Why range 4 – 20 mA?
Since a lot of years the use of 4-20 mA standard range has been popularized because it has many advantages that we are going to discuss in this article. At the end you will understand why we use this range.
Many of its advantages are the consequence of raising the 0 point to 4 mA for having a “live zero”.
1- We can detect the wire break when we are below 4 mA. Values under 4 and over 20mA can be used for detecting signal failures. Diagnosis is, no doubt, a great advantage.
2-Two wire field transmitters, the most common, don’t need to be powered, since they do it through the 4-20 mA loop. This is one of the biggest advantages, especially in installations with many instruments. If zero point were 0mA we couldn’t power the transmitter when we were under 3,6mA.
3-Range 4-20 has a 1:5 ratio which is the same that range 3-15psi, very usual in pneumatic. It makes the calculations easier.
4-Range 4-20 mA can transmit HART digital data through the same wires without any interference between both of them.
5-Range 4-20 mA can be used for intrinsically safe signals in hazardous areas because it allows line check.
6-A current signal, generally, is more immune to electric noise than any other voltage signal (0-10 VDC, 1-5 VDC) and also can work in long distances (more than 1 km if nominal power is 24 VDC; raising the nominal value can allow longer distances).
7-In long distances and with intermediate junction boxes there are voltage drops that make impossible and even inadvisable to work with voltage ranges.
8-Using a standard multimeter we can detect failures or check the 4-20 mA loop easily. Besides there is no personal risk if we touch the wire because voltage is 24VDC and the dangerous current threshold for the heart is 30 mA.
9-If we need to convert the signal to a voltage range, for instance inside the control cabinet, in order to wire it to multiple devices, we can do it very easily with a 250Ω resistor. So the signal is converted to 1-5 VDC range.