Patent ID: 6323597
Filing Date: 2001-11-27
Classification: H05B

Abstract:
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.