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Signal Isolator

04/11/2017 1:10 AM

Hello Everyone, i wanted to know what is signal Isolator and why it is used in between transmitter and AI card of PLC??

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#1

Re: Signal isolator

04/11/2017 1:42 AM

To eliminate ground loops.

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#2

Re: Signal isolator

04/11/2017 6:02 AM

On the "as built" issue drawing of the circuit will be the names of the individuals who designed and commissioned it. Surely a word in these individuals' ears is the best place to start? After all, the drawing cannot be seen from here.

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#3

Re: Signal isolator

04/11/2017 10:00 AM

The clue is in the name...
It isolates the signals from unwanted voltage levels. Something like an opto isolator allows you to have some sort of sensor on (say) a high voltage circuit providing a signal into a 5v logic circuit. the signal being transmitted optically across a gap in the integrated circuit to provide electrical isolation.

Mind, I may be barking up the wrong tree as I have no idea what your appliction is or what an AI card is... artificial intelligence? Alien Invasion? Artificial Insemination?
Ah! Me sir me sir I know... is it analogue input?
Del

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#4
In reply to #3

Re: Signal isolator

04/11/2017 10:25 AM

It surely bears no resemblance to draw testing.

Depending on what the "noise" in the environment is, this could be a band pass filter. Or, as you stated, it could be an optoisolator to separate signal from a very high voltage offset from the low values tolerated by either logic or an analog input.

For all we know from information supplied in the initial post, this could be nothing more than a shield of woven wire around the analog signal wire and its common ground wire.

just one example, that provides galvanic isolation in this instance.

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#6
In reply to #3

Re: Signal isolator

04/12/2017 8:08 AM

PLC's have AI, AO, DI, DO, and some others lesser used cards. Analog Input, Analog Output, Digital or Discrete Input and of course Output. Standard acronyms in the control world.

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#8
In reply to #6

Re: Signal isolator

04/12/2017 8:39 AM

D'uh... but not everyone is in "The control world"
Grrrr hisss ftzzzzzz

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#9
In reply to #8

Re: Signal isolator

04/12/2017 8:51 AM

That's why I gave you an explanation. The OP assumed we were in his "world". I just happen to be there too. We don't think when we speak of PLC cards - we just use the terminology we use everyday.

Be careful with that grrrr and hissss - that avatar is "Nittany" and you don't want to excite her - she takes aggressive cats out of the yard every time they enter. She will try to go right through our sliding door to the deck if one gets up on the deck. She even holds her own against 30 pound dogs.

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#10
In reply to #9

Re: Signal isolator

04/12/2017 12:23 PM

Our current cat Emily is a bit poorly, she had an accident, her tail doesn't work any more, her bladder control isn't fully back to normal and she has some lumbar irritation which is making her bit at her back. It's a work in progress, but she's happy in herself.
She was missing for 3 days, but dragged herself home.
Prob' going to have her tail amputated next week
Del

PS. I was a tad confused by your statement:-

"It surely bears no resemblance to draw testing." which meant absolutely nothing to me... I recognise each word, but the only draw testing I'm familiar with relates to longbows

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#11
In reply to #10

Re: Signal isolator

04/12/2017 2:24 PM

That is because I knew about your bow testing rig. Your draw tests. The only signal that matters is the draw, not the twang, or if it breaks, the thunk sound.

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#13
In reply to #11

Re: Signal isolator

04/12/2017 3:08 PM

Ah! That explains it... you are ahead of me
Del

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#12
In reply to #10

Re: Signal isolator

04/12/2017 2:50 PM

That is a quite a bit like the old stray my mother took in while I was in college. He'd go out for a week and come back all beat up. He'd stay until he healed well enough to go again. He had partial ears, from fights, his face was always covered in scars, and his tail drooped a bit, but he soldiered on to about 13 years old. Best incident was the broken leg when he was gone over 2 weeks. We had to have the Vet. break it and reset it. It extended the full length of the hind leg and beyond a bit. One of us stayed with him day and night to pick him up to go relieve himself, as he could barely hobble around on the cast and despite our allowing him to stay in the house, he was still an outdoors cat, so that is where he went - no litter box. Didn't dare let him out by himself, or he would have tried to go hunting on that cast.

The things we do for our beloved little friends......

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#5

Re: Signal Isolator

04/11/2017 11:00 PM

Without a signal isolator, you could end up accidentally feeding in a high voltage into your computer / PLC, which could damage it.

Even an incoming signal level is only, say 3.3V, you don't know what it is referenced to!

For example, the "ground" or zero volt level might be at a different potential between your computer's motherboard and the other device that is sending or receiving a signal.

You could have a 100V difference or more between the main DC ground of your computer and the other device.

Without a signal isolator, you could blow up your computer!

For example, Ethernet interfaces are magnetically isolated. Each wire pair is connected to small transformers on both ends ("magnetics"). This, by the way allows for transmitting power in Ethernet cables (PoE).

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#7

Re: Signal Isolator

04/12/2017 8:23 AM

In my 25 years of building/installing control panels on process skids, it has been my experience that they are there for something to do during system testing. EVERY one that was requested by a customer, or installed in a customer provided panel, are sitting in a box in my largest desk drawer. We removed them all during final testing of the system, at the customer's request. Now I'm sure there are times they function properly and are required, but so far, I've not encountered that.

