Eaton MTL-644 Galvanic Isolator

The MTL-644 is a loop-powered galvanic isolator designed to provide electrical isolation and signal conditioning for 4-20mA current loops. It is widely used in process industries to break ground loops, eliminate noise, and protect sensitive control room equipment from voltage transients in the field.

Operating Principle

The MTL-644 requires no external power supply; it derives its operating power entirely from the 4-20mA input signal. It uses transformer isolation (with proprietary modulation technique) to transfer the signal across an isolation barrier, while maintaining accuracy and low output ripple. Input and output are electrically isolated to 1kV (AC or DC).

Performance Specifications

The isolator has an accuracy of better than 0.1% of span, with linearity and hysteresis errors below 0.05%. Temperature drift is less than 0.02% per °C. The output follows the input with a typical response time of 50ms (to 90% step). It can operate with input loop voltages as low as 6V (at 20mA) and supports up to 30V loop supply. The output voltage drop is approximately 6V at 20mA, allowing integration into most control loops.

Isolation and Safety

Galvanic isolation eliminates ground loops and provides protection against common-mode voltages up to 250Vrms. The unit meets the requirements of IEC 61010-1 for insulation and is suitable for installation in safe areas. It also complies with EMC directives (IEC 61326) for industrial environments. The isolation barrier withstands 1.5kV for 1 minute (test).

Mounting and Connections

The MTL-644 is housed in a compact, UL94 V-0 rated plastic enclosure measuring just 12.5mm wide. It snaps onto standard 35mm DIN rail (EN 60715). Terminals are plug-in screw type, accommodating wires up to 2.5mm². LED indication (green) confirms loop power status. The unit is polarity protected and can be wired in series with the transmitter or receiver side.

Environmental Ratings

Operating temperature range is -20°C to +60°C, with storage down to -40°C. It is rated IP20 for the terminals. Humidity tolerance up to 95% non-condensing. The design is robust against vibration and shock typical of industrial control panels.

Applications

Typical uses include isolating 4-20mA signals from transmitters, PLC analog inputs, temperature controllers, and data acquisition systems. It is also employed to protect DCS cards from ground faults and to enable signal sharing between multiple instruments without interaction.