DMS-40LCD Series
4�½ Digit, LCD Display Digital Panel Voltmeters
APPLICATIONS
5. Process Control (4-to-20mA) Measurements:
In many common
process-control applications, a 4-to-20mA current loop is used to
transmit information. Because DMS-40LCD meters have such high
input impedance, a simple shunt resistor across the meter’s input can
be used to convert the loop current to a voltage. See Figure 7. The
value of the shunt resistor is a function of the scaling requirements of
the particular application and can be calculated using the following
equation:
R
Shunt
= R1 = V
Fsr
/ I
Fsr
Where:
V
Fsr
= Full scale reading (in Volts)
I
Fsr
= Relative full scale current (in Amps)
Example: For a meter with a 2V full scale input (1.999 full
scale reading) and a desired full scale display reading of
1000 (with an input of 20mA), V
Fsr
= 1.000 Volts
R
Shunt
= 1.000V/(0.020 – 0.004)A
R
Shunt
= 1.000V/0.016A = 62.5 Ohms
To calibrate the circuit of Figure 7, perform the following:
1. With 4mA applied, adjust the 50k potentiometer (R2) to
display a reading of "0000" (assuming that is the desired
reading).
2. With 20mA applied, adjust the gain-adjust potentiometer on
the back of the meter to display a reading of "19999".
For different full scale readings, alter the value of R
Shunt
accordingly.
11
(+) IN HI
10
ANA COMM
12
(–) IN LO
1
+5V SUP
2
RANGE
3
5V RET
6. Power Supply Monitoring:
A popular application for DATEL’s low-
power LCD meters is monitoring the supply voltage in battery-operated
portable equipment. Figure 8 demonstrates how a 9V-powered DMS-
40LCD can be used to monitor its own supply. The meter used is the
DMS-40LCD-0/1-9 configured for its ±2V input range (pin 2 connected
to pin 1). A three-resistor voltage divider is used to attenuate the bat-
tery voltage and also to satisfy the requirement that the input voltages
applied to pins 12 and 11 be at least 1.5 Volts above and below the
battery voltage applied to pins 1 (+BATTERY) and 3 (–BATTERY).
The divider should be designed so that 1/10th the battery voltage falls
across the inputs to the meter.
R2
= 0.1
(R1 + R2 + R3)
Therefore, the 9V battery voltage appears to the meter inputs as 0.9V.
With the decimal point moved to its DP2 position (pin 5 tied to pin 3),
the meter reads 9.000 Volts.
The circuit can be calibrated by first measuring the actual battery volt-
age with another meter and then adjusting the gain-adjust potentiom-
eter on the back of the DMS-40LCD until a similar reading is obtained.
If possible, the resistors in the divider should be ±1% metal-film types
with TCR’s less than 50ppm/°C.
DMS-40LCD-0/1-9
11
(+) IN HI
R1
45.3k
12
(–) IN LO
R2
10.1k
DMS-40LCD-0/1-5
1
+BAT
2
RANGE
+
3
–BAT
5
DP2
R3
45.3k
+
9V
BATTERY
–
4-20mA
R1
R2
Figure 8. Power Supply Monitor
(9V-Powered Models)
–
50k
120 VAC
DATEL
UPA-5/500
AC to DC Converter
Figure 7. 4-to-20mA Current Loop Operation
(5V-Powered Models)
www.murata-ps.com
Technical enquiries
email: sales@murata-ps.com, tel:
+1 508 339 3000
MPM_DMS40LCD.C02
Page 5 of 6
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