Originally posted by: gadge
I've never worked with 3 wire instruments before so this is a new one to me.
I'm installing 3-wire gas sensor's in a haz area that will go direct into our siemens S7 PLC (iSP haz area I/O).
Apparently the loop power is not enough so i have to supply the sensor with external power and use a 4-wire AI card abd set the sensor as source and not sink powered.
Has anyone got a loop of how this should be wired?
2 of the wires provide power to the instrument +24V DC & 0V DC. The third wire provides the current drive to the card 4-20mA. So you are correct that the instrument is the source. Make sure that both the instrument and the I/O card use the same power supply.