5

Applications

3 ½

DIGIT ,

LED

DISPLA Y

DIGIT AL

P ANEL V O L TMETERS

DMS-20PC

Figure 8. 4.5V-18V Power Supply Monitor

Figure 10. External Gain Adjustment

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

Figure 9. DC Ammeter Circuit

The LM-2931 was chosen because it has the following on-chip

protection features: reverse polarity, short circuit and thermal

runaway. When using other, higher-power, DMS-20PC 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 circuit shown in Figure 10 for applications

needing external gain adjustment. Calibration is performed with

a precise, near-full-scale, input voltage.

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-20PC-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.

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

13

+5V SUP

5V RET

DATEL

UPA-5/500

120 VAC

AC to DC Converter

DMS-20PC-1

12

(–) IN LO

10

ANA COMM

11

(+) IN HI

R1

–

4-20mA

50k

R2

8

7

REF OUT

REF IN

2

DISPLAY

ENABLE

5

7

8

+

–

DMS-20PC-2-RL

12

11

1

4.5 - 18Vdc

LM2931Z-5

GND

IN

OUT

+5V SUP

3

DP2

REF

IN

REF

OUT

(–) IN LO

5V RET

2

DISP EN

(+) IN HI

22µF

10V

+

6

3

+

–

DMS-20PC-0

11

0.150V

1

1.5 Amp (Load Current)

5Vdc

7

8

+5V SUP

5V RET

DP1

(+) IN HI

REF OUT

REF IN

2

DISP EN

0.1

W

Load

(3.3 )

12

(–) IN LO

W

12

VOLTAGE

CALIBRATOR

OUT

10

(+) IN HI

(–) IN LO

ANA

COMM

= 10 to 20 Turns

7

REF

IN

11

732, 1%

8.06k, 1%

5V RET

Connections

for ±2V, ±20V

and ±200V models

Connections

for ±200mV

models

COM

17.4k, 1%

2k

24.3k, 1%

3

12

DMS-20PC-1

8

NC

200

+5V IN

1

1

1

1