How to Choose a 4-20mA Isolator Application Note
Very often the simple task of choosing an isolator requires more than just comparing the number of channels and the cost per point. To help you with this exercise, here are a few concepts to know when specifying a 4-20 mA isolator.
The lowest cost approach is a Two-Wire System using Output Loop Powered devices such as our 651T-0600. This module steals power from the output loop to operate and control the 4-20mA loop to the load. Since the module requires an external output power source, it is characterized as having a Sinking Output. For loads that have a powered input connection, the 651T-0600 provides the compatible circuit. See example # 1 below.
If there is no power source on the output loop, then a 4-20mA isolator that powers the output loop is required. This type of isolator is characterized as having a Sourcing Output and there are two types. The lowest cost approach is a Two-Wire System using Input Loop Powered devices such as our 671T-0600. This module steals power from the input loop to operate and power the 4-20mA output loop to the load. If this device is not appropriate, then a four-wire device is available such as our 631T-0500. See example # 2 and #3 below to select the correct module.
To determine if the modules are properly matched to an application, we need to know what is the voltage level of the external power source, the total input impedance of all loads and if other peripheral devices are connected to the input loop.
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