Analog Circuits Training - Analog 4-20mA Current Circuit Types - 2 Wire and 4 Wire Current Source
4-20mA devices can be put into two categories. 2-Wire transmitters or 4 wire transmitters.
In our Analog Circuits Training - Understanding Analog Circuits 0-10VDC 4-20mA lesson you learned the basics principles of a 4-20 mA signal. Now you will learn how to wire 2 Wire "Current Loop" analog circuits and 4 Wire "Current Source" analog devices.
Current 2 Wire Loop Powered - Loop powered devices are usually identifiable because the they only have two wires attached to them. The are powered by the 4-20mA circuit.
It has the advantage of simplicity and the analog device's low power consumption many times means they carry hazardous area approvals such as Intrinsically Safe (I.S.) and Non-incendive (N.I.) which allows them to be used in explosion proof environments.
However, despite their simplicity, they can be confusing to understand how to wire and troubleshoot. Power must be connected from the positive of your power supply to the positive of your first analog device in the loop. Then connect the negative of that device to the positive of the next device, continuing to do this until the last device where you will connect the negative of that device to the negative of your power supply to complete the loop. This is very confusing in text format so pay close attention to the "+" and "-" signs in the image to the right.
Also note that the order of analog devices in the loop does not affect the functionality of the loop. Also you can have multiple devices reading the same analog device simply by adding them to the loop. Remember that every device consumes some power and adds impedance to the network which will eventually increase the circuit resistance to the point that it cannot drive a 20mA signal.
Current Loop 2 Wire - Where the loop powered circuit requires an external power supply to be wired in the loop, a current source or 4 wire circuit creates its own "source" of power for the 4-20mA loop. It will be externally powered and may be powered by any voltage and either AC or DC such as 24VDC, 110VAC, 220VAC, 480VAC, etc. 4 wire analog transmitters will usually have additional features that can make them more versatile than their 2 wire counterparts such as specific analog scaling or scaling based off of a different set of units than the default. They are also capable of having features beyond that of analog such as relays to close a dry contact at a specific analog signal, connection to an automation network, etc.
While 4 wire analog devices seem like the clear winner, they cost more, will usually not have any type of hazardous classification for explosion proof environments, are not suitable for many harsh environments, and require individually isolated analog inputs which most economical PLCs lack.
Simulating a 4-20 mA Current 2 Wire Loop and a 4 Wire Current Source
Current 2 Wire Device Simulation
1. Connect 24VDC to the black terminal of the Analog Simulator
2. Connect a wire from the red terminal of the Analog Simulator to terminal 2 (mA+) of the trainer's analog option.
3. Connect a wire from terminal 3 (mA-) of the trainer's analog option to 0VDC.
4. Turn on the Analog Simulator and select "Current 2 Wire" mode.
5. Now you can use the UP/DOWN arrows to vary milliamp signal to the meter.
Current Source 4 Wire
1. Connect a wire from the red terminal of the Analog Simulator to terminal 2 (mA+) of the trainer's analog
2. Connect a wire from the black terminal of the Analog Simulator to terminal 3 (mA-) of the trainer's analog option.
4. Turn on the Analog Simulator and select "Current Source.
5. Now you can use the UP/DOWN arrows to vary milliamp signal to the meter. The Analog simulator generates the milliamp signal from it's 2 AA batteries.