A PTC /Positive-Temperature-Coefficient/ thermistor placed in series with each branch load of a multiload circuit prevents power loss in parallel branches. This thermistor may be used in any circuit requiring current limiting or intended overload resetting

Leverone, H.

Mandell, N.

English

NASA Technical Reports Server

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November 1964	 Brief 64-10281 
NASA TECH BRIEF	 4W 
This NASA Tech Brief is issued by the Technology Utilization Division to acquaint 
industry with the technical content of an innovation derived from the space program. 
PTC Thermistor Protects Multiloaded Power Supplies 
The problem: Prevention of power loss in parallel 
branches of a multiload circuit, should an overload 
occur in one of the branches. The protective device 
must be self-resetting, capable of handling short-dura-
tion transients, and must withstand shock and vibra-
tion. 
The solution: A properly selected PlC (positive-
temperature-coefficient) thermistor placed in series 
with each branch load. The thermistor should be se-
lected so that (1) its nominal resistance is negligible 
compared to the branch load, (2) its response time is 
long enough to endure switching transients, and (3) its 
critical (Curie) temperature is well above the ambient 
temperature range in which the equipment operates. 
How it's done: At temperatures below the critical 
temperature (typically about 120° C), the nominal re-
sistance of a PTC thermistor is relatively low. In this
temperature range, the thermistor's rise of resistance 
with temperature is very small. Above the critical tem-
perature, however, the temperature coefficient of re-
sistance of a PTC transistor may be as much as + 10% 
per °C and under conditions of overload its resistance 
can increase to as much as fifty times the nominal 
value. 
An overload condition in any branch will cause ex-
cessive current to flow through the thermistor in that 
branch, and the corresponding I R increase will raise 
the temperature of the thermistor. The temperature 
rise is a function of the nominal resistance of the 
thermistor, its dissipation constant, and the overload 
current. The time required to reach the critical tem-
perature is a function of the thermistor material. The 
increase in resistance accompanying the temperature 
rise decreases the current flowing through the branch 
load to an equilibrium value which will be maintained 
Anil,
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States Govern-
ment, nor NASA, nor any person acting on behalf of NASA: A. Makes 
any warranty or representation, express or implied, with respect to the 
accuracy, completeness, or usefulness of the information contained in
this document, or that the use of any information, apparatus, method, 
or process disclosed in this document may not infringe privately-owned 
rights; or S. Assumes any liabilities with respect to the use of, or for 
damages resulting from the use of, any information, apparatus, method, 
or process disclosed in this document.
https://ntrs.nasa.gov/search.jsp?R=19640000077 2020-03-11T17:05:27+00:00Z
as long as the overload persists. When the overload is 
corrected, the reduction in current will cause the PTC 
thermistor to revert to its nominal resistance. 
Notes: 
1. The PTC thermistor can be used in any circuit re-
quiring current limiting or intended overload re-
setting. PTC thermistors with appropriate charac-
teristics may also be used as fuses to open branch 
circuits when overloads exceed prescribed values. 
2. To minimize the effects of ambient temperature on 
the thermistor, vacuum encapsulation is suggested. 
3. Individual applications will determine the neces-
sary thermistor parameters. These devices are 
available with the following characteristics: 
(1) nominal resistance values from approximately
I ohm to 10,000 ohms; (2) resistance change at the 
critical temperature from about 2.5 to 5 times nom-
inal resistance; (3) response time from less than a 
second to 100 seconds; and (4) power dissipation 
from a few milliwatts to 2 watts. 
4. Inquiries concerning this innovation may be di-
rected to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland, 20771 
Reference: B64-10281 
Patent status: NASA encourages commercial use

of this innovation. No patent action is contemplated. 
Source: Nathan Mandell and

Harry Leverone (GSFC-236) 
/ 
Brief 64-10281	 Category No. 01


PTC thermistor protects multiloaded power supplies

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