Current output transmitters

The most common analog style transmitter is the current output device. This device will convert the signal from the probe into a scaled output that is transmitted on a 4 to 20 milliamp output. In a typical transmitter system, the transmitter reads the device input and calculates what the appropriate scaled output should be.

As an example, a 0 to 500°F Temperature input would be scaled from 4 to 20 ma. This means that a temperature input of 100°F would be transmitted down the wires as a current of 7.2ma. Current loop systems work over long wire runs, up to 10,000 feet, and are fairly immune to noise induced on the wires. They are also fairly economical.

On the receiving end the control system/computer must convert this signal back into something it can use.

The most common method is to flow the current thru a precision resistor and measure the voltage generated across the resistor with a data acquisition card.