The exception would be intrinsically safe barriers which would technically fall into this category as well as the discussed noise filters. Those must remain to keep voltage/current in a circuit below the level needed for arcing in a hazardous atmosphere application.

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#14
In reply to #7

Re: Signal Isolator

04/12/2017 3:49 PM

Very interesting timing. The customer built panel we are testing on a process skid today, much to my surprise, had signal isolators on the analog signal to and from a motor mounted VFD. I totally missed that when going over the drawings to layout our wiring plan. These seem to function fine, and I can see why an engineer threw this in - a VFD is a very noisy item. I know this same panel has worked before without them, but it can't hurt to have them. Wiat - just got word that the analog circuits in question are not working - more on this later after I go do some circuit analysis.

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#15

Re: Signal Isolator

04/14/2017 6:57 AM

Signal Isolators Information

Show all Signal Isolators Manufacturers

Signal isolators provide electrical (galvanic) isolation between the input and output circuits. They couple the signal to the output through a transformer or optical isolator. Signal isolators also break the direct electrical galvanic path between two or more loop points. They protect against dangerous measured-variable voltages and increase protection from surges and spikes.

Signal isolators are closely related to signal converters and signal interfaces. These devices are often used to share, split, boost, protect, step-down, linearize, and digitize process signals.

The most important role of a signal isolator is to break the galvanic path between circuits that are "grounded" to different potentials. A galvanic path is defined as a path in which there is a direct electrical connection between two or more electrical circuits that allow current to flow.

For example, if a differential pressure transmitter is sending a 4-20mA measurement to a receiver, such as a recorder, and the two ground points are different, an additional and unpredictable amount of current can be introduced into the loop, distorting the true measurement. This current path is known as a ground loop and is a very common cause of signal inaccuracies, along with the current path having two grounds, the ground at different potentials, and a galvanic path between the grounds. In order to remove the ground loop, the galvanic path between the grounds must be removed.

Signal isolators are especially important since the other two causes for ground loops cannot always be removed safely. The ground may be there for safe operation of the electronic device.

Function

There are two prevalent methods for galvanic isolation.

Optical isolation uses light to transfer a signal between elements of a circuit. The isolator is usually in a small module mounted on a circuit board. The isolation circuit is composed of an LED and a photo-sensitive detector, such as a phototransistor. The insulating air gap between the LED and the photo transistor serves as the galvanic separation between the two circuits.

The advantages of optical isolation are: the device is small, can sometimes provide higher levels of isolation than transformer isolation, and the device has better common-mode noise rejection. Disadvantages of optical isolation are that each element needs its own power supply and the signals on both sides must be quite small.

Transformer isolation is also referred to as electromagnetic isolation. It uses a transformer to electromagnetically couple the desired signal across an air gap or non-conductive isolation gap.

Transformers are very efficient at transforming AC signals. This is a disadvantage since many process control signals are DC and must be changed into an AC signal so they can pass across the transformer. Once passed, they have to be rectified and amplified back into the desired DC signal output.

Advantages of a transformer circuit include: the device does not need a power supply and it works well with AC signals such as audio. A disadvantage is that a transformer does not work with DC signals since at some frequencies the device will begin to act as a filter for AC signals.

Specifications

Signal isolators are often specified by what the isolation levels are from input to output. Two-way (input-to-output) isolation is used to describe a 2-wire transmitter since it is powered from either its input or output terminals.

Three-way isolation is defined as input-to-output, power-to-input, and power-to-output isolation. It is important to note if the isolator is powered by a DC supply which could cause a problem with common mode noise, or failing to switch power supply, creating unwanted output signal errors.

If the signal isolation is a 4-wire device it may require 24Vdc, 110Vac, or 220Vac to operate its circuits. For these devices, it is important to ensure the isolator has full three-way isolation.

Each type of signal isolator should provide signal isolation between the input, output, and power source. A typical signal isolator is capable of withstanding 1000 V, or more common-mode signals at the input.

The level of isolation is also important when selecting a signal isolator. Level of isolation describes the level of voltage needed to create an arc from one side of the circuit to the other. Optical isolators have the advantage here as the electrical parts of the circuit can be designed to be further apart.

Environmental Factors

Due to the widespread application of signal isolation many devices are available with built-in safety features for use in various environments.

For example, intrinsically-safe, nonincendive, and explosion proof options are all available for hazardous areas within a plant. Hazardous conditions also include the amount of heat the electronics will be exposed to while installed in the plant.

Other devices may need protection from radio frequency interference (RFI) and electromagnetic interference (EMI) which can cause unpredictable and non-repeatable degradation in instrument performance.

Variable frequency drives (VFDs) are also known to cause noise that interferes with signal isolators and are common in plants. This interference can be avoided by installing a 4-wire isolator to filter the unwanted common mode noise.

Applications

Signal isolators are used in industrial, medical, and other environments in which electrical isolation is essential for safety. Signal isolators can also be used to amplify signals, enable instruments to be added to an overburdened loop, or to step down dangerous, high-voltage signals to safer levels.

Specific examples of applications include resistance input for use in RTD, slidewire, strain, and potentiometer transmitters, and current/voltage isolators for use as alarm tripping and deviation alarm notification.

References

Signal Isolators, Converters and Interfaces

Using Signal Isolators to Reduce Ground Loop Problems

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