Google Patents CN104519298A - VGA connector-based ground loop isolator for high-definition video ( Register here and scroll down to select your choice of eNewsletters free.CN104519298A - VGA connector-based ground loop isolator for high-definition video Edited by Peter Welander, process industries editorĬontrol Engineering Instrumentation & Sensors eNewsletter Signal isolators are available from Moore Industries and Dataforth. This article was prepared with information and graphics supplied by Moore Industries-International. The isolator can operate with powered inputs from both sides, thus restoring normal operations on the loop. When neither device can be eliminated, the solution is a signal isolator. Solve “bucking power supplies” : When two devices (such as a 4-20 mA transmitter and a DCS) are trying to source power to a loop, the result is a non-functioning loop. Signal conversion : An isolator can perform signal conversions, such as from 1-5 V to 4-20 mA, to allow devices with incompatible signal types to interface with one another.ĭivert and protect signals : Using a signal isolator, you can send the output from one transmitter to a second location, protect expensive monitoring/control equipment by eliminating common electrical paths, or create a buffer between devices to allow interruption of one leg of a loop without impacting the other.Īmplify (boost) signals : If a signal is weak, or if additional instruments need to be installed on a loop, a signal isolator can boost the signal. Isolators can perform several tasks, besides eliminating ground loops: Four-wire isolators work well in applications where power is readily available. Some isolators have an input impedance as low as 2 ohms which helps maintain voltage.Ī four-wire isolator (Figure 2) is powered by an external source, such as 117 V ac or 24 V dc. If the loop power supply cannot support this burden, the loop will become weak or overloaded. At 20 mA, this can represent a full 5 volt drop on the loop. Many devices have input impedances that are around 250 ohms. When evaluating isolator specs, look at the input impedance or burden to the loop. This makes wiring an OLP isolator to a two-wire transmitter very simple.įigure 2: Four-wire isolator application. Output loop powered isolators are more convenient because most control system current input cards offer optional power for two-wire transmitters. Most two-wire isolators are output loop powered (OLP), but there are two-wire isolators that are input-loop powered (ILP) too. Two-wire isolators (Figure 1) draw power from the 4-20 mA process signal loop, so they require no additional power supply or lines which can save on wiring costs. There are two basic types of signal isolators: Moreover, an isolator also eliminates another problem: ac continuity noise, otherwise known as common mode voltage. Without this path, there is no way for any stray current or voltage to reach the receiving device. These devices break the galvanic path (the dc continuity) between grounds that are at different earth potentials. When multiple grounds cannot be eliminated, the solution is to use signal isolators. (Ground loops are also common in audio equipment, including home sound systems, where they turn up as annoying hums.)Įliminating grounds may not be possible for some instruments (e.g., thermocouples) because they require a local ground for accuracy, or they must be grounded for personnel safety. The I/O point receiving the signal can’t differentiate between the desired and corrupted signal, so the readout or DCS will not reflect true process conditions. Ground loops cause problems by adding or subtracting small current or voltage levels to or from the process signal. Usually the circuit is completed when the process signal wire is connected from the transmitter (ground potential #1) to the receiving device (ground potential #2). These grounds must have a galvanic path or circuit established between them. Ground loops are particularly troublesome since they are capable of serious signal disruption and their effect can be intermittent.įor a ground loop to occur two things must be present:Īt least two different grounds that are at different potentials. Engineers that have been trained to troubleshoot digital networks may not know how to handle these systems, so we offer this tutorial as a help for this older but still critical technology. Analog wiring is still very common, with 80+% of plants saying they have 4-20 mA communication with instrumentation. Analog wiring from a sensor to I/O point is prone to certain types of problems than can disrupt accurate readings.
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