|There are essentially 3 general configurations of Galvanic isolator2|
| loop powered where the isolator is powered from the signal itself, the most common being for 4-20mA loops and digital output signals.
Output voltage can be limited as the loop current has to power the interface
| 2 Port isolated where a separate power supply (usually 24vdc) connection is required but this is not galvanically isolated from the signal lines.
Consideration must be given to the electrical configuration of any circuit the non-hazardous area signal terminals are connected
|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|
Due to their complexity they must be designed specifically for the application circuit or signal type, Analogue, Digital, input, signal type etc.
These are conventionally shown as 'positive' voltage but, unlike Zener barriers, they are isolated, so the concept of positive and negative barriers becomes irrelevant which could become critical if two or more circuit loops are joined.
Unless allowed in a specific certificate, as fault conditions must be considered i.e. it must be assumed they could be linked in both series and parallel and the parameters added accordingly. e.g two off 28v 93mA isolators linked would result in output parameters for the circuit of 56v 186mA which would almost certainly cause issues.
The basic output circuit of isolation interfaces tends to be like that used in Zener barriers but, particularly for custom applications, output circuits can be complex and use other techniques and components for power limitation ↩
Shown here with a 'representative' Zener diode and resistor output which is not necessarily the actual circuit used and may vary between isolator types. ↩