In my last post, I started a string on 4-20 milliamp current loops by looking at why we use them in the first place. In this post, I’ll take a look at how they work.
The image below illustrates a basic 4-20 milliamp current loop.
There are a number of good resources out on the web that explain the details of how a current loop works, including the Pumps and Systems article I mentioned in a previous post, a primer from Datel and DDC On Line so I’ll just give a quick overview here.
In general terms, current loops use a transmitter between the sensor measuring the process variable you are interested in and the input device associated with the control system. The transmitter generates a dc current that is directly proportional to the measured process variable. For instance a 4-20 milliamp transmitter rated for 0-100°F would generate 4 milliamps at 0°F, and 20 milliamps at 100°F.
Most control systems work with voltages rather than currents, so most current loop signals ultimately are converted to a voltage at the controller or indicator they serve. That is the function of the load resistor in the picture above. The varying current through the load resistor causes a varying voltage, which is picked up by the controller electronics. The table below illustrates the voltages produced by the two most common scaling resistors in use. If you have been around controls for a while, you will notice those voltages are also common input standards.
Obviously, the tolerance of the resistor is important. Going to Radio Shack and picking up a few resistors probably won’t do the job if you find you need a scaling resistor in the field. (That usually happens when you accidentally drop one into the conduit entering the bottom of the control panel you are working in.) Rather, you will need to find an electronics supply house that sells precision resistors (usually in multiples of 25). Or, if you work with this stuff a lot, you probably will end up carrying a few spares with you (having bought 25 when you needed 1 the first time dropped one in the field).
One of the cool things about current loops is they can serve multiple devices. You simply wire the loop in series through the load resistors and as long as the power supply has enough voltage to drive 20 milliamps through the sum of the resistances (including
the wire), you are good to go. You can figure out how much resistance you can drive through by a simple application of ohms law. For instance, a DC power supply rated for 24 volts can drive 20 milliamps through 1,200 ohms as illustrated below.
That should give you a feel for how a current loop works. The next post will look at how you interpret the information the provide and some of the calibration issues associated with them.
Senior Engineer – Facility Dynamics Engineering