inc.
• Inventors: Janning, John Louis
• Primary Examiner(s): Wong, Don
• Assistant Examiner(s): Dinh, Trinh Vo
• Application Number:
  US09/612,885
• Time in Patent Office: 505 Days
• Field of Search: H05/B 3.700, 315/185.S, 315/185.R, 315/75, 315/122, 315/323, 315/123, 315/125, 307/10.8, 307/100, 307/157
• US Class Current: 315/185.00S
• CPC Class Codes: H05B 39/105 with a spare lamp in the circuit, and a possibility of shunting a failed lamp lamp changing devices H01R33/00, H01R43/00; incandescent lamps with a reserve body H01K

Abstract

Use of NTC (Negative Temperature Coefficient) thermistor devices having base electrical resistances in the range of about 400 to 2200 ohms is disclosed. A string set of series-connected, low voltage incandescent lamps can be configured in which each of the lamp filaments in the set is provided with a shunt circuit that includes one of the temperature sensitive NTC thermistor devices which operates as a filament shunt. The thermistor devices may have relatively low conductivity when connected to a source of operating potential during normal operation of the string set. The devices, however, may become fully conductive in response to an increase in the voltage that exceeds the device'"'"'s normal voltage drop. Remaining lamps in the string can continue to receive nearly rated current and nearly rated voltage and further continue to be illuminated at nearly the same illumination even though other lamps in the string are inoperative due to burning out, being loose in their sockets or being missing from their respective sockets all together. It is desirable for the NTC thermistor devices to use at most, a base resistance that would allow initial operation at or near the point of maximum power dissipation by the device upon failure of its shunted lamp. It is also desirable that the base resistances be high enough to limit “leakage” current so there is no more than a 5% increase in string current compared to the same string without shunts.

Claims

1. A string set of series-connected incandescent lamps for use with particular rated values of electric operating potential and current, to illuminate the lamps in the string set by providing substantially the rated value of current flow through each of the lamps, wherein substantially all the lamps in the set are each provided with a respective heat sensitive shunt circuit comprising a negative temperature coefficient thermistor device having an electrical impedance dependent upon temperature in the thermistor device, where the temperature in the thermistor device is due at least in part to the ambient operating temperature and current flowing through the thermistor device, the thermistor device being connected in parallel with the associated lamp being shunted by the thermistor device, where the thermistor device has a base electrical resistance in the range of about 400 to 2200 ohms and presents an electrical resistance during normal operation of the string set twenty or more times the normal operating electrical resistance of the lamp being shunted by the thermistor device, so that leakage current through the thermistor device is substantially limited when the lamp being shunted by the thermistor device is operating normally in the string set, where the thermistor device changes to a relatively lower resistance state upon an increase in voltage across the thermistor device resulting from inoperativeness of the respective lamp being shunted, such as by reason of the lamp being shunted burning out, being put in its socket crooked or being loose in or missing from its socket, and where the thermistor device in the relatively lower resistance state has a voltage drop across the thermistor device equal to or greater than the value of the voltage drop across the associated lamp being shunted when the lamp is normally illuminated, whereby the voltage drop across each remaining illuminated lamp in the series-connected string set of lamps remains substantially unchanged with the continued flow of substantially the rated value of current flow through the string set and with substantially unchanged illumination by remaining operative string set lamps, despite the occurrence of a failure or loss of a lamp or removal of a lamp from its socket in the string set.

2. A string set of series-connected incandescent lamps for use with a particular value of operating potential to illuminate the string of lamps by providing substantially rated current flow through each of said lamps, wherein substantially all the lamps in the set are each provided with a respective heat sensitive shunt circuit consisting of a negative temperature coefficient thermistor device wherein the base electrical resistance is in the range of about 400 to 2200 ohms, and the electrical resistance is generally dependent upon temperature changes in the device due to current flowing through the device, which device presents an electrical resistance level during normal operation when shunted across a normally operating lamp and changes to a relatively lower electrical resistance state upon an increase in voltage across the device resulting from its respective lamp being inoperative or being loose or missing from its socket, said device in said relatively lower impedance state having a voltage drop equal to or greater than the value of the voltage drop across its respective lamp when illuminated, whereby the voltage drop across each illuminated lamp remains substantially unchanged with the continued flow of substantially said rated current flow through the light string and with substantially unchanged illumination by remaining operative string set lamps, despite the occurrence of a failed lamp or removal of a lamp from its socket in the string set.

