Negative-temperature coefficient device /thermistor/ in series with a lamp filament alleviates high filament surge current during initial application of power. The thermistor should be selected for a cold resistance approximately equal to one fourth of the normal hot resistance of the filaments to be protected

Macomber, G. A.

English

NASA Technical Reports Server

August 1970	 Brief 70-10483 
NASA TECH BRIEF 
NASA Tech Briefs announce new technology derived from the U .S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
A Conceptual Current Surge Protector for Incandescent Lamps 
The problem: 
It is estimated that 95 percent of all incandescent 
lamp failures are attributed to the high filament surge 
current during the initial application of power. The 
surge current is due to the "low cold resistance" of 
the filament relative to the resistance at the normal 
operating temperature. 
How it's done: 
The thermistor could be made an integral part of 
the incandescent lamp socket or ,a disc type of device 
could be supplied which would fit within existing 
sockets and would form electrical contact between 
the lamp and socket (see Figurel). 
The thermistor should be selected for a cold resis-
tance (77 0 F)
 
approximately equal to one fourth of the 
normal hot resistance of the filaments to be protected. 
For example, a 100 watt lamp used in a 117 V ac circuit 
will have a hot resistance of approximately 130 ohms; 
the thermistor should have a cold resistance of ap-
proximately 32 ohms. Upon initial application of 
power, the cold resistance of the thermistor plus the 
cold resistance of the filament will be sufficient to 
prevent a destructive current surge. The thermistor 
resistance will immediately start to decrease due to 
self heating; the filament resistance will increase for 
the same reason until a state of equilibrium is reached. 
Assuming a lamp socket stabilization temperature of 
400°F, the thermistor resistance at that temperature 
will be approximately 0.3 ohms and the power dis-
sipated will be a minimal 0.2 watts.
Ceramic Insulated Body 
Thrm,otnr	 I	 Threaded Portion 
Figure 1(A). Thermistor as Integral Part of Lamp Socket 
Disc Type 
Insulating 
Material 
Figure 1(B). Thermistor Disc to be Installed in Socket 
This device should be extremely useful for household 
lighting systems. Even though the initial cost of in-
candescent lamps is relatively low, the high failure 
rate becomes costly and the "nuisance factor" of 
replacement is worth eliminating. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B70-10483 
Patent status: 
No patent action is contemplated by NASA. 
Source: George A. Macomber of 
North American Aviation Corporation 
under contract to 
Marshall Space Flight Center 
(MFS-16658) 
Category 01 
The solution: 
Insert a negative-temperature coefficient device, 
such as a thermistor, in series with the lamp filament. 
•	 The initially high resistance of the device will prevent 
the surge current. 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States	 will be free from privately ownedrights.
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A conceptual current surge protector for incandescent lamps

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