Pressure Transducer and Transmitter Wiring Explained

In this article, we’re going to show you how to wire a few different pressure transducer and transmitter types to digital and analog PLC input modules.

We’re also going to analyze a typical instrument field-wiring drawing to show you how PLCs are properly configured for different transducer and transmitter types.

Transducer and transmitter

The terms transducer and transmitter have become confusing because they are used interchangeably by different people. In particular, the term transducer is tossed around as a name for many devices and instruments.

Just for clarity, let’s look at the differences between a Transducer and a Transmitter.

A transducer is a device that converts one form of energy to an electrical or electronic signal.

Pressure switch

A Pressure switch is a two-part device consisting of a sensing transducer and an electrical switch.

A pressure switch is a digital device because it has only two possible conditions – open or closed.

It is therefore connected to a PLC digital input module.

Pressure transmitter

A Pressure Transmitter is a two-part device consisting of a sensing transducer and a circuit that transmits a standard instrumentation electrical signal representing a physical variable being measured. In most applications, the electrical signal is 4 to 20 mA.

Pressure transmitter wiring

A Pressure Transmitter is an analog device and is therefore connected to a PLC analog input module.

Many people refer to this device as a Transducer. For the sake of simplicity, we’ll continue to refer to it as a Transmitter.

A Pressure Transmitter requires a power supply to operate.

Most Pressure Transmitters are two-wire types. Transmitters are also available as four-wire types.

As the name suggests, a two-wire pressure transmitter has only two wires. These two wires provide power for the transmitter and are also the signal lines!

A four-wire transmitter has two wires connected to a power supply and two signal wires connected to the PLC. The power supply can be AC or DC depending on the vendor and model.

Wiring to PLC modoules

1) Pressure switch

Alright, let’s wire a Gems PS41 series pressure switch to Input 3 of a ControlLogix 1756-IB32 digital input module.

All of the GND terminals on the 1756-IB32 module must be wired together and connected to the -DC terminal of the 24-volt DC Power Supply.

So, we need to connect terminals 17,18,35 and 36, and wire them to the -DC terminal of the power supply.

Because the pressure switch is not voltage-dependent, it doesn’t matter which wire goes to the +DC terminal of the power supply and which wire goes to terminal 4 of the input module. So, take your pick and connect the two wires.

The 1756-IB32 module is capable of supporting up to 32 digital input devices. Each additional device would be wired the same as the Gems pressure switch.

One terminal of the new device would connect to the +DC terminal of the power supply, and the other device terminal would connect to any of the remaining open inputs on the digital input module.

2) Pressure transmitter

OK. Let’s move on to wiring Pressure Transmitters to a PLC Analog Input Module. We’ll look at wiring a two-wire Wika E-11 Pressure Transmitter to a ControlLogix 1756-IF8 analog input module.

On the Transmitter, the Red lead is Current in, and the black lead is current out.

The analog module can be wired in two possible modes: Single-ended or Differential.

The selection is important as it determines how the module is physically wired. Our module is configured for Single-Ended mode.

In Single-Ended mode, we can connect 8 two-wire transmitters sharing one power supply. Each transmitter connection is referred to as a Channel.

The 1756-IF8 module is configured for current input during programming in the I/O configuration.

We will connect the Wika E-11 two-wire transmitter to Channel 5 of the analog module.

Terminals 9,10, and 13 of the analog module are connected and then wired to the -DC terminal of the 24V DC Power Supply.

The Red lead of the Wika E-11 pressure transmitter is wired to the +DC terminal of the Power supply. The Black lead of the transmitter is wired to Terminal 14 on the analog module.

As we said earlier, more two-wire transmitters sharing the same power supply can be connected to the analog module.

Let’s add another pressure transmitter to the 1756-IF8 module. We will connect another Wika E-11 two-wire transmitter to Channel 6.

Terminal 15 is wired to Terminal 13 of the analog module. The Red lead of the pressure transmitter is wired to the +DC terminal of the Power supply. The Black lead of the transmitter is wired to Terminal 16 on the analog module.

Details on how to wire more transmitters can be found on the Rockwell Automation website.

Wiring example

Field wiring diagrams vary between companies. In the following example, two channels of the 1756-IF8 are wired in Single-Ended Mode.

Pressure Transmitter ZT-P3606 is connected to Channel 0, and ZT-P3610 is connected to Channel 1.

Both transmitters share the same +24V DC Power Supply.

Summary

That should do it for this article. Let’s review:

– in process control and instrumentation, the terms Transmitter and Transducer are often used interchangeably

– A transducer is a device that converts one form of energy to an electrical or electronic signal.

– A Pressure switch is a two-part device consisting of a sensing transducer and an electrical switch.

– A Pressure Transmitter is a two-part device consisting of a sensing transducer and a circuit that transmits a standard instrumentation signal which in most applications is 4 to 20 mA.

– Most Pressure Transmitters are two-wire types having two wires that provide power for the transmitter and are also the signal lines!

– PLC analog input modules can be wired in two possible modes: Single-ended or Differential. The selection is important as it determines how the module is physically wired.

– In Single-Ended mode, two-wire transmitters can share a single power supply.

If you have any questions about the Pressure Transducer and Transmitter or about measuring instruments in general, add them in the comments below and we will get back to you in less than 24 hours.

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