Development of electric discharge circuit damping element which increases in resistance with current and time is described. Damping element is resistor made of tungsten wire which has large resistance-temperature coefficient. Specifications of tungsten resistor and incorporation into circuit are explained

Cheng, D. Y.

English

NASA Technical Reports Server

September 1972	 B72-10304 
.
NASA TECH BRIEF 
Ames Research Center 	 W 
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 National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Control of Oscillations in a Discharge Circuit 
The problem: 
A crowbar system is frequently included in a 
pulsed, undamped capacitor-discharge circuit to ar-
rest oscillations. The systems customarily used for 
this purpose require active elements such as thyra-
trons and spark-gap triggers, which are costly and 
complex; a simple, economical alternative was desired. 
The solution: 
Include in the discharge circuit a series damping 
element which increases in resistance with current 
and time. 
How it's done: 
The damping element is a resistor made of tung-
sten wire; tungsten has a high melting point, low 
specific heat, and a large resistance—temperature co-
efficient. The resistance of the tungsten wire is low 
during the first quarter cycle of oscillation while the 
current is increasing, but since the temperature of the 
wire increases as heavy currents pass through it, its 
resistance will be very high during the time (second 
quarter cycle) when the discharge current of the cir-
cuit is decreasing. Thus, the circuit progresses from 
an underdamped state during the early portion of 
the discharge to a critically damped or overdamped 
state in the late portions of the discharge while the 
current is falling. The required resistance during the 
decay of the discharge can be obtained only if the 
resistive element has a positive temperature coeffi-
cient of resistivity and if its temperature increases 
rapidly because of joule heating. Since the duration 
of a pulsed capacitive discharge is short, heat gen-
erated in the resistor is not dissipated, and its tem-
perature rises exponentially as a function of time. 
The required length and diameter of the tungsten 
wire are computed by equations obtained from an 
analysis of the behavior of an oscillatory circuit in 
which the resistance is a function of current and 
time. In actual performance tests, the results of the 
mathematical analysis were verified with a T-tube 
plasma generator and accelerator powered by a bank 
of charged capacitors. Although the impedance of the 
plasma load varies during operation, it is low with 
respect to the computed critical damping resistance; 
thus, it is possible to obtain meaningful comparison 
between experimental results and theory. 
Consideration has been given to the possibility of 
using a precooled superconducting wire as the damp-
ing resistance to obtain minimum risetime and maxi-
mum peak current in a discharge. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 72-10304 
Patent status: 
No patent action is contemplated by NASA. 
Source: Dah Yu Cheng

Ames Research Center

(ARC-10556) 
Category 01 
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Control of oscillations in a discharge circuit

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