Loop Powered Transmitters

A loop-powered transmitter gets its operating power from the minimum terminal voltage and
current available at its two terminals. With the typical source voltage being 24 volts DC, and the
maximum voltage dropped across the controller’s 250 ohm resistor being 5 volts DC, the transmitter
should always have at least 19 volts available at its terminals. Given the lower end of the 4-20 mA
signal range, the transmitter should always have at least 4 mA of current to run on. Thus, the
transmitter will always have a certain minimum amount of electrical power available on which to
operate, while regulating current to signal the process measurement.
Internally, the loop-powered transmitter circuitry looks something like this:

All sensing, scaling, and output conditioning circuitry inside the transmitter must be designed
to run on less then 4 mA of DC current, and at a modest terminal voltage. In order to create loop
currents exceeding 4 mA – as the transmitter must do in order to span the entire 4 to 20 milliamp
signal range – the transmitter circuitry uses a transistor to shunt (bypass) extra current from one
terminal to the other as needed to make the total current indicative of the process measurement.
For example, if the transmitter’s internal operating current is only 3.8 mA, and it must regulate loop
current at a value of 16 mA to represent a condition of 75% process measurement, the transistor
will bypass 12.2 mA of current.
Early current-based industrial transmitters were not capable of operating on such low levels
of electrical power, and so used a different current signal standard: 10 to 50 milliamps DC.
Loop power supplies for these transmitters ranged upwards of 90 volts to provide enough power
for the transmitter. Safety concerns made the 10-50 mA standard unsuitable for some industrial
installations, and modern microelectronic circuitry with its reduced power consumption made the
4-20 mA standard practical for nearly all types of process transmitters.