MTL7706+ for 'smart' 2-wire 4/20mA transmitters

MTL7706+ for 'smart' 2-wire 4/20mA transmitters        

The MTL7706+ is a 1-channel shunt-diode safety barrier, with builtin electronic overvolt protection, for energising a 2-wire, 4/20mA transmitter in a hazardous area. It is powered from a positive supply of 20–35V dc and delivers a 4/20mA signal into an earthed load in the safe area. It is proof against short circuits in the field and in the safe area and is extremely accurate. The MTL7706+ will pass incoming communication signals up to 10kHz from a ‘smart’ transmitter, while in the outgoing direction it will pass signals of any frequency likely to be encountered.

Since the MTL7706+ has no return channel for energising the load, the entire output of the single ‘28V’ channel is available to power the transmitter, providing high output capability. This channel is negatively polarised, and the safe-area signal is in fact the very current that returns through it from the hazardous area, the novel circuit being energised by a built-in floating dc supply derived from the external dc source of power.

To prevent any leakage through the zener diodes and maximise the output voltage available at 20mA, the floating supply is given a rising voltage/current characteristic. A separate circuit limits the current to protect the fuse in the event of a short circuit in the hazardous area.

With a 20V supply, the barrier will deliver 16.2V minimum at 20mA for the transmitter and lines and consumes typically 45mA at 24V operation.