Hazardous circuit ohmmeter is of sufficiently low energy output that it may be used to test extremely sensitive circuits safely, reliably, and accurately. A polyurethane-foam-lined aluminum case provided protection for the unit assembly

Platt, L. W.

English

NASA Technical Reports Server

July 1968
	 Brief 68-10269 
•UF AEC-NASA TECH BRIEF -IV 
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Low Energy Ohmmeter Can Be Used To Test 
Sensitive Circuits, Other Meters 
1/10 9
IIfT \ \	 1/7 / 200
L300Q
 
io/iooc2 3 8 
ADJ
cw 3'l 
100/1 KQ
10
cw 
0	 1 I	 1 
Cw I 
I
0.2251 
__ 
10^
CW 2Q 
Multiplier	 10 
100
El 
Press 
- to 
T Read
TEST CIRCUIT 
The problem: 
To develop an ohmmeter of sufficiently low energy 
output that it may be used to test extremely sensitive 
circuits (such as explosive switches or other meters) 
safely, reliably, and accurately. 
The solution: 
A hazardous circuit ohmmeter measures resistance 
from 0.01 to 500 ohms on three ranges with an energy 
level of 31.25 microwatts delivered to the external
circuit when a 0.2 ohm resistance is measured on the 
1 X range. The energy is lower for all other measure-
ments by a factor of the range multiplier on the other 
ranges. 
How, it's done: 
A 1.34-volt mercury cell provides the power for the 
test circuit, shown in the schematic. The possibility of 
damage to the item being tested from overvoltage or 
(continued overleaf) 
This document was prepared under the sponsorship of the Atomic	 liability resulting from the use of the information contained in this 
Energy Commission and/or the National Aeronautics and Space document, or warrants that the use of any information, apparatus, 
Administration. Neither the United States Government nor any per-	 method, or process disclosed in this document may not infringe pri-
son acting on behalf of the United States Government assumes any vately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19680000269 2020-03-12T06:10:05+00:00Z
overcurrent as a result of component failure is mini-
mized in two ways: (1) limiting resistors are installed 
in the circuit at each end of the cell, and (2) the 
meter, which is connected directly to the ohmmeter 
terminals, is paralleled by at least two resistors. The 
test circuit for each range is a resistance network 
which, including the resistance of the meter, is equal 
to the center scale value. A rheostat is provided in 
each network so that it may be adjusted precisely to 
that value for each of the three ranges. The battery 
also has a rheostat in the circuit which permits the 
current to be set for full-scale deflection of the meter 
for each range. The three ranges, times 1, times 10, 
and times 100, are selected by changing the resistance 
network, including the resistance of the meter, by the 
factor of the multiplier and increasing the control re-
sistance to provide full-scale deflection of the meter. 
The safety of the design is ensured by two or more 
resistors on all series and parallel resistor circuits to 
preclude the danger of short circuit failure. The coil 
of the meter is in parallel to avoid the possibility of 
excessive voltage being applied to the terminals be-
cause of an open resistor. Since the meter is connected 
directly to the terminals of the ohmmeter, a check for 
"set to infinity" performed before each test will indi-
cate any component failure before the unit is con-
nected to the unknown resistance. Therefore, the only 
hazard is failure of a component between the set-to-
infinity check and the actual test. 
Reliability is achieved by using a ruggedized, taut 
band meter and packaging the assembly in a shock-
absorbing carrying case to protect it against vibration, 
shock, dust, and moisture. In addition, all components 
except the meter are operated at less than 25 percent
of their rating. The set-to-infinity check verifies re-
liability because failure of any component will make it 
impossible to adjust to infinity. 
A 2-percent meter with 0.5-percent scale linearity 
and an ohmmeter scale can be tested for accuracy with 
a scale calibration chart. The network rheostat assures 
further accuracy. Additional information, including 
maximum and typical error charts, are available from 
Sandia Corporation. 
Notes: 
1. A polyurethane-foam-lined aluminum case pro-
vides protection for the unit assembly. Its dimen-
sions are approximately 9 by 13 by 7 inches, and 
its weight is about 12 pounds. 
2. The highest currents on the three ranges with a 5-
millivolt meter and a low range resistance of 0.2 
ohm at midscale are 25 milliamps on the lx 
range, 2.5 milliamps on the lOX range, and 0.25 
milliamp on the bOX range, all delivered at zero 
ohms. The highest voltage delivered to the external 
circuit is 5 millivolts at infinity on any range. 
3. Inquiries concerning this innovation may be di-
rected to: 
Sandia Office of Industrial Cooperation 
Org. 3413 
Sandia Corporation 
Post Office Box 5800 
Albuquerque, New Mexico 87115 
Reference: B68-10269 
Patent status: 
No patent action is contemplated by AEC or 
NASA.
Source: Laurence W. Platt 
(SAN-10013) 
IS—CAS-42D 
RU. 1313	 KSC uqs. 
Brief 68-10269	 Category 01


Low energy ohmmeter can be used to test sensitive circuits, other meters

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server