4
DMS-40PC
4 ½ DIGIT , LED DISPLA Y DIGIT A L P ANEL VOL TMETERS
Applications
which low calibration drift is desirable. When using other, higher-
power, DMS-40PC models in combination with three-terminal
regulators, be sure to consult the regulator manufacturer's data
sheet to ensure the device is being utilized safely and correctly.
Figure 4. 4.5-18V Power Supply Monitor
4. Floating Signal Source Measurements: A floating input is a
signal that, before it is applied to the DMS-40PC's inputs, has no
galvanic connection (direct current path) to the meter or the
meter's power supply. The circuit shown in Figure 5 illustrates
the necessary connections for measuring floating inputs. The
1.5V battery represents a true floating input signal since it
initially has no connection whatsoever in common with the
meter. Real-world floating inputs typically originate from power
supplies which are transformer isolated from the DMS-40PC’s
+5V supply.
The connection of pin 12 ((–) INPUT LO) to pin 10 (ANALOG
COMMON) is required in order to provide a bias return for the
meter’s input amplifier. This is because neither pin 11 nor pin 12
are tied to any reference voltage inside the DMS-40PC (see
Figure 1). These connections are not made internally in order to
give the meter the ability to make differential measurements as
described in a previous section.
Figure 5. Floating Input Measurements
3. Power Supply Monitoring: One of the most widely used digital
panel voltmeter applications is monitoring the output voltage of a
system power supply — often the same supply that also powers
the meter. The low-power, red LED DMS-40PC-2-RL, with its
excellent 0.001Vdc resolution, can be configured to monitor
power supplies with outputs in the range of 4.5-18Vdc. The
circuit in Figure 4 uses a low-drop-out, three-terminal regulator
(LM-2931T-5, in a T0-220 package, 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
runaway. The DMS-40PC-3-RL can monitor voltages up to
±200Vdc, provided a separate +5V power source is used since
many three-terminal regulators cannot operate with supply
voltages greater than 24V. Red, low-power LED models, with
their very low self-heating, are recommended for applications in
Figure 3. Differential Input Configuration
2. Differential Input Configurations: Differential inputs can also
be measured with DMS-40PC meters as shown in the circuit of
Figure 3. Differential inputs must also originate from power
supplies that have a common ground with the meter's 5V
RETURN (pin 3). However, differential inputs usually have both
terminals above and/or below 5V RETURN. Figure 3, though not
necessarily a typical real-world application, does serve the
purpose of illustrating the concept of a differential signal.
The voltages developed across R1, R2 and R3 are equal to each
other and measure approximately +1.6666Vdc or 1/3 of the +5V
power source. More importantly, while the signal across R3 is
single-ended, both ends of R1 and R2 are well above ground
and are described here as being differential. Please note that
while the DMS-40PC can measure the voltages across either R2
or R3, it cannot measure the +1.6666 volts across R1! The
voltage at the lower end of R1 is approximately 3.333V and this
exceeds the common mode voltage limit of ±2V.
1
1 2
3
+ 5 V S U P
( ) I N L O
5 V R E T
1 1
( + ) I N H I
D P 1
A C t o D C C o n v e r t e r
R 2
R 1
R 3
1 k
1 k
1 k
D M S - 4 0 P C - 1 - G S
D A T E L
D M S - P S 1 - C M
8 5 - 2 6 4 V a c
DMS-40PC-2-RL
5
12
+
–
11
1
4.5 - 18Vdc
LM2931T-5
GND
IN
OUT
+5V SUP
3
5V RET
DP2
(+) IN HI
(–) IN LO
22µF
10V
+
1
3
+ 5 V S U P
5 V R E T
6
D P 1
A C t o D C C o n v e r t e r
D M S - 4 0 P C - 1 - G S
1 2
( ) I N L O
1 1
( + ) I N H I
1 . 5 V
C E L L
1 0
A N A C O M M
D A T E L
D M S - P S 1 - C M
8 5 - 2 6 4 V a c
English ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
