Myron L 750 II Series - Converting a Current to a Voltage

4. CONVERTING A CURRENT
TO A VOLTAGE
Current measuring devices actually measure voltage, but have
aninternalresistorasshowninguresIII.B.1&III.B.2andare
scaled to display in current. If you have a voltage input instru-
ment and you wish to utilize the current from the 4A (4-20mA)
Module, the following will help you to make this conversion.
To convert a 4-20 mA current output to operate or drive a volt-
age input device it is necessary to install a LOAD resistor across
theinputterminalsasshowninguresIII.B.6&III.B.7.The
value of the resistor is chosen to match the input voltage range,
i.e.0-10Voltsrequiresa500Ωresistorandwillproducea2to
10inputvoltage.Thisoatingzeroisusefultoindicateabroken
4-20 input wire when the indication is zero volts.
For other input ranges, divide the input voltage range by 0.02,
the answer will be in ohms.
NOTE: The tolerance of the load resistor directly affects the
accuracy of the resulting voltage, i.e. 5% resistor = 5% error.
26
VOLTAGE INPUT INSTRUMENT
+ 35 VDC
MAXIMUM
VOLTAGE INPUT INSTRUMENT
Figure III.B.7
(+)
(-)
+
-
(+) SIGNAL OUT
POWER OUT
POWER IN
+
Remote - Powered
EXTERNAL LOAD
RESISTOR
Figure III.B.6
(+)
(-)
+
SO PO PISO PO PI
-
(+) SIGNAL OUT
POWER OUT
POWER IN
+
Self - Powered
N
EXTERNAL LOAD
RESISTOR
N
For an input voltage range of:
10.0 Volts the resistor value is 500 ohms indicates 2.0 V @ 4 mA
5.0 Volts “ 250 ohms “ 1.0 V @ 4 mA
1.0 Volts “ 50 ohms “ 0.2 V @ 4 mA
100 millivolts “ 5 ohms “ 20 mV @ 4 mA
10 millivolts “ 0.5 ohms “ 2 mV @ 4 mA