Current Loops

Current loop signals (i.e. 0-1 mA DC, 4-20 mA, 10-50 mA etc) ride on the supply or signal voltage supplied by a power supply. This can be a separate device, such as the back of panel mounting Deltron 112A, which will convert a standard 120VAC or 240VAC line input into a stable 24V DC output @ 1.2 amps (1200 milliamps). Or the power supply might come from a smart digital panel meter, such as the Red Lion PAXP0000. The meter is powered by 120/240 and outputs a 24 VDC @ 50 milliamps. The Deltron example could power up to 60 separate 4-20 mA signals (1200 milliamps divided by 20 milliamps = 60); the Red Lion however only two. Sizing should really be based on at least 25 milliamps per device.

The power supply's 24 Volt DC output will be reduced by the voltage drop of each instrument in the series connection of the loop, and by the length and gauge of the wire utilized. Care must be taken that the voltage is not dropped below the minimum operating range of the instruments used in the loop. The signal voltage in the loop is the difference between the voltage at one terminal (referenced to ground) and another terminal (see diagram above).

Fortunately, most Digital (about 20 ohms) and Analog Instruments (about 3 ohms) have a very low resistance. Therefore, their utilization of voltage in the series circuit is usually minimal. One exception is if a receiver, such as a chart recorder or other such device, is really a 1-5 VDC device, and takes the 4-20 mA signal only through a 250 ohm resistor mounted on the signal input terminals of the device. According to Ohm's Law, the maximum drop for such a device is 5 VDC (250 ohms x 20 milliamps or .02 amps = 5)

Ohm's Law: Voltage = Current x Resistance or E=I x R

Current =Voltage Divided by Resistance or I=(E/R)

Also fortunately, the length and gauge of instrument wire utilized has also usually a minimal effect on the voltage drop. Let us take a common situation with a single sensor and readout, and a distance between the two of 1000 feet:

A common instrument signal wire is 22 Gauge. The resistance of this wire is only 0.0165 per foot (each leg must be considered). A thousand foot run from the sensor to the power supply would provide the following resistance: 1000 x 2 x 0.0165 = 33 ohms.

By Ohm's Law, Current times Resistance (I x R) = Voltage (E).

20 milliamps (.02 amps) x 33 ohms = 0.66 volts.

If you have a 24 volt DC power supply, you would still have 23.34 volts left over. Even 10,000 feet would only take 6.6 volts away.

Caution: If you remove an instrument from the ioop, the entire ioop goes down. Another consideration is the prevention of GROUND LOOPS. Please click on the underlined for a discussion on this topic.