Figure 4 shows the case where current is also measured at the high-
side of the load but the meter is powered from an independent supply.
Because the meter is now independently powered, it can now measure
voltage and power over the full range of 0.5 to 72 VDC. In this configura-
tion the shorting block on jumper J1 should be connected across pins
J1-1 and J1-2 so that the meter measures the voltage at the +IN (TB1-3)
terminal. In this configuration, the meter measures the sum of both the
load power and the power dissipated by the shunt resistor.
Figure 5 shows a high-side shunt configuration in which the power
dissipated by the shunt is excluded from the measurement. In this case,
the meter’s +V (TB1-1) terminal is connected to the -IN (TB1-4) terminal
and the meter both derive power and the voltage measurement from
this point in the circuit. Measuring voltage through the +V (TB1-1) pin
requires that the shorting block be connected across pins J1-2 and J1-3
of jumper J1. Because the meter requires very little operating power, its
addition to the load will typically influence neither the current nor power
measurements to a significant degree.
Figure 6 shows a configuration where the shunt is inserted into the
load return line and measures the current through that connection. In
this case, the meter must be powered from the DC line though the +V
(TB1-1) input, as the +IN (TB1-3) input only detects the voltage across
the shunt (< 100 mV). For this reason the jumper shorting block should be
placed across pins J1-2 and J1-3 of jumper J1 to enable measurement of
voltage from the +V (TB1-1) input. The total power reported in this configu-
ration is the sum of the load power plus the shunt power dissipation.
11. Noisy Power Supplies: Some power supplies contain high-frequency
switching devices that may conduct and/or radiate significant noise onto
the low-level signal developed across the internal shunt. Even though
DCM20 ammeters incorporate built-in input filtering, some portion of this
noise may be amplified and subsequently measured by its sensitive input
circuitry. The amplified noise introduces errors that are particularly no-
ticeable at zero load current (i.e., the ammeter function may not display
a relatively steady “000” reading). Connecting an external, non-polarized
capacitor across the +IN and -IN inputs can help reduce noise-related
display errors. In certain situations, the use of twisted pair or shield wir-
ing may be required. As a general rule, avoid using excessively long leads
between the shunt and the meter’s +IN (TB1-3) and –IN (TB1-4) inputs.
Figure 4. High-side current measurement, independent meter supply
METER
SUPPLY
9 - 72 VDC
LOAD
SUPPLY
72 V MAX
LOAD
+V
–V
+IN
–IN
1
2
3
4
123
TB1
J1
SHUNT
+
+
–
–
– –+ +
DCM20
Measurement Configuration Figure
Voltage
Measured at . .
Jumper
Across Pins
High-side current measurement 3 +IN (TB1-3) 1-2
High-side current measurement,
independent meter supply
4 +IN (TB1-3) 1-2
High-side current measurement,
shunt power excluded
5 +V(TB1-1) 2-3
Low-side current measurement 6 +V (TB1-1) 2-3
*Product shipped with jumper across pins 1 and 2.
Table 2. J1 Jumper Settings*
Figure 3. High-side current measurement, meter powered from DC line
DC
SUPPLY
9 - 72 VDC
LOAD
+V
–V
+IN
–IN
1
2
3
4
123
TB1
J1
SHUNT
+
–
– –+ +
DCM20
Figure 5. High-side current measurement, shunt power excluded
POWER
SUPPLY
9 - 79 VDC
LOAD
+V
+IN
–IN
1
2
3
4
123
TB1
J1
SHUNT
+
–
–
–
+
+
DCM20
–V
Figure 6. Low-side current measurement
POWER
SUPPLY
9 - 72 VDC
LOAD
+V
–V
+IN
–IN
1
2
3
4
123
TB1
J1
SHUNT
+
+
–
–
–
+
DCM20
www.murata-ps.com/support
DCM20 Series
Three-Function DC Power Meters
MPM_DCM20Series.A01.D5 Page 4 of 6
