5
3 ½ D I G I T , L E D D I S P L A Y D I G I T A L P A N E L V O L T M E T E R S
DMS-30PC
Figure 10. External Gain Adjustment
Figure 8. 4.5-18V Power Supply Monitor
Figure 9. Basic DC Ammeter Circuit
Figure 7. 4-to-20mA Current Loop Operation
To calibrate the circuit of Figure 7, perform the following:
1. With 4mA applied, adjust the 50k
Ω
potentiometer (R2) to
display a reading of "000" (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 "1000". For
different full scale readings, alter the value of R
Shunt
accordingly.
Example: For a meter with a 2V full scale input (1.999 full
scale reading) and a desired 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
6. Power Supply Monitoring: One of the most common digital
panel meter applications involves monitoring the output voltage of
the system power supply — often this supply also powers the
meter itself. The low-power, red LED DMS-30PC-2-RL can be
configured to allow power supply monitoring over the range of
4.5-18Vdc. The circuit in Figure 8 uses a low-drop-out, three-
terminal regulator (LM-2931Z-5, available from National
Semiconductor) to provide regulated 5V power to the meter. The
LM-2931 was chosen because it has the following on-chip
protection features: reverse polarity, short circuit and thermal run-
away. When using other, higher-power, DMS-30PC models with
three-terminal regulators, be sure to consult the regulator
manufacturer's data sheet to ensure the regulator is being
utilized safely and correctly.
7. Digital Ammeter: Digital ammeters are finding ever-increasing
usage because analog-style ammeters (moving-vane types) now
cost roughly the same as their digital counterparts. Additionally,
analog ammeters are not nearly as rugged as modern digital
panel voltmeters. Figure 9 illustrates a typical ammeter
application. The circuit uses a ±200mV input meter
— the preferred range for most ammeters — to measure the
voltage developed across a 0.1
Ω
current shunt. The circuit
shown represents a basic ammeter connection diagram. A
detailed application note describing digital dc ammeters is
included in DATEL's new Digital Panel Meter Databook.
8. External Gain Adjustment: Connect REFERENCE OUT
(pin 8) to REFERENCE IN (pin 7) for normal, factory calibrated,
operation. Use the +1.23V REFERENCE OUT (pin 9) for
applications needing external gain adjustment. Figure 10 shows
the wiring configuration for each model. Calibration is performed
with a precise, near-full-scale, input voltage.
Applications
1
3
5V RET
DATEL
UPA-5/500
120 VAC
AC to DC Converter
DMS-30PC-1
12
(–) IN LO
10
ANA COMM
11
(+) IN HI
R1
+
–
4-20mA
50k
R2
8
7
REF OUT
REF IN
9
1.23V
REF
5V SUP
DMS-30PC-2-RL
5
7
12
+
–
11
1
4.5 - 18Vdc
LM2931Z-5
GND
IN
OUT
+5V SUP
3
5V RET
DP2
REF
IN
(+) IN HI
8
REF
OUT
(–) IN LO
22µF
10V
+
DMS-30PC-0-YS
6
3
+
–
1
1.5 Amp (Load Current)
5Vdc
7
8
+5V SUP
5V RET
DP1
REF OUT
REF IN
11
0.150V
(+) IN HI
0.1
Ω
R
Shunt
Load
(3.3 )
12
(–) IN LO
Ω
9
(+) IN HI
(–) IN LO
ANA COMM
10
1.23V REF
1k
10 to 20 Turns
8
REF IN
11
1k
10 to 20 Turns
8.45k, 1%
8.45k, 1%
5V RET
Connections
for ±2V, ±20V
and ±200V models
Connections
for ±200mV
models
7
NC
12
3
DMS-30PC
12
VOLTAGE
CALIBRATOR
OUT
COM