Two terminal device protects dc electronic circuits and experimental solid state devices, and replaces fuses and circuit breakers directly. The device consists of two transistors and two resistors and draws its necessary supply voltage from power source being protected. The limiter acts as a voltage regulator

Doubt, L. S.

English

NASA Technical Reports Server

October 1970	 Brief 70-10232 
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. 
Two Terminal Current Limiter 
This simple and inexpensive device offers current 
limiting protection for dc electronic circuits, and can 
be used to replace fuses and circuit breakers directly 
with great speed, accuracy and reliability. 
In many phases of electronic development, the 
power supply source can be overloaded or delicate 
components can be damaged when excessive currents 
flow due to accidental shorting, component failure, 
or circuit changes. Fuses and circuit breakers are 
commonly used for protection, but these devices are 
neither fast enough to cope with instantaneous over-
currents nor accurate or reliable enough to provide 
suitable protection for some power sources and some 
solid state experimental units. 
A two terminal current limiter, which could be 
fabricated in the size of a type 3AG fuse, can be used 
instead of a fuse to provide sensitive protection for dc 
power supplies or experimental solid state devices 
under test and development. The device consists of two 
transistors and two resistors. The necessary supply 
voltage for operation is obtained from the power 
source being protected, with the load functioning 
as the return to the power source from the device. One 
of the transistors acts as a series element which allows 
current, up to a desired maximum, to flow to the load. 
One resistor provides a suitable bias for the series 
transistor to permit such current flow. A second sens-
ing resistor interposed between the series transistor 
and the load, provides bias for the second transistor. 
Normally this bias is low enough to prevent conduc-
:ion by the second transistor. The collector of the 
;econd transistor is connected to the base of the series 
ransistor to reduce the bias applied to the series tran-
istor. When current in excess of the desired maximum 
ows through the sensing resistor as a result of a cir-
cuit malfunction or a short across the load, the voltage 
drop across the sensing resistor rises to a value which 
is sufficient to bias the second transistor into conduc-
tion. 
When the second transistor turns on, it reduces the 
bias of the series transistor to limit the amount of 
current that can flow through the series transistor 
so that it cannot exceed the desired maximum. The 
magnitude of the desired maximum current flow can 
be varied by changing the value of the sensing resis-
tor. 
The figure shows a comparison of the two terminal 
device and an ordinary limiting resistor used in series 
with the load as a protective device. As current in the 
12 
11	 -
Source Two Terminal 
10 Current Limiter 
9- 
8 
Volts
Resistor 
4 560 
0
0 10	 20	 3032 
MA 
Comparison of Two Terminal Device 
and an Ordinary Limiting Resistor
load rises, the voltage available for the load using 
a limiting resistor drops in accordance with the load 
current. With the two terminal device, the voltage 
remains reasonably constant until the current limiting 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately ownedrights. 
https://ntrs.nasa.gov/search.jsp?R=19700000223 2020-03-17T00:11:36+00:00Z
point is reached, at which the voltage drops sharply. 
Thus the two terminal device exhibits some of the 
characteristics of a voltage regulator. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: B70-10232
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: L.S. Doubt of

Caltech/JPL
under contract to

NASA Pasadena Office

(N PO- l 1350) 
Brief 70-10232	 Category 01


Two terminal current limiter

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