At the same time, many of the best sensors in the industry are only available as a 4 to 20 mA output. Many low cost HVAC/R controllers do not include a dedicated power supply or a factory configured input for a 4 to 20 mA signal. 2-Wire Transmitter with External Power & External Resistor To properly design a current loop you must know the value of the controller’s resistor. While ANSI/ISA-50.1-1982 (R1992) Compatibility of Analog Signal for Electronic Industrial Process Instruments says that the load resistor shall be 250Ω, resistor values of 50 to 1,000Ω may be used. The load resistor can be internal to the controller or external.
![250 ohm resistor color code 250 ohm resistor color code](https://i.stack.imgur.com/Tacsq.png)
At 4 mA, a 250Ω resistor drops 1 volt at 12 mA, a 250-Ohm resistor drops 3 volts and at 20 mA, a 250-Ohm resistor drops 5 volts. Most commonly a 250-Ohm resistor is used to convert the loop current back to a voltage. The transmitter consumes 7 to 15 VDC of loop voltage, depending on model, to power itself.įinally, the controller’s load resistor converts the loop current to a voltage for easy signal processing. Table 1 shows the resistance of common wire. The voltage drop in wire is directly proportional to the current flowing through it by Ohms law, current x resistance equals voltage. The voltage drops occur in the wire, the transmitter and load resistor.
![250 ohm resistor color code 250 ohm resistor color code](https://images-na.ssl-images-amazon.com/images/I/41EIXjsKQoL._SX342_.jpg)
Designing 4 to 20 mA current loops is just managing the voltage drops around the loop. Current loops are ideal for data transmission because of their inherent insensitivity to electrical noise.
![250 ohm resistor color code 250 ohm resistor color code](https://i.ebayimg.com/images/g/G3EAAOSwhjNeqLDW/s-l640.jpg)
The 4 to 20 mA current loop is a very robust sensor signaling standard.
#250 OHM RESISTOR COLOR CODE HOW TO#
This document describes how to manage the voltage drops in a 4 to 20 mA current loop, and how to determine the proper power supply voltage for the loop.