Myron L 729II - Manual (Page 30)

4. CONVERTING A CURRENT
TO A VOLTAGE
Current measuring devices actually measure voltage, but have
an internal resistor as shown in figures 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 figures 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 floating 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 PI
SO 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