4mA-20mA loop behaviour when out of range

Question

I have a temperature sensor that records temperatures from 10ºC to 30ºC, but in my project they only want temperatures to be read in the range of 15-25 with 4ma when 15 and 20ma when 25.My problem is what is supposed to be the behaviour of a 4-20 loop when the sensor records <15 and > 25.Should it freeze on the minimum and maximum value or should it take 0 as value? thanks

Answer 1

It's not clear if you will implement a receiver or the transmitter for the current loop. Suppose you will make a 4-20mA transmitter. Usually the 4-20mA loop would give you 4mA at minimum and 20mA at maximum. Those 4mA serves to for a wire break detection and/or to supply the transmitter from the loop.

However I have seen industrial transmitters that they also have control of over range- too low or too high, it means the signal goes little below 4mA and little higher than 20mA.

Now, this is Siemens PLC 4-20mA transmitter, don't look DAC values dec&hex those are specific to that PLC. It goes from 0mA to 22.81mA, If you intend to build a tranmitter that is powered from the loop, so called passive transmitter (2 wire connection), then you won't be able to output less than cca. 3.2mA - it depends on power consumption of your transmitter. If you intend to build a active transmitter (4 wire: 2 wire loop + 2 wire supply), then it's up to you to decide if you want to go down to 0mA output (wire break can't be detected) or to stop diving at certain low value output.

Answer 2

You can go slightly beyond the rated range and still assume the numbers are reasonable, which is more or less a standard from the analog world, but also hard transition to a presumed failure might be done these days.

Below about 3.6mA or about perhaps 22mA can be considered a sensor or loop failure. A sensor failure into a working loop-powered transmitter might not be able to reduce its current much below 3.6mA because some current is needed to internally to power the circuitry. On the other hand, if the wiring fails the current will drop to zero.

For example, a broken thermocouple would typically manifest itself at the transmitter output as >>20mA current, which would cause (through a controller) the heat to be turned off, which is usually (but certainly not always) the safer alternative to turning it on. The exact limiting depends on the design but I've seen >30mA from a commercial transmitter.

Interpreting (say) > 20mA as 500°C on a 0~500°C transmitter would not be a good approach, in my opinion. For example, if the controller setpoint is set to 500.1°C the heat might never turn off, even as the plant melts.