Fundamentals of loop-powered devices.

What is a loop-powered device?

Loop powered devices are electronic instruments that can be connected In a transmitter loop, without having a separate or independent power supply.

These devices are arranged to use the power from the current flowing in the loop. Common loop-powered devices include sensors, transducers, transmitters, indicating meters, data loggers, PLCs, and many more filed instruments.

Loop powered devices are essential because in some systems it is difficult to supply separate power to all the devices and instruments in the loop.

One of the most significant types of current loops is the 4-20 mA loop. The 4-20 mA loop is an accepted method to convey sensor information by the current.

Characteristics of loop-powered meters.

Loop-powered devices come with two input connections a positive (+) and a negative (-). The current signal enters through the positive (+) terminal and leaves via the negative (-) terminal.

Hence the term ‘loop-powered’ is synonymous with the term ‘2 wire’ (meaning that only 2 wire connections are involved in any of the connections between the transmitter, the power source, and the output device).

In addition, being easy to wire and requiring very little power, there are a few other defining characteristics of loop-powered devices.

They generally do not feature LED displays nor mechanical relays. Process outputs and serial communications capabilities are quite limited Loop powered devices, however, are normally less expensive, smooth to install, and will usually feature hazardous area approvals such as Intrinsically Safe(I.S.)and Non-incendive (N.I.). Explosion-proof loop-powered devices are also common as they undoubtedly simplify the installation.

Advantages

There are a few advantages to utilizing the power supplied for the current loop to power your devices rather than incorporating an additional power supply into your system.

First and foremost, it might not be feasible to supply additional power to your system if your application is in a remote area, for example, in the case, your transmitter might be powered by a battery, solar cell power, or some other power source and the other devices in the system would derive their power from the transmitter.

Easy and smooth wiring:

The expenditure of running additional lines may also make additional power other than just the 4-20mA signal cost-prohibitive.

Because there is only one power source for the entire system (often supplied by the transmitter which is outputting the 4-20 mA signal), loop-powered devices are incredibly simple to set up.

This is in contrast to three and four-wire setups, which incorporate an external power supply into the mix. This makes installation power supply into the mix. This makes installation power supply into the mix. This makes installation complexity minimal.

Cost-effective:

Loop-powered devices are generally much lower cost than other process control devices with built-in high power electronics. This is simply because the expensive components that could be included in these devices such as power supplies, mechanical relays, or advanced digital or analog signal output components are left out in order to limit the amount of power necessary to operate the device.

Many loop-powered devices are approved for use in hazardous areas as Non-ignitable (N.I.) or Intrinsically Safe (I.S.). Both of these approvals require the device to consume such little power that it cannot cause any kind of damage.

Combination under normal operating or fault conditions, respectively. Since the power consumption of loop-powered devices is so low by necessity, they are usually easy candidates for these approvals.

Characteristics of loop-powered meters.

  • Check the voltage drop across each device in the loop to ensure the power supply distributes enough voltage. That means the power supply must support the additive voltage drop.
  • Check the output specification of the loop-powered device as they deliver low power output signals. Usually, need supplement devices.
  • Check the operating temperature ranges, as low temperatures affect the LCD. Below -20 deg C temperature LCD stops working. Available only at -40 deg C to +75 deg C.

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