MTL Barries - MTL7766PAc - STRAIN-GAUGE BRIDGES (cont)

The MTL7766pac is a shunt-diode safety barrier that uses a Zener diode circuit to limit energy and prevent ignition in hazardous areas. Its working principle is to protect intrinsically safe circuits by connecting them to safe areas through a shunt-diode barrier, which clamps any overvoltage to a safe level (e.g., 10V). It powers two strain-gauge bridges and is designed to keep the voltage across the hazardous area circuit low.

How it works

• Energy limitation: The MTL7766pac contains a shunt-diode circuit that acts as a safety valve for electrical energy.
• Voltage clamping: It connects a power source from a safe area to a field device in a hazardous area. If the voltage in the hazardous area circuit exceeds the Zener voltage (e.g., 10V), the diodes conduct, shunting the excess current to ground and keeping the voltage at a safe, non-incendive level.
• Powering two bridges: The "pac" in its name indicates it's a packaged version with a power feed facility. In this case, the MTL7766pac can provide a power supply for two strain-gauge bridges simultaneously.
• Intrinsically safe (IS) operation: By clamping the voltage, the barrier ensures that the energy transferred to the hazardous area is below the level required to cause a spark or heat that could ignite a flammable atmosphere.

STRAIN-GAUGE BRIDGES (cont)

Key features and applications

• Two-channel power supply: It is designed to power two 75-ohm strain-gauge bridges, which is useful for applications monitoring two load cells.
• Removable terminals: It has removable terminals for easy installation and maintenance.
• Bussed power: It can be used in conjunction with other barriers that have a bussed power feed facility to share a protected power supply, like the MTL7798 power feed module.
• Applications: It is used in applications requiring power delivery to two load cells or other sensors in hazardous locations, such as oil and gas, chemical plants, and mining operations.

The MTL7766Pac is a shunt-diode safety barrier that works on the principle of intrinsic safety. Its primary function is to safely connect electrical equipment in a hazardous area to a safe area by limiting the voltage and current that can enter the hazardous environment, thereby preventing an explosion.

Working Principle

The barrier is a passive component that uses a combination of Zener diodes and series resistors with a fuse to achieve its safety function.

1. Zener Diodes: These are connected between the signal lines and the safety earth connection. Under normal operating conditions, the voltage across the diodes is low, and they do not conduct current. If a fault occurs in the safe area, causing an overvoltage, the Zener diodes break down and safely divert the excess current to the dedicated high-integrity safety earth, preventing the voltage in the hazardous area from rising to an unsafe level.
2. Series Resistors: These are included in the circuit to limit the current that can flow into the hazardous area under fault conditions. The resistance value is carefully selected to ensure that even the maximum possible voltage cannot cause enough current to generate a spark capable of igniting a flammable atmosphere.
3. Fuse: A fuse is placed in the line to permanently disconnect the circuit if a prolonged overcurrent condition occurs, ensuring the safety components (diodes and resistors) are not damaged and the safety function is maintained.

Key Features and Application

• Intrinsic Safety (IS): The core principle is "prevention rather than containment" of explosions, allowing for "live maintenance" in the hazardous area.
• Application: The MTL7766Pac is typically used for load cell applications (e.g., weighing systems). It provides power to the load cell bridge while other barriers (like the MTL7761Pac) handle the sense and pick-off signals, offering the advantage of a lower voltage drop.
• Installation: It is a DIN-rail mounting barrier designed for quick and easy installation, securing a reliable IS earth connection through the rail itself.
• Non-Polarized: The 'ac' in the model number (MTL7766ac, which the Pac version is based on) indicates it is a non-polarized, dual-channel barrier, which retains the 'earth-free' nature of the signal and is unaffected by earth faults on the primary element.

 Full Datasheet