The signal isolator is used to provide galvanic isolation from one signal (input) to another (output) and usually combines the converter function.

For example;

• Input: 0-10mA.
• Output: 4-20mA.

This type of fault occurs whenever the return path of least resistance for the signal is via the earth or ground.

The tank diagram provides an example of this type of fault. Note: a transmitter with input/output isolation would eliminated this fault.

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Current looping problems arise whenever a path of least resistance for the current loop is not the intended path.

The looped powered transmitter example shows two receiving instruments supplied from the same 24Vdc supply that have a common input and supply negative.

In this illustration the current flows into REC 1 where it finds the path of least resistance to the supply ground is return via REC 1's negative supply rail, thus bypassing REC 2's input.

This problem is eliminated by isolating the 24Vdc supply from REC 1's input.

Dangerous high voltages can be present at sensor level, as in the case with dc shunts.

This voltage will be conducted through a non-isolated transmitter creating a threat to humans with a floating load or damage to receiving devices.

Often the signals from sensors and transducers are small voltages and susceptible to noise pickup from motors, variable speed drives and general switching spikes carried by the electrical reticulation system.
If there is isolation between input and output, the common mode noise (present on both input terminals with reference to ground) will be prevented from passing to the output even if it is relatively high in magnitude.

APCS has a complete range of isolators to suit every application. Three main categories of isolators can be identified, they are Input signal powered, Output loop powered and Externally powered.

Input signal powered isolators derive operating power from the 4-20mA input signal which is isolated, conditioned and sent to the output load.

Circuit loss which is characteristic of input signal powered isolators equate to a resistance of 250 ohm and therefore for linear operation, the input must be capable of driving the 4-20mA signal into a load which is 300 ohm more than the output load.

Signal powered isolators are usually used as a quick fix to unexpected isolation problems or as signal splitters to create extra isolated signal channels. Because they require no separate power supply, there is minimal effect on the wiring resulting in the lowest installed cost.

They cannot be used if the input signal does not have sufficient drive capability. While APCS isolators have losses considerably lower than others do, it is strongly advised that the output load and input drive be checked first.

All modern two wire transmitters have an output of 4-20mA. They are called two wire because the output signal that they control provides the power for their operation on the same pair of wires.

A typical loop consists of a regulated dc power supply, the two wire transmitter and a receiving device all connected in series as shown. The 4-20mA output signal powers the electronics of the transmitter. The input is filtered and conditioned and then passed across an isolation barrier by converting the input signal to a frequency, then passing it to the output through either an opto-coupler or transformer, before reconverting the frequency to the required signal.

Output loop powered signal isolators, converters or transmitters are usually used to provide isolation for the analogue inputs into PLC or other receiving devices. Installation costs is low as there is no need to run a separate pair of wires to the instruments for power. These are also compact units with channel widths as low as 22mm.

APCS output loop powered isolators are capable of excepting supplies in the range of 8 to 40Vdc and take only 8Vdc from the output current loop. With a typical supply of 24Vdc this enables them to operate with an external loop load up to 800 ohm. The output is load independent and the adjustable zero and span controls are non-interactive.

Load for 4 - 20mA output: RL max = ???[ohm]

Four wire transmitters can have uni-polar (e.g. 4-20mA) or bi-polar (�10Vdc) outputs. They are called four wire because the output signal and the power to the isolator are provided on two separate pairs of wire. The terms three port isolation is often used to describe these transmitters because input, output and power ports are all isolated from each other.

The input is filtered and conditioned and then passed across an isolation barrier by converting the input signal to a frequency, then passing it to the output through either an opto-coupler or transformer, before reconverting the frequency to the required signal.

Four wire signal isolators and converter can be used on both input and outputs of PLC or wherever full isolation between two pieces of equipment is required.

Four wire signal isolators will work in all applications and because they incorporate their own power supply, drive capability is higher than a two wire transmitter.

The output is load independent and depending on the model is adjustable via non interacting zero and span controls.

Most of the APCS range includes a 24Vdc auxiliary power supply on the input to power a two wire transmitter. This allows these units to provide complete single loop integrity because no loops share a common loop power supply.

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