Stable, low duty cycle transistorized emitter follower load cell controls and absorbs large currents at low voltages. The use of energy storage in capacitors reduces auxiliary power source requirements. Low duty cycle pulse mode of operation reduces the average power handling requirement of all components

Breitwieser, R.

Manista, E. J.

English

NASA Technical Reports Server

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September 1970	 Brief 70-10470 
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. 
Load Cell for Thermionic Converter Tests 
A variable energy heat sink that can control and 
absorb large currents at low voltages is required in 
thermionic converter tests. Several types of variable 
loads have been developed for this purpose including 
variable resistances, transformers, and electronic load 
cells. The advantage of an electronically controlled 
load cell is the ease with which the steady state oper-
ating point can be set (the starting point for a voltage 
and current excursion) and in the simplicity with 
which the range of loading can be controlled. How-
ever, some electronic loads exhibit undesirable in-
stabilities when connected to the highly nonlinear 
characteristics of thermionic converters. Also, the 
electronic loads often require large auxiliary power
sources so that the converter can be driven into the 
positive bias condition. 
A stable, low duty cycle transistorized emitter fol-
lower load cell that has evolved from those used in 
converter testing is shown in the figure. Auxiliary 
power source requirements have been reduced through 
the use of energy storage in capacitors. The low duty 
cycle pulse mode of operation reduces the average 
power handling requirement of all components, al-
leviating to a large extent the drifting and instability 
problems caused by thermal effects in components. 
A negatively-going pulse is required to drive the 
load cell. The early stages of amplification reduce the 
power requirement of the driver (the input shown 
(continued overleaf) 
in 
IMF
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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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lightly loads a wave generator whose specifications 
are 15 volts into 600 ohms). This stage is coupled to 
the main output stages through the 500 p fd capacitor 
to guard against bias changes and hence base width 
clipping under heavy current loads. 
The degenerative coupling of the paralleled tran-
sistors provides load balancing, stability, and wave 
shaping. At high currents, the rate of change of cur-
rent and the magnitude of the voltage excursion are 
both reduced, a desirable feature in these kinds of 
tests. The output voltage swing referenced to the emit-
ter is a positively-going pulse rising off the dc level 
of the bias voltage. The amplitude of the pulse is eas-
ily controlled by the amplitude of the input drive. 
The duty cycle in this application is dictated by the 
thermal inertia characteristics of the thermionic con-
verter and is about 1 to 5 percent. The main bank of 
power transistors can easily pass a triangular current 
pulse of over 300 amperes peak with a base width 
of 2 to 3 milliseconds. The nonlinear characteristics
of thermionic converter loads permit larger base 
widths at this current level. The energy in the current 
pulse can be increased by increasing the size of the 
energy storage capacitors. 
Notes: 
I. Although specifically designed for thermionic con-
verter testing, the load cell adapts to other similar 
high-current, low-voltage uses. 
2. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Lewis Research Center 
21000 Brookpark Road 
Cleveland, Ohio 44135 
Reference: B70-10470 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. Breitwieser and E. J. Manista 
Lewis Research Center 
(LE W-1 1068) 
Brief 70-10470	 Category 01


Load cell for thermionic converter tests

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