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MTL7700 SERIES BARRIER APPLICATIONS
►Analogue inputs (high level)
2-wire transmitters, 4/20mA, conventional and smart
The recommended barrier for use with 'conventional' and 'smart' 4/20mA transmitters (fed by a 26V regulated supply) is the MTL7787+.
This provides up to 12.9V (14.6V for MTL7787P+) at Vwkg and 20mA for a transmitter and its lines as well as 5V for the typical 250Ω load.
This application and this barrier is suitable for use with the optional power bus facility.
The MTL7706+ is recommended for applications where an unregulated supply of up to 35V is used. It provides 16.0V for conventional and smart transmitters at 20mA, as well as 5V for a typical 250Ω load.
With the MTL7706+ terminal 3 is negative with respect to earth, so the connections to terminals 3 and 4 should be reversed.
The 3-wire transmitters used with vibration monitoring equipment are invariably supplied by a –24V dc power supply – hence the recommended barrier choice is the negatively-polarised MTL7796–.
►Analogue inputs (low-level)
Thermocouples and mV sources
The recommended barrier for thermocouples and mV sources is the MTL7760ac. This 2-channel non-polarised barrier retains the ‘earthfree’ nature of the signal and, providing the receiver’s input ‘floats’, rejects common-mode ac and dc interference up to at least 7V and is unaffected by earth faults on the primary element.
AC sensors, photocells, microphones and turbine flowmeters
The MTL7760ac is the recommended choice for these devices. While many of these are designated ‘simple apparatus’ and thus do not need certification, note that some ac sensors may be subject to a significant level of inductance and will therefore need to be designed and certified for hazardous-area locations.
Slidewire displacement transducers
The simplest choice is the MTL7760ac. This barrier supplies power and brings back a unipolar signal.
For 3-wire RTDs, a single MTL7755ac barrier is the most economical choice. This is suitable for use with a floating bridge – the two leads from the bridge arms are protected by the barrier with the third (supply) lead being earthed through the barrier. The barrier has a low end-toend resistance of only 19Ω/channel to minimise span changes and its channels track within 0.15Ω (between –20°C and +60°C) to minimize zero shift with temperature.
If the bridge circuit is already earthed, the third barrier channel provided by an MTL7756ac is needed. For extreme accuracy, 3 channels and an earth-free bridge can be used, a configuration that cancels out the small errors due to barrier leakage.
Channels 1 and 2 (those between terminals 1 & 2 and 3 & 4 respectively) track to within 0.15Ω (between –20°C and 60°C).
4-wire constant-current circuits do not need matched barrier resistances and can be protected by two MTL7761ac barriers. If the increase in loop resistance is too great, use two MTL7755ac barriers instead.
Single strain-gauge bridges
This shows an arrangement using two or three barriers, which is safe in IIC gases. With the MTL7761ac, the circuit is powered from a 14V, 230Ω source; if the bridge resistance is 230Ω, then the bridge voltage is 7V. If the bridge resistance is 350Ω, then the bridge voltage is 8.4V.
An MTL7764ac can be used to sense the bridge supply voltage.
An MTL7761ac is used here for the mV output.
An MTL7766Pac provides 12.3V for a 350Ω bridge with a 20V supply.
MTL7761Pac's can be used for the sense and pick-off circuits.
►STRAIN-GAUGE BRIDGES (cont)
Double strain-gauge bridges
Quite frequently there is a demand to monitor two load cells, and a possible circuit, safe in IIC, is shown.
Here, the lower voltage drop of the MTL7766Pac is an advantage.
The MTL7766Pac supplies power to the bridge(s) while two MTL7761Pac barriers interface with the sense and pick-off circuits.
Using 350Ω bridge systems, the following voltages are available from an MTL7766Pac with a ±10V supply:
1 bridge: 13.11V
2 bridges: 9.75V
Controller outputs (I/P converters)
The single-channel MTL7728+ with a voltage drop of 6.66V at 20mA is the recommended choice for most controller outputs. Higher-power versions are available: the MTL7728P+ (5.1V drop) is suitable for IIC applications; the MTL7729P+ (3.68V drop) for IIB applications.
