0-10VOLTS AND 4-20 mA

[Tom Jenkins]

I'm pretty sure 4-20 mA came first, and then loop powered devices followed as electronics improved. Loop powered devices, indicators or transmitters, run the current through a diode to create a voltage drop (or I suppose sometimes a resistor, but usually a diode so you have a constant voltage). This voltage is used to power the electronics.

It isn't the first or the last mA, just all of the mA. Obviously, if you had 0-20 mA you couldn't have loop powered devices. However, I believe the loop power took advantage of the 20% zero offset, the offset wasn't created to allow loop power.

As I hinted above, the old pneumatic controls had a 20% zero offse as well, 3-15 psig. This may have had a benefit in providing energy for moving bellows etc. at a low percentage of full scale signal, but I don't think that is why they went 3-15 psi and I doubt that is why 4-20 mA emulated it.

The big reason, from what I have ben told and read, is that the 4 mA gives you a way to distinguish between a true zero process variable and an open circuit. Ganutenator was right.

 

[rogerhollingsworth]

4-20mA

The 1-5v and 4-20mA makes it possible to determine when your signal is lost. The 0-5 and 0-20mA could not destinguish whether you were operating at zero or you had a zero due to a lost signal.
In the steel mill where I worked all the thermocouple devices would go high when a thermocouple was lost.

 

[godfrey]

The voltage supply for the loop power is usually 24 VDC. I've noticed in the manuals of certain loop powered devices that the voltage should be at least about 16 VDC. Some devices post a chart which shows the effect of the supply voltage on the maximum resistance that the loop can drive through.

 

[f mc caffrey]

4-20 mv

Thanks to you guys for replying.

I posted this question only after looking on the internet and looking through various books,I would call this investigating. I did not just get the Question and decide to ask someone else.

I already was pretty sure about the 4 - 20 mA part but found it difficult to find info on the 0 -10 v.

If you are not prepared to give me an answer, don't bother replying,I don't need lectures.

By the way. We in Ireland drive on the left side of the road, just for information purposes, not trying to be smart.

I first used this website when i asked the question, Hope you can view the replies i got

Let me know if you can't and how i can let you see them

Thanks

 

[rsdoran]

If you are not prepared to give me an answer, don't