What is the best analog input? 1-5V or 4-20mA 2

[josedavidch]

Hi

I have been working several years in PLC programming, but only doing analog input application since last 3 months. I am wondering what is the best analog input to work with?
0-10V
1-5V
4-20mA

Please advice based on experiences.

Regards

 

[controlsdude]

0-10v if your analog is in the same panel with your card
If you still want to use in the field, make sure you use shielded cable and ground shield on one end. I would look at a twisted pair cable that can go the distance since 0-10v will drop voltage if your using a crappy marginal cable.
recommend = Black Box EDDC cable spec, used this for rs232 for long distances but could be for the 0-10vdc. make sure that you do not put this cable near other ac or high dc voltage cables. A lot of times you can run these outside of conduit and wire tie the cables.

1-5v = never use

4-20ma = good for long distances.

biggest problems is watch out for ground loops. Mainly caused by shielding on both ends grounded.

 

[HCBFlash]

4-20mA

 

[ScottyUK]

4-20mA is the industrial standard for a reason. Of the traditional technologies, it works better than the alternatives.


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

A historic background:

0-20 mA was used for many years when instruments actually needed some current to work. Moving coil recorders and analogue instruments had a 20 mA (sometimes 60 mA) F.S. input and wire resistance was a problem when sending a voltage signal to such an instrument. There were usually little adjustable resistors delivered with those recorders and instruments so that the total series resistance could be adjusted to nominal value.

The idea of using a very (infinitely) high series resistance caught on and since an infinitely high resistance is equal to a current source Thevenin equivalent, the current signal was born. First 0-20 and 0-60 mA and later the now standard 4-20 mA signal.

The reason for the infinitely high resistance was based on the fact that you can add anything to infinity and still have infinity. So, the different wiring resistance wouldn't influence the signal at all. No calibration necessary.

The 20 mA current was recommended for all long wire applications and for good reasons. It has then earned an undeserved reputation for being less unsensitive to transients and HF interference than a 0-10 V signal. That is not true. I have had 4-20 signals being very sensitive to inverter interference where a 0-10 V signal (which very often has a well-filtered sending unit and receiving unit) was completely unaffected. It is a lot depending on actual hardware and application. There is no general rule saying that 4-20 is better than 0-10 V in "long distance" applications - only an opinion.

But, old thinking lives on, and the opinion usually is that 4-20 mA is a lot better than 0-10 V (or 0-5 V). I have used all three for decades. In paper mills, steel works, power stations, anywhere. And I still cannot say that one is "better" than the other.




Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[FrancisL]

4-20mA can power a tranmitter as well as returning the signal, that it why it is used, all with 2 wires. It is also compatible with Intrinsically safe circuits.
But many 4-20mA inputs are actually 1-5V inputs, by simply passing the current through a 250 Ohm resistor.
Francis

 

[Bardes]

An other reason for using 4-20mA is that it is much more immune to noise (resistance 50-250 Ohm) compared to fx. 0/2-5/10V signals , where the plc input resistance card is in range 0,1-1MOhm).

Bardes

 

[mchenney]

Another reason to use 4-20 ma is for explosion proof applications, with fusing and voltage limiting as to stay below max VA power limits for group and division clasification. Source series fused at say .05 A and followed by a Transorb AC clamp diode, intrinzic safty block.


Dennis McHenney
mchenney@mindspring.com

 

[MrMiyagi]

That's right. 4..20mA has several things going for it over a voltage signal.

1. You can power an instrument directly using a two-wire loop. (Loop-powered instruments)

2. Because of the 'live' zero, cable breaks and instrument faults can be detected fairly easily.

3. 4..20mA is great for controlling valves. By dropping the current to 3.5mA or raising it to 20.5mA, the valve can be sure to be fully closed or fully open.

4. You can use HART over a 4..20mA line to increase functionality and operator comfort, as well as do remote calibration and diagnostics.

On the other hand...

A major drawback of any current loop circuit is that the circuit has to be broken to measure the signal with a multimeter or other diagnostic device. There is a work-around with either a diode in series, or with a purpose-built switch in the circuit, but both inclease costs.

Also, 3- or 4-wire current loop circuits are susceptible to 'forking' of the current signal in poorly-designed measurement circuits. Many PLCs are notorious for this.

A seperate option - Have you considered one of the field-bus systems? On large, distributed applications, ProfiBus, ASi, or Foundation Fieldbus can offer significant cost savings with the total cost of ownership in the longer term.

Regards,
Mike O

 

[ScottyUK]

Fluke just introduced a clamp-on instrument specifically for process loops which does not require the loop to be broken. I'm thinking of getting one, just for the convenience it brings. Maybe next year with a new budget!

http://us.fluke.com/usen/products/Fluke+771.htm

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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

A friend of mine just bought one of Fluke's "clothesclips". He talks a lot about how easy things are nowadays.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Hi Gunnar,

What is his opinion of his new toy?


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If we learn from our mistakes I'm getting a great education!

 

[danw2]

Looks to be a handy unit for loop checks. I finally saw one of the Fluke's live at a trade show last week.

It has a white LED 'flashlight' on it to light the area by the pincer sensor clips. Backlit LED display can be turned on or off.

It is not rated for hazardous areas, so safe areas only.

I stood on the opposite corner, watched and counted 24 guys over 10 minutes who came up and handled the 3 meters on display. A very popular item for a PLC show (Rockwell's Automation show).

It's not a calibrator, it seemed to be good to ±.03mA (comared to the source's digital readout) but they aren't claiming it is, either.

I budgeted for one.

Dan