Circuit converts an rms, ac voltage to a proportional dc voltage with good accuracy over a voltage range of approximately 6-1. It incorporates a pair of vacuum thermocouples in a dc feedback circuit

Cremer, H. C.

English

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June 1970	 Brief 70-10076 
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. 
Improved Low Cost AC-to-DC Converter
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Figure 1 
A low-cost circuit has been designed to convert an 
rms, ac voltage to a proportional dc voltage with 
good accuracy (± I to ± 1.5 percent) over a voltage 
range of approximately 6:1. The converter, which 
incorporates a pair of vacuum thermocouples 
(V-T/C's) in a dc feedback circuit, exhibits a tempera-
ture coefficient of approximately +0.01 to —0.04 per-
cent over the temperature range from 20° to 165°F. 
This converter can be built at about one-quarter the 
cost of a similar commercially available circuit which
operates at a ±1 percent accuracy over a 10:1 range 
and should be of interest to manufacturers and users 
of instruments, such as X-Y plotters and data re-
corders, that operate on dc voltages. 
Figure 1 shows a circuit schematic with illustrative 
component values. The input T/C 1 is a 1 mA type 
used for current-to-voltage conversion in the input 
loop; T/C 2 is a 5 mA type used in the feedback loop. 
Each of these T/C's is contained in a glass bead and 
is electrically insulated from the heater to 100 volts 
(continued ovcrleaf) 
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peak. The T/C's are closely matched to avoid non-
linearity between input and output. A damping 
and filtering network in the negative feedback loop 
consists of a 150 kilohm resistor (9 6 ) in series with 
a 2.2 tF, 25V capacitor (C 1 ). Transistor Q 1 provides 
additional loop gain and also provides a voltage 
shift to allow the use of a grounded power supply. 
Resistor R4, protects T/C 2 from burnout. A 10 
kilohm potentiometer (P 3.) serves as a dc offset 
adjustment. A 0 to 5 mA dc meter may be placed in 
series with T/C 2 as a permanent monitor. If a meter 
is not used, the terminals must be shorted. The out-
put terminals of the circuit are connected in series 
with resistor R 5 .and calibration potentiometer P 2 . The 
latter is adjusted to provide a I V dc output when a 
E(
(a)
 I(rm
(b) 
Figure 2.
normal maximum of 3 V rms is applied at the input, 
and P 1 (Figure 2a) is adjusted for a current of 1.2 mA 
through T/C 1. Zener diodes Dl and D2 (Figure 2a) 
protect the T/C 1 heater against burnout. Although 
T/C 1 has a nominal current rating of 1 mA, the 
slight overcurrent (1.2 mA) will ensure a higher T/C 
output voltage and greater overall accuracy. 
Figure 2b shows an rms current-input circuit that 
can be used in place of the rms voltage input to the 
converter circuit. A current transformer with a ratio 
X A / T/CI (where XA = the maximum current to be 
measured) is substituted for the input circuit of Fig-
ure 2a. 
To remove noise from the output signal (Eo t), a 
filter network may be added(R 7 C 3 in Figure 1). Such 
a network should maintain or reflect a low impedance 
(< 2 kilohms) to the output terminals. The dc voltage 
output of this converter circuit is proportional to the 
rms current input. Because the thermocouples are 
insulated from the heaters to 100 V peak, there are 
no common-mode problems, and the output can be 
connected directly to the desired equipment (e.g., 
data system or recorder). 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena. California 
Reference: TSP 70-10076 
Patent status: 
No patent action is contemplated by NASA. 
Source: H. C. Cremerof 
Caltech/JPL 
under contract to 
NASA Pasadena Office 
(N P0-11055)
. 
S. 
. 
Brief 70-10076	 Category 01


Improved low cost ac-to-dc converter

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