3. An electric lamp assembly comprising an electric lamp shunted by a temperature sensitive NTC thermistor device connected in parallel with the lamp, in which the electric lamp is a 2.4-3.5 volt mini-light and the NTC thermistor device has a base resistance in the range of about 400 to 2200 ohms.

4. The electric lamp assembly of claim 3 in which the NTC thermistor device has at most a base resistance that would allow its initial operation at or near the point of maximum power dissipation by the NTC device upon failure of the electric lamp.

5. The electric lamp assembly of claim 4 in which the electric lamp is a 2.4 volt mini-light present in a fifty light series string, and in which the NTC thermistor device has a base resistance in the range of about 400 to 600 ohms.

6. The electric lamp assembly of claim 4 in which the electric lamp is a 3.5 volt mini-light present in a thirty-five light series string, and in which the NTC thermistor device has a base resistance in the range of about 600 to 900 ohms.

7. The electric lamp assembly of claim 4 in which the base resistance the NTC thermistor device is sufficiently high enough to limit “

leakage”

current through the device, so that there is no more than a 5% increase in total current through the device and the lamp, compared to the same lamp operating without the NTC device present.

8. The electric lamp assembly of claim 4 in which the base resistance of the NTC device is twenty or more times the resistance of the shunted lamp when the lamp is operating normally.

9. The electric lamp assembly of claim 8 in which the base resistance of the NTC device is less than one hundred eighty times the resistance of the shunted lamp when the lamp is operating normally.

10. A lighting apparatus comprising:

a string of lamps connected in series; and

a heat sensitive shunt circuit connected in parallel with one of the lamps, said heat sensitive shunt circuit comprising a negative temperature coefficient thermistor device having an electrical impedance dependent upon temperature in the thermistor device.

11. The lighting apparatus of claim 10 wherein, when the one of the lamps is functioning normally, the thermistor device presents an electrical resistance at least twenty times a normal operating electrical resistance of the lamp and leakage current through the thermistor device is substantially limited.

12. The lighting apparatus of claim 11 wherein, when the one of the lamps is not functioning normally, the thermistor device changes to a relatively lower resistance state in response to an increase in temperature of the thermistor device.

13. The lighting apparatus of claim 12 wherein, when the thermistor device is operating in the relatively lower resistance state, a voltage drop across thermistor device is at least equal to a voltage drop across the lamp when the lamp is functioning normally.

14. The lighting apparatus of claim 13 wherein, when the thermistor device is operating in the relatively lower resistance state, the remaining lamps continue to substantially normally operate.

15. The lighting apparatus of claim 14 wherein the thermistor device has abase electrical resistance between about 400 ohms and 2200 ohms.

16. The lighting apparatus of claim 15 wherein the base electrical resistance of the thermistor allows the thermistor to operate initally at a substantially maximum power dissipation when the lamp is not functioning normally.

17. The lighting apparatus of claim 10 further comprising an additional number of heat sensitive shunt circuits, each of the additional number of shunt circuits connected in parallel with one of the lamps.

18. A lighting apparatus comprising:

a light socket comprising a lamp, a first terminal and a second terminal, the first terminal and the second terminal for supplying electrical current to the lamp; and

a heat sensitive shunt circuit connected between the first terminal and the second terminal, said heat sensitive shunt circuit comprising a negative temperature coefficient thermistor device having an electrical impedance dependent upon temperature in the thermistor device.

19. The lighting apparatus of claim 18 wherein, when the lamp is functioning normally, the thermistor device presents an electrical resistance at least twenty times a normal operating electrical resistance of the lamp and leakage current through the thermistor device is substantially limited.

20. The lighting apparatus of claim 19 wherein, when the lamp is not functioning normally, the thermistor device changes to a relatively lower resistance state in response to an increase in temperature of the thermistor device.