Current Loop Transmitter Impedance - Automation Direct DL405 User Manual

Current Loop
Transmitter
Impedance
Standard 4 to 20 mA transmitters and transducers can operate from a wide variety
of power supplies. Not all transmitters are alike and manufacturers often specify a
minimum loop or load resistance that must be used with the transmitter.
The F4–16AD–1 provides 250 ohm resistance for each channel. If your transmitter
requires a load resistance below 250 ohms, then you do not have to make any
adjustments. However, if your transmitter requires a load resistance higher than
250 ohms, then you need to add a resistor in series with the module.
Consider the following example for a transmitter being operated from a 36 VDC
supply with a recommended load resistance of 750 ohms. Since the module has a
250 ohm resistor, you need to add an additional resistor.
R + Tr * Mr
R + 750 * 250
R w 500
DC Supply
0V
+36V
NOTE: Make sure the power dissipated in the resistor does not exceed its power
rating. (P=VI) In the example above, P=36 * 0.02 = 0.72, or 3/4 Watts.
F4–16AD–1 16-Channel Analog Input
R – resistor to add
Tr – Transmitter Requirement
Mr – Module resistance (internal 250 ohms)
+ –
Two-wire Transmitter
Module Channel 1
–
250
R
ohms
+
6–7