Hook-up:
4 – 20 mA loops are series connected elements (like flashlight batteries). A simple overview of the loop
concept might help.
The elements of a current loop are connected in series. An easy way to remember the connections is (+)
-> (–), (–) -> (+), then reverse the power supply connections (+) -> (+), (–) -> (–). Everything is
connected in series. Although this looks easy when it is drawn like below, mistakes are often made when
we go through junction boxes.
What if I need a PLC input? This is a common question. To add a PLC or computer input, just insert it
in the loop. Remember (+) -> (–), (–) -> (+) except at the power supply.
Adding a computer or PLC input is done as follows:.
Grounding:
It is important to note that there can only be one ground in a 4 – 20 mA system. Multiple grounds will
cause erroneous readings.
Compliance Voltage or Loop Power Supply Voltage:
Compliance of loop power supply voltage required must be greater than the maximum voltage in the loop.
In the diagram above the available compliance voltage is the power supply voltage. There are three
items that must be checked when determining loop compliance or power supply voltage.
1. The compliance voltage must be great enough to fully power all of the loop devices at 20 mA. It
is usually better to assume 25mA max to allow yourself some “head room”. You must also have
enough voltage available to account for the drop in the wires. (250’ of # 20 instrument wire =
5. Voltage drop at 25mA = 125 mV).
2. The loop power supply voltage must not exceed the maximum voltage rating of any device in
the loop. (For instance the transmitter above has a maximum loop voltage of 30 VDC.)
3. Be aware of safety. If you are using low voltage wiring, you generally want to keep the loop
voltage below 28V DC.