For controllers with an output circuit separated from the 0V rail by the control transistor, the 2-channel MTL7787+ is the preferred choice as the return channel can handle up to 26.6V allowing the control signal to be turned off completely. The voltage drop is 8.1V at 20mA. A higherpower version of the latter, the MTL7787P+, is also available. The return channel of these barriers handle up to 26.4V and the maximum voltage drop is only 6.38V.
The MTL7787+ and MTL7787P+ are also suitable for controllers containing a resistor which enables the return current to be monitored for high-integrity operation.
►Digital (on/off) inputs
The normal choice is the MTL7787+/7787P+ with a regulated supply.
The MTL774X modules are recommended for applications where an unregulated supply of up to 30V for relay output modules, or 35V for solid state output modules, is used.
The MTL7789+ offers a dual channel passive barrier for switch inputs where the input current for each channel is <10mA.
Switches / Proximity detectors
MTL’s range of new switch/prox input barriers provide the user with a choice of relay and solid state outputs in single and dual channel versions.
The MTL7741 is single channel with a changeover relay output.
The MTL7742 has a single channel solid state switch that can be configured to switch from a power rail or down to ground. This is also ideal for high switching frequency applications.
The MTL7743 and MTL7744 are dual channel versions affording very high packing densities. Power must be provided to these modules using the power bus facility.
The MTL7745 is a single channel proximity input (or switch input if ‘end of lines’ resistors are fitted) with relay contacts providing switch and line fault status. The LFD relay contacts close when a fault is detected.
►Digital (on/off) outputs
Alarms, LEDs, solenoids valves, etc
For these applications, the MTL7728+ is recommended. Higherpowered versions are available: the MTL7728P+ is suitable for IIC applications; the MTL7729P+ for IIB applications.
If the control switch is to earth, then the 2-channel MTL7787+ barrier should be used, or, alternatively, the MTL7787P+ higher-power version.
If the supply is poorly regulated use the MTL7707+.
The MTL7707+ is recommended for applications where an unregulated supply of up to 35V is used.
►POSITIVE DC SYSTEMS
Low-level to 12V dc systems
The two channels of the MTL7764+ and MTL7767+ can be combined safely in IIC.
The MTL7764+ can be used for low-level logic return signals whilst the MTL7767+ is used for 6V dc and 12V dc systems.
18V dc systems
The single-channel MTL7722+ is recommended for 18V dc systems.
►AC AND DC SYSTEMS
High-level ac and dc systems
The versatile star-connected MTL7765ac and MTL7778ac allow Vwkg to be developed from each channel to ground but only allow Vwkg to be developed between channels. This provides some common-mode voltage capability and can allow higher cable parameters to be used.
►NEGATIVE AND FLOATING POWER SUPPLIES
Digital (on/off) outputs
The MTL7728– is used with a negative power supply and positive earth. Typically used for digital inputs or outputs, as shown.
The MTL7728– can also be used with floating power supplies, for transmitters.
►SPARE CABLE CORES AND SCREENS
The MTL7799 dummy barrier is used primarily for securing and earthing unused cables and screen connections. Hazardous area terminals 3 and 4 are internally connected to the DIN-rail mounting/earth connection. It also provides a power bus connection for direct connection of power for modules such as the MTL7743 and MTL7744 where no power supply screw terminal is provided.
►POWER BUS APPLICATIONS
The PB7700 power bus is invaluable for saving installation time and wiring when connecting a 24V dc power source to a number of barriers.
Typical applications include hazardous-area switches, 4/20mA transmitters and proximity detectors. The diagram illustrates the configuration for 4 barriers but up to 40 barriers can be served by this method.
The MTL7798 power feed module would normally be used with standard barriers such as MTL7787+ and MTL7787P+ because the current/voltage trip protection mechanism of the MTL7798 protects the fuses in the barriers.
The MTL7799 dummy barrier can be used instead of the MTL7798 for direct ‘feed-through’ connection of a 24V dc supply onto the power bus. Looping the power feed to each end of the bussed power allows the removal of individual barriers without loss of power to others in the chain.
Other units that can use the power bus facility:
Contact Person: Ms. Steven