Electropneumatic rheostat maintains a constant direct current in each of several high-power parallel loads, of variable resistance, across a single source. It provides current regulation at any preset value by dissipating the proper amount of energy thermally, and uses a column of mercury to vary the effective length of a resistance element

Haacker, J. F.

Jedlicka, J. R.

Wagoner, C. B.

English

NASA Technical Reports Server

ASO- 3//
 
October 1965	 Brief 65-10299 
NASA TECH 
NASA Tech Briefs are issued by the Technology Utilization Division to summarize specific 
technical innovations derived from the space program. Copies are available to the public from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia, 22151. 
Electropneumatic Rheostat Regulates High Current 
U-Tube 
Resistance 
Elemen 
Air Line 
Transdu 
Reservoir
Cooling Water
Electrical 
Connector I	 Spirally Wound Plastic Spacer 
Mercury
	
Load Current 1 	 i'". Load Current 
	
I	 I	 Level selector
Av Supy 
Output	 7t 
EIeornc.ToPnevn,atic	
—	
1 
Transducer	 22.5V 
FIG. 2 
FIG. 1 
The problem: To maintain a constant direct cur-
rent in each of several high-power (megawatt range) 
parallel loads, of characteristically variable resistance, 
across a single source. It was desirable to use a single 
available dc generator to avoid purchasing separate 
power supplies, rectifiers, or rotary machines. 
The solution: A simple, automatically-controlled 
rheostat system that will provide current regulation 
at any preset value by dissipating the proper amount 
of energy thermally. The electropneumatic rheostat 
uses a column of mercury to vary the effective length 
of a resistance element.
How it's done: The resistance element of the rheo-
stat is a U-shaped stainless steel tube, internally 
cooled, immersed in a mercury reservoir, each leg be-
ing centered within a standpipe of low electrical re-
sistance by a spiral insulating spacer. Air pressure 
from an electric/pneumatic transducer forces the 
mercury in the reservoir to rise into the area between 
the standpipes and the resistance element. Because 
the resistance element is short circuited to the stand-
pipes at the surface level of the mercury, the current 
bypasses that portion of the element below the surface 
level and flows through the low-resistance standpipes. 
The resistance of the rheostat is, therefore, inversely 
(continued overleaf) 	 - 
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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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proportional to the applied air pressure. The control 
circuit senses changes in the load current and activates 
a commercially available electric/pneumatic trans-
ducer to convert a 0 to 5 milliamp dc signal to a 15 to 0 
psi (inverse acting) pneumatic output that allows lin-
ear control of the rheostat. The load current level 
selector, a small potentiometer in the control circuit, 
serves as voltage divider in parallel with the load cur-
rent shunt and permits manual changes in the control 
value of the load current. A plastic tube between the 
rheostat and transducer pneumatic output provides 
electrical isolation of the rheostat from the control 
circuit. 
Notes: 
I. This control system has been used to maintain con-
stant current in each of five parallel dc circuits con-
taining five electric arc loads, all supplied from a 
single dc source. A different current level could be 
set in each of the circuits. Because the loads are 
dc-arc discharges, which characteristically exhibit 
fluctuations in the load current, the control circuit 
is provided with ac filters that render the system 
insensitive to load fluctuations down to 10 cps with 
amplitude equal to the dc amplitude.
2. For this particular application, the rheostat units 
have a maximum resistance of about 0.8 ohm and 
are rated at 200 amperes for 1,000 psi cooling 
water. 
3. Inquiries concerning this innovation may be di-
rected to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California, 94035 
Reference: B65-10299 
Patent status: NASA encourages commercial use 
of this innovation. No patent action is contemplated 
by NASA.
Source: Cleo B. Wagoner, Jack F. Háacker, 
and James R. Jedlicka
(A RC-44) 
Brief 65-10299	 Category 01


Electropneumatic rheostat regulates high current

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