Isolation Interfaces

loop powered isolator 2 port isolator 3 port isolator

Galvanic isolation interfaces now dominate the market, they work on the principle1 as zener barriers with the addition of isolation between input and output circuits.

There are essentially 3 general configurations of Galvanic isolator2

  1. loop powered where the isolator is powered from the signal itself, the most common being for 4-20mA loops and digital output signals.
  2. 2 Port isolated where a separate power supply (usually 24vdc) connection is required but this is not galvanically isolated from the signal lines.
  3. 3 Port isolation is probably the most common with signals and power supply isolated offers a greater flexibility and reduces the possibility of hook up problems.

These are conventionally shown as 'positive' voltage but, unlike zener barriers, they are isolated so the concept of positive and negative barriers becomes irrelevant.

Due to their complexity they have to be designed specifically for the application circuit or signal type

  1. The basic output circuit of isolation interfaces tends to be similar to that used in zener barriers but, particularly for custom applications, output circuits can be complex and use other techniques and components for power limitation 

  2. Shown here with a 'representative' zener diode and resistor output which is not necessarily the actual circuit used which will vary between isolator types.