Analogue signal isolators still play important role

Paul Boughton

The pressure is on manufacturers to reduce cost whilst improving performance and flexibility. Here Paul Wade looks at the continuing role of analogue signal isolators.

The ‘humble’ loop powered DIN rail analogue signal isolator has a strong and growing part to play in modern plant instrumentation, claims Status Instruments.,

The company’s SEM1000 series of isolators are suitable for both OEM and retrofit applications.

The compact nature of the devices means that many more units can be installed in existing cabinets, while their high accuracy permits the addition of isolation to systems without significantly degrading the overall system performance.
If necessary the span and zero potentiometers can be used to remove any system errors.

System versatility

These products provide great system versatility. For instance where PLCs or similar systems are installed their high level non-isolated inputs can cause problems.

Inserting the SEM1000 into an existing powered (4–20)mA loop provides a simple low cost means of providing complete signal isolation. If required, a resistor can be placed in the output loop to provide a voltage signal.

In another typical application, the isolator solves the problem where a field transmitter and control room device may often have different earths. Inserting the SEM1000 into the loop enables both earths to be connected to the signal lines without the introduction of earth loops.

The SEM1010 isolator is typically used to provide isolation between a field mounted, two wire 4–20mA transmitters and a PLC type device with a common power supply.
The required supply for the two wire transmitter is obtained from the power supply connected to the output terminals of the 1010 and total isolation is maintained between these and the transmitter.

Many systems have a number of inputs that require isolation. Using conventional isolators, a separate power supply is required for each loop in order to completely isolate each input from each other.

However by using the SEM1010, a single power supply at the system side produces a separate isolated power supply for each field transmitter as well as isolating and repeating the transmitter signal.

In certain applications there can be a number of devices in a loop, each requiring some volts from the loop. Often there is a requirement for more volts than the loop can supply, but by using the SEM1020 loop boosting isolator, an additional load up to 900ohm can be driven from a single 2V supply.

The input and output loads can be driven from separate supplies to maintain isolation.

Hazardous areas

The 1020 also offers important benefits when field transmitters are in hazardous areas. These are normally protected by the inclusion of a zener barrier system. x

However zener barriers have a typical series resistance of approximately 300ohm and so it can be difficult on occasions to provide sufficient drive capability for several safe area devices.

The SEM1020 provides an economic means of creating an additional drive capability of up to 900ohm from a single 24 V supply. This low cost isolator requires power supply connections on both the input and output terminals. If true isolation is to be maintained, the two power supplies used must be fully isolated from each other.

The most common use for the SEM1020 is to provide extra drive capabilities from an existing (4–20) mA current loop. The input circuitry only drops 2.7 V from the existing current loop and providing a 24 V isolated power supply is used on the output side, the isolator will drive up to 900 ohms. 

Paul Wade is with Status Instruments Ltd, Tewkesbury, Gloucestershire, UK. www.status.co.uk

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