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Isolated DC Signal Conditioners Applications

October 23, 2013 John Wilkerson AC Voltage and Current, DC Voltage and Current, DC/AC Isolators, DC/DC Isolators, Signal Conditioners No comments

Value of Isolated 4ma to 20ma Signal Conditioners

DC signal conditioners with 4mA to 20mA outputs provide the ability to send signals over long wires to more than one instrument. The current from the signal conditioner is a constant current proportional to the signal into the signal conditioner. The constant current allows long leads. The level of the measured signal is proportional to the voltage drop across the resistor in the input of the instrument receiving the signal. Voltage drops across the long wires have no effect on the constant current. The constant current allows several instruments to have their inputs wired in series and the current through each input is identical.

Isolated Signal Conditioners provide a further important value. The isolation circuit has no direct electrical connection between the input and output circuits. The input signal is passed through a transformer as an AC signal or through an optical isolator as a light beam.

These two types of circuits make the signal conditioner able to have a high voltage common mode signal while still processing a very low level signal from a sensor or other instrument.

The isolation from input to output also keeps extraneous currents from odd sources from flowing on the common signal lead through the signal conditioner. This extraneous current is usually called a “ground loop”. The isolation breaks this ground loop.

Prevent Ground Loops

A typical ground loop is created when a thermocouple has its welded junction grounded to earth through its mounting hardware. The thermocouple signal is very low in amplitude. If the thermocouple wire has a completed circuit from the thermocouple mount to the earth at the signal conditioner and a current flows in the thermocouple wire, it will impose a voltage on the thermocouple signal. The typical ground loop noise is caused by AC current flowing in the wire. With an isolated signal conditioner, there is no path for current to flow to earth at the conditioner. The isolator breaks the ground loop. The thermocouple signal is amplified and passed on through the isolation circuit to the conditioner output with no noise from the ground loop.

Process Small Signals On High Common Mode Signals

A common mode signal is a signal which is connected to both inputs of a signal conditioner and the conditioner has an output equal to a zero input. An example would be a thermocouple welded to the positive terminal of a 138V battery so the temperature of the battery terminal can be ascertained. The conditioner processes the thermocouple voltage but is not affected by the 138VDC common mode voltage from the battery.

Another example would be a battery with a resistor in series with the + terminal. A signal conditioner can measure the voltage drop across the resistor so the current through the resistor can be determined. The measurement is unaffected by the battery 138V common mode voltage.

A great value of isolated signal conditioners is that common mode ranges of 1000V to 2000V are not difficult to find.

Split One Signal into Many Separated Individually Isolated Signals

A common requirement of 4mA to 20mA outputs from DC input isolated signal conditioners is a need to measure 1 sensor and send the signal to a number of locations in the 4mA to 20mA form.

One method of creating several isolated signals is to connect the inputs of several 4mA to 20mA input/output signal conditioners in series. Each conditioner will provide an isolated output.

Multiple output conditioners are available. If dual output conditioners are used as above, one gets 2 outputs per each input.

Another method is to use one 4ma to 20mA input conditioner. Using voltage input conditioners with 4mA to 20mA outputs, connect the voltage inputs in parallel across the input of the current input conditioner. 250 ohm loads are common inputs for current inputs. This creates 1 to 5V drop across the 250 ohm resistor.

Isolator Creates a Separately Isolated Signal From An Existing Signal With No 2nd Power Supply Required

In areas where power is lacking and another isolated signal is required, a loop powered isolator can do the job. A loop powered isolator uses the 4mA to 20mA current from a signal conditioner to power another circuit to create another isolated output.


Mighty Module (MM)	DR Series	DIN-Mod (DM)	SR Series	TW8 Series

Ground Loop Primer

August 22, 2012 John Wilkerson DC/DC Isolators, Ground Loop, Installation Help No comments

Ground Loop Primer Printer Friendly PDF

One of the most frustrating problems of the measurement and control industry is that of the ground loop. Its effects can appear and disappear with no apparent reason and can range from mere annoyance to downright destructive. It seems that ground loops carry some mystical connotation to the point that our industry has made it a “catch all” culprit for anything that cannot be explained.

While ground loops can be complex problems and may not always be predictable they can be understood and dealt with if we have a little insight into just what they are and how they can affect a transmitted signal. Let us first refer to Figure 1.

This Figure depicts what we might consider to be a typical measurement loop. It has a transmitter sending a signal to a receiver, some finite distance away, over a pair of wires. One side of the signal current has become grounded via internal circuitry and ultimately is tied to earth ground usually via the instrument case.

As depicted in Figure 1, this measurement loop would probably work fine and not have any influence from ground loop currents. However, reality sets in and we have to abide by plant safety procedures, the National Electric Code, etc. Safety procedures almost always will mandate that each piece of equipment be grounded to earth at its respective installed location. This is where the trouble starts.

Once we ground two pieces of equipment at two different locations we have set the stage for ground loop problems. If we could take a volt meter (Figure 2) with very long leads and measure the voltage between the ground points of the transmitter and the receiver we would measure some voltage.

It may measure in millivolts or it could be many volts. Either way, if there is a potential difference, then current will flow between these two points. Since the earth presents itself as a resistor between these two ground points the amount of current that flows between the points will be directly proportional to the voltage difference and inversely proportional to the resistance.

For those who are fans of Ohms Law you will recognize this equated as I=E/R. I being the ground current; E being the voltage between the ground points; and R being the resistance of the earth between the two ground points.

Figure 3 shows that we now have two currents that can flow through the wiring between the transmitter and receiver. If it all stopped here we could just calibrate the measurement loop to nullify the effects of the ground current and go on about our business.

Many times this is exactly what happens. A technician calibrates the loop and comes back a few days later to find that his calibration is no longer accurate. What has happened? It could be a lot of things. Maybe it rained and the resistance of the earth changed. Suffice it to say there are many phenomenons, either natural or man-made, that can change the resistance or the voltage between the two ground points thus effecting the calibration of the loop.

It is apparent that we are “fighting a losing battle” thinking we can anticipate the interactive affect of ground loop currents on our measurement loop. What can we do to get around this problem? The answer is to provide DC isolation between each component in our loop. (Figure 4) DC isolation can be accomplished either in the transmitter, the receiver, or with a third component as shown in Figure 4.

Figure 4 shows DC isolation being accomplished by using a transformer. An isolator module, of course, is much more than just a transformer, but it is the transformer component in a signal isolator that, in fact, provides the isolation since DC cannot pass through a transformer. Now that we have inserted this “transformer” into the circuit, the ground loop between the transmitter and the receiver no longer exists, thus eliminating its effects on the signal current.

The Wilkerson product line provides several options for implementing DC isolation depending on how the isolator is powered. There are three basic ways of powering an isolator.

These are listed below with the respective modules:

1. Input Powered or Loop Powered

DM4391-1 DM4391-2

2. Output Powered or Two Wire

TW810X

3. External Powered

MM4300 Series MM4380A

DM4300 Series DM4380A

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