STRAIN-GAUGE BRIDGES- MTL7761ac

　　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 12V (±6V), 214Ω (107Ω + 107Ω) source. If the resistive bridge elements are 230Ω, then the voltage applied to the bridge will be 6.2V, and if the bridge resistances are 350Ω, then the bridge's applied voltage will be 7.4V. An MTL7764ac can be used to sense the bridge supply voltage. An MTL7761ac is used here for the mV output.

　　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

　　ANALOGUE OUTPUTS

　　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.