|
MTL Barries - MTL7755ac - 2 or 3 Wire RTDs (floating bridge)
The MTL7755AC is a shunt-diode safety barrier that protects hazardous area circuits by using Zener diodes to limit voltage and a terminating resistor to limit current in case of a fault on the safe side. A fuse provides an additional layer of safety by protecting the diodes, and two stages of voltage limitation ensure continued protection even if one stage fails. The "ac" in its name signifies it is a non-polarised barrier, suitable for AC and DC signals.
How it works
- Energy Limitation: In the event of a fault in the safe area (like an overvoltage), the Zener diodes on the barrier are designed to clamp the voltage down to a safe level.
- Current Limiting: A series resistor then limits the current that can flow through the circuit, preventing a spark that could ignite a flammable atmosphere.
- Fuse Protection: A fuse is in place to blow if a fault causes a sustained overcurrent, further protecting the diodes and the rest of the circuit.
- Two-Stage Safety: The barrier uses two stages of voltage limitation to ensure safety even if one of the stages fails.
- Non-Polarised ("ac"): This type of barrier can accept and/or deliver both positive and negative voltages, making it suitable for AC and DC signals.
| RTD Connection |
 |
Key features of the MTL7755AC
- Intrinsically Safe: It is designed to keep the energy in the circuit below the level required to ignite a hazardous atmosphere, meeting "ia" and "IIC" requirements for explosive atmospheres.
- Low Resistance: It has a low end-to-end resistance (19Ω) to minimize impact on the measurement signal, which is important for applications like RTD (resistance temperature detector) measurements.
- Application: It is commonly used for 2- or 3-wire RTDs and floating strain-gauge bridges.
The MTL7755AC is a shunt-diode safety barrier designed for use with 2- or 3-wire RTDs (Resistance Temperature Detectors) in hazardous areas. Its working principle is based on the concept of intrinsic safety (IS), which involves limiting the electrical energy (voltage and current) that can enter the hazardous area to a level incapable of igniting flammable atmospheres.
Working Principle
The barrier operates on a simple, passive principle using Zener diodes and a fuse:
- Energy Limitation: The primary function is to restrict the voltage and current passing from the safe area (where power sources are located) to the hazardous area.
- Zener Diodes: A combination of Zener diodes is connected between the signal lines and an integral safety earth connection. Under normal operating conditions, the voltage across the diodes is below their Zener breakdown voltage, so the current flows through the process loop.
- Voltage Clamping: In the event of a fault in the safe area that causes an overvoltage, the Zener diodes break down and conduct the excess current to the safety earth, thus clamping the voltage at the hazardous-area terminals to a safe, low level.
- Current Limiting Fuse: A fuse in series with the input limits the current to a safe maximum level in the event of an overcurrent fault. If the current exceeds the fuse's rating, the fuse blows, permanently breaking the circuit and preventing excessive energy transfer into the hazardous area.
- Earthing: The effectiveness of this principle relies on a highly reliable, low-resistance connection to a dedicated safety earth. The MTL7700 series barriers clamp securely onto a standard DIN-rail to achieve this high-integrity earth connection.
Application (MTL7755AC specific)
The "AC" designation indicates that the channels of this specific barrier can withstand voltages of both polarities (AC or DC application). It is designed for low-level analogue input applications using 2- or 3-wire floating bridge RTDs, with a working voltage of approximately 3V and a maximum current of 300mA.
In summary, the MTL7755AC ensures safety by acting as a "safety clamp" that diverts excess energy harmlessly to earth and uses a fuse to limit current, thus preventing potential explosions in flammable environments.
For more technical details, you can consult the official MTL7700 Series Datasheet from Eaton (MTL Instruments' parent company).
 Full Datasheet
|
