How to Connect a 2 Wire Transmitter to a PLC
To connect a 2 wire transmitter to a PLC, you’ll first need to connect a power source to the two wire transmitter. This power source is usually the process signal loop. Two-wire devices receive power through a pair of cables and transmit the data through the same pair of cables. These devices will usually require a 24Vdc power supply.
Two-wire devices receive power from the process signal loop
The power for two-wire devices comes from a loop that is powered by the process signal. The current flowing in the loop powers the device, which in turn transmits the signals without loss. The power used to energize the device comes from an external power supply, or sometimes from the loop power itself. The power travels through the same wires as the information signal.
A two-wire instrument operates with a 24 VDC power supply, and consumes less than 0.1 W of power. It also features a safety barrier, making it ideal for use in hazardous areas. This feature eliminates the need for separate safety components. However, the low power requirements of two-wire devices make them less suitable for use in sophisticated flow devices. In particular, two-wire flowmeters are not suited for applications requiring large voltage drops.
A device powered by a loop has two input connections: positive (+) and negative (-). The positive (+) terminal receives the current, while the negative (-) terminal receives the signal. The loop’s power supply needs to be capable of powering all the devices in the loop, without voltage drop.
They transmit data through the same pair of cables
A two-wire transmitter transmits data through a pair of cables using the principle of power-line communication (PLC). The device receives power from the receiving device and converts it into an analog signal. The data is transmitted without any loss or degradation and is thus known as an analog signal. In the process instrumentation industry, a two-wire transmitter is used. The data signal is transmitted using the same pair of cables as the power.
They act as a load regulator
A 2 wire transmitter acts as a load regulator when it senses changes in the current flow between two different voltage levels. Typically, the output voltage is between four milliamps and twenty milliamps, with the voltage of the power supply being the constant source. Depending on the voltage source, the transmitter can be used to measure the full scale input of a variety of processes.
There are three basic types of transmitters. Type 2 has two wires for power supply, while Type 3 has four wires. The power supply voltage for Type 2 is 24VDC, while the output signal is four to 20 mA. A 250-Ohm load resistor is used as the load resistance.
A two-wire transmitter is the most common type of control loop these days. It converts process signals to a 4-20 mA DC current and transmits them without loss. The power source is connected in line with the transmitter. The transmitter, controller, and power source are all connected in the same loop.
They require a 24Vdc supply
Most transmitters require a 24Vdc power supply, which can be either local or provided by the associated control panel. The wiring for these devices varies, depending on the type of transmitter and the design of the associated control panel. Most modern transmitters can be wired in a source or sink configuration. This is determined by the location of the link in the transmitter, or by the terminals. Some transmitters are limited to a specific configuration, but the technical manual will indicate which options are available.
Loop-powered devices are similar to 2 wire transmitters. They are powered by a current loop and require two wires for the positive (+) and negative (-) connections. The positive (+) wire enters the current loop, while the negative wire is used to exit.
A 2 wire pressure transmitter requires a 24Vdc power supply and outputs a 4-20mA signal. Many people are unaware of the fact that a 24Vdc power supply is required for the 4-20mA output signal. A simple way to determine whether your transmitter is using a 24Vdc power supply is to test the output. Using a voltmeter or an ammeter, measure the current in a signal wire to make sure the current is within the desired range. If it’s displaying a 0 or 1 volt, the current is within the acceptable range.