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A
4
C
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Models 940 and 945 DC Transmitters
http://www.calex.com
2401 Stanwell Drive
Concord, CA 94520-4841
(510) 687-4411 Fax (510) 687-3333
MK940 Application
Isolated 0 to 20mA Output
NOTE:
R6 IS A 1% METAL FILM 1/4 W
RESISTOR.
ZERO CURRENT MAY BE 100μA
refer to locations labeled on the MK940. All fixed value
resistors should be metal film 1% type. Where a potentiometer
is shown, use a multiturn trim pot with a CERMET resistance
element. The hole pattern is for industry standard
potentiometers such as Bourns “3006Y”, Spectrol “43”, and
Beckman “89” series.
Applications
The 940/MK940 may be configured for 4 to 20mA output, as
a constant current source, as a 0 to 1mA current source, 0
to 5V output, voltage input and 1 to 5V output, or an isolated
voltage input and 4 to 20V output with a single pole 10Hz input
filter. Figures 3 through 8 illustrate these configurations with
the Model 940/945 and MK940 mounting kit. The R-numbers
MK940 Application
Isolated Constant Current Source
FIGURE 3.
Shows an isolated 0 to 20mA current loop. In this application the ZERO pot is not used and is replaced by a wire jumper. R6 parallels an internal
50 ohm resistor to change the scale factor. The zero current may be from 10nA to 100μA. Zero current must be checked with a short between pins
F and H, or 0 Volts input. Do not leave the input open.
FIGURE 4.
Is a potentiometer adjust 0 to 20mA constant current source which is isolated from the power supply. The jumper in place of R4 ensures that the
input is zero volts. Do not apply any control voltage to the input. The output will drive zero to 1000 ohms with a 15 volt power supply.
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