What is a signal isolator and what does it do?

The signal isolation is an inevitable part of process control, mostly it does the isolation of unwanted current loops, ground loops, and it also protects the operator and the machine. Isolation is the process in which there won’t be any electron flow between two circuits. The isolation breakdown voltage for the equipment is the voltage needed to cause flashover or a breakdown in isolation, in such a circuit. Isolation is mostly done by passing signal over a barrier using magnetic or optical coupling then converting the signal into the required output. While doing the isolation we must consider three isolation paths and they are supply to output, supply to input, and input to output.

What does, a loop-powered isolator and signal powered isolator do?

The loop power isolator will do the isolation for the input-output, and input-supply paths but not for the output supply paths. While the signal power isolator will provide isolation input-output, and output supply paths but not for the input supply paths because the signal is the supply.

Why do we isolate?

The major reason for doing isolation in an electrical circuit is to block the transfer of high or hazardous voltages and also to break ground loops.

  • Isolation can prevent the transfer of high or hazardous voltages between circuits
  • Isolation can do the protection from electric shocks
  • It can reject high common-mode voltages
  • Isolation can break the conduction path for high transient voltage noise between circuits, like voltage spikes that are generated from switching inductive loads, lightning, etc.
  • It can break potential ground loops, in an electric circuit, a potential difference or the voltage is the force that will drive current flow through it.

Why not isolate all transmitters?

Mostly transmitters are isolated from input to output, but in certain cases, it is not possible. Some transmitters must produce high sensor current or have high output drive, and in other types of transmitters, they use sensors that are highly insulated so to reduce the cost, isolation could be an option.

What are the types of signal isolators and how does it work? What are Industrial signal isolators?

The important part of isolation is how we add electrical isolation to block unwanted signals and to transmit our required signal through the circuit without providing a direct path for signal conduction.

Magnetic isolation

Magnetic isolation or DC/DC isolator must be selected in a way that it must match to the transformer within the circuit. The parts of magnetic isolators are a transformer that has a driver at the primary and a receiver at the secondary. The driver would encode the input signal into an AC waveform that then couples from the primary to the secondary. The receiver would decode the waveform and reconstructs the signal at the output.

Optical isolation

In the optical isolation, it would use a light-emitting diode and a photodiode to transfer information across an isolation barrier. The measured input voltage signal will be converted to current, which would activate an LED within an optical coupler. The light-sensitive transistor will convert the light signal back to a current that the instrumentation amplifier can handle.

Capacitive isolation

A capacitor would allow the AC current to flow but it would block the DC, so it can be used to couple AC signals between circuits at different DC voltages through a varying electric field. The measured signal to be isolated will be modulated and coupled through the capacitor to the receiving side. On the receiving side, the AC signal is demodulated to restore the original signal.

What are the applications of the signal isolator?

  • Field interface device
  • Isolation of field signals
  • Distribution of signals
  • Translation of signals
  • Factory automation
  • SCADA
  • DCS
  • Impedance matching of transmitters and receiver instruments
  • Powering a field transmitter
News Reporter
post-graduate in Electronics & communication