What is a Loop Signal Isolator?
The DAT511 and it sibling the DAT511H are passive loop powered Signal Isolators. A Signal Isolator removes the ground loops that may be present when instruments have different potentials on their grounds. If both the field Instrument (EG Magnetic flow meter) and the control room receiving Instrument (PLC) have different grounds, most likely there will be differential voltage across these grounds.
The current will flow from one side to the other which will disturb the 4-20mA signal from the field instrument to the control room equipment.
Sometimes there may not be a power supply ground on the field instrument but the field instrument may be physically attached to a building and may use the steel structure as a potential ground.
There are many Datexel Isolators that can eliminate this flow of current. The simplest Datexel Isolator is the DAT511 and it provides 3000 Vac of isolation and is self powered using the 4-20mA as its own power supply. It can be fitted on a din rail mounted in the control room or in the field.
The author has seen ground loops to be sometimes erratic. If a certain motor or conveyer belt is running this could affect the ground on the field instrument, hence it may only be a problem whilst the motor is running. It is always good practice to allow the 4-20 mA signal to be isolated from the ground. The DAT511H is the Hart version. Sometimes there are several signals coming into a field instrument such as a mass flow unit. It is advisable to isolate each and every loop.
Is there ever a problem with Loop Powered Isolators?
Yes, they could draw too much power. Even though the DAT511 only has a volt drop of 3V (the lowest in the industry) it could draw too much energy to indicate the correct reading at the control room (PLC). If that is a problem we would then recommend the DAT4135.
The DAT4135 could be used if the DC power is coming from the control room side of the Isolator. This will actually boost the output so it can be driven into an impedance of 1000 Ohms. The isolation of the DAT4135 is 2000Vac. The DAT4135 can also be used as a signal conditioner. Not only does it isolate between the 4-20mA’s it can convert Thermocouples, Rtd’s, Voltages, mV, Potentiometers and Resistances into current as well. The input impedance is only 50 Ohms. Furthermore it is available for intrinsically safe loops.