Power Monitors, Transducers & Repeaters

Signal Isolation Basics

When dealing with scenarios that involve many different control circuits and input and output signals, distortion can occur. This distortion can corrupt the signals, rendering the data virtually useless. For example, if there is a transmitter sending a 4-20 milliamp signal to a PLC and the transmitter is located several feet away in a different building, both devices have the possibility of having different ground potentials. This difference in potential can cause the current flowing between the two points to distort the control signal being sent. This current flow between two different ground potentials is referred to as a ground loop. Ground loops can be fairly common in industrial control applications, and because of the issues that they cause, must be controlled. This is where signal isolation comes into play.

Signal isolation

Signal isolation provides electrical isolation between input and output circuits and is sometimes referred to as galvanic isolation. This physical break in the galvanic path of the control signal offers a way to eliminate the effects of ground loops. There are two different methods of signal isolation that are used today: optical isolation and transformer isolation.  

Optical isolation

Optical isolation uses a module and a small circuit board composed of an LED and photosensitive detector with an insulating air gap, that creates the physical separation. The advantage of using optical isolation is that the size of the unit is quite small and can provide higher levels of insulation than other methods. A disadvantage of using optical isolation is that these devices need to be powered separately from the signal itself and the signals need to be relatively small.  

Transformer isolation

Transformer isolation utilizes a transformer to provide the isolation gap, while still allowing the signal to be transmitted. This type of isolation works well with AC power signals. An advantage of using this type of isolator is the that it does not require its own power source. It can simply transmit the signal passively.