Two-Wire Current Loop
Celesco currently offers both two-wire and three-wire current loop transducers. This discussion is about our two-wire devices. A typical configuration consists of the transducer, a DC power supply and either a panel meter display or an analog card to a PLC as shown above.
Because all the devices in the loop are connected in series, not all of the voltage supplied by the power supply gets delivered to the transducer. Your PLC (or Panel Meter) and your system wiring act as a load on your power source which needs to be calculated to ensure that enough voltage is available for the transducer.
There are four simple steps below to determine the proper supply:
Step 1: PLC, Display or Panel Meter
Typically the largest voltage drop will be caused by the internal resistance (impedance) of your panel meter or analog card. Values can range from 50 ohms upwards to 200 ohms or more. Please check manufacturer's specifications.
Some applications require multiple I/O devices on a loop such as a display and a PLC. In such cases, you need to record the voltage drop across EACH device!
Step 2: System Wiring
Though wire resistance is usually measured to be only milliohms per foot, a run of a modest distance can quickly add up to enough resistance to cause critical voltage loss. The table below shows the resistance values for stranded copper wire.
|Max Resitance @68°
ohms per 1000 ft.
When measuring the distance of the system wiring, make sure to count the total amount of wire to complete the entire current loop. If for example, you have a 300 foot run from the transducer to your PLC, the total distance of that leg would be 600 feet: 300 down and 300 back!
Step 3: Transducer Voltage
All of our 4...20 mA two-wire transducers require a minimum of 12 volts (DC) to operate correctly. While 12 volts is the minimum, we highly recommend a few extra volts to spare to help insure that all the calculations were carried out and just in case there's something on the loop that is using more voltage than planned for. Breathing room is beneficial.
Warning! the absolute maximum our sensors should ever receive is 40 volts--anything greater and you'll have toast.
Step 4: Calculating the Minimum Voltage Required
Simply add the voltage drop across the PLC plus the drop due to the wiring plus 12 volts and you're done!
If you've got any questions please don't hesitate to give us a call and one of our Applications Engineers would be glad to help.