Limiting circuit protects the main power supply of electronic modules and limits the current drawn by each module should a short circuit occur. It limits current within one mA when used with direct current of either polarity, or with pulse or ac power sources from direct current to 100 kHz

Hilbert, E. E.

English

NASA Technical Reports Server

September 1970	 Brief 70-10414 
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. 
Transistor Current and Voltage Limiting Switch 
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Limiting-Switch Circuit 
The problem: 
A main power supply, used to power a series of 
electronic modules undergoing extended life tests, 
requires an overload circuit to protect the main supply 
and to limit and control the current drawn by each 
module in the event of a shortcircuit. The entire se-
quence must be automated due to the long and un-
attended nature of the test operation. 
The solution: 
A limiting circuit in a balanced configuration that 
limits current within I mA when used with direct
current of either polarity or with pulse or ac power 
sources from direct current to about 100 kHz. 
How it's done: 
For an isolated power supply whose output is a 
50-V-peak 2.4-kHz square wave, the circuit (see fig.) 
consists of two identical stages that alternately per-
form the limiting opeation or bypass the current 
through diode Dl, depending on the polarity of the 
supply voltage. Resistors R 1 and RC, are chosen so 
that, for normal load current, QI is saturated; D-,, Rb, 
and R 1 form a voltage- divider network that feeds 
(continued overleal) 
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back part of the voltage across Qi and R to the base 
of Q,. When Qi is saturated, and with normal load 
current through Re, Q, is biased off. The current limit 
is controllable within I mA by variation of Re. 
As load current through R increases above the 
selected safe level, the voltage at the base of Q, rises, 
causing Q, to turn on; thus Q is turned off, and re-
generation then keeps Q-, on and Q I off. By switching 
of the high resistance of Rc, in the path of the load 
current, both load and power supply are protected. 
The circuit automatically resets to the on state when 
the supply voltage is dropped to zero and raised again; 
capacitor C2 ensures that Q1 turns on first when volt-
age is reapplied. By use of two stages and bypass 
diodes Dl, this circuit limits current for direct current 
of either polarity or for a non-dc type of supply. 
In this particular case, the load of the transformer's 
primary is considerably inductive; therefore, zeners 
Z I and Z2 are used to limit inductive spikes that would 
occur if the circuit limited. The collector-to-emitter 
breakdown voltage of Q and power rating of R, 
are chosen so that even with a worst-case power-
supply failure, resulting in a supply output of 120 V 
rrns (170-V peak), the transformer is not damaged.
The circuit also protects the power supply and elimi-
nates the chance of fire resulting from direct shorting 
of the outputs; it is capable of limiting continuously 
more than 120 V rms supplied to a direct short at the 
output. The switch's limiting speed for this circuit is 
about 5 p sec; therefore it can be used for supply-voltage 
frequencies from direct current to about 100 kHz. The 
frequency range could be increased, if desired, by use 
of high-frequency transistors if the high collector-to-
emitter breakdown voltage were not required. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP70-10414 
Patent status: 
No patent action is contemplated by NASA. 
Source: E. E. Hilbert of 
Caltech/JPL 
under contract to 
NASA Pasadena Office 
(NPO-1 1166)
. 
El 
.. 
Brief 70-10414	 Category 01


Transistor current and voltage limiting switch

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