Sensing circuit in which a tunnel diode controls a series regulator transistor protects a low voltage transistorized dc regulator from damage by excessive load currents. When a fault occurs, the faulty circuit is limited to a preset percentage of the current when limiting first occurs

Airth, H. B.

English

NASA Technical Reports Server

Load 
July 1966
	 Brief 66-10292 
NASA TECH BRIEF
	
_1a 
NASA Tech Briefs are issued to summarize specific innovations derived from the U. S. space 
program and to encourage their commercial application. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Circuit Protects Regulated Power Supply Against Overload Current 
Load	 Q2 2 
The problem: 
To protect a low voltage transistorized dc regulator 
from damage by excessive load currents. In some ap-
plications, a single load fault can disable an entire 
system by disabling the regulators. Current threshold 
detectors have employed zener diodes and the voltage 
characteristics of transistor base—emitter junctions 
but these have not achieved sharp detection and cur-
rent limiting. 
The solution: 
A sensing circuit in which a tunnel diode controls 
a series regulator transistor. When a fault occurs, the
faulty circuit is limited to a preset percentage of the 
current when limiting first occurs. 
How it's done: 
R4, Q, and CR3 form the regulator series stage 
driver and Q2 is the regulator series element. The over-
load circuit is composed of Ri, R2, R3, CR1, CR2, and 
Qi, and functions by shunting the base current of Q2 
through Qi in case of overload, thereby shutting off 
Q2 and limiting the fault current. The volt-ampere 
characteristics of CR1 are used to provide the voltage 
threshold detection. The voltage across RI is used to 
detect the magnitude of the load current.
(continued overleal) 
Adak 
Now
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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 owned rights.
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When the load current is just below the limiting 
level, current through R3 plus current through R2 is 
just below the threshold point of CR1. The base-to-
emitter voltage of Q  is the sum of voltages across CR1 
and CR2 and the current through R3 is such that the 
voltage across CR2 is about 400 m y . The voltage across 
CR1 is at 50 m y
 and the base-emitter voltage of Q  is 
450 m y
 which is not sufficient to turn on Qi. This is 
the normal mode of overload curcu it/ regulator 
function. 
When the load current causes the peak-point cur-
rent of CRi to be exceeded, it causes the base-emitter 
voltage of Q I to turn that transistor on. As a result, 
current through R3 now flows into the base of Q  and 
the collector current of Q3 flows into Qi rather than 
the base of Q2 so that Q2 turns off and limits the cur-
rent to the regulator, the collector-emitter (saturated) 
voltage of Qi being less than the threshold base-emit-
ter voltage of Q2. The regulator series element being 
turned off, current through R1 decreases, allowing 
current through CRi to decrease. When the current 
through CR1 drops below its valley-point current, the 
overload circuit returns to its original state. If the 
overload is still present, the cycle is repeated, alter-
nately cutting off Q2 and continuing to limit overload 
current to the regulator until the fault in the load is 
corrected. Value of the capacitor controls frequency 
of the series stage cycle.
Notes: 
I. Typical changes of the threshold detection current 
are =00% over a range from 00
 to +70°C. Any 
change with temperature in the base-emitter volt-
age threshold of Qi is compensated for by a like 
change in the threshold voltage of CR2. 
2. This circuit provides sharp detection of overload 
currents at very low voltage levels and has limited 
short circuit currents to less than 10% more than 
the detector (CR1) threshold current. 
3. The circuit shown uses a germanium tunnel diode 
but will perform satisfactorily with one of silicon. 
4. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B66-10292 
Patent status: 
No patent action is contemplated by NASA. 
Source: H. B. Airth 
of Westinghouse Electric Corporation 
under contract to 
Goddard Space Flight Center 
(GSFC-453) 
Brief 66-10292	 Category 01


Circuit protects regulated power supply against overload current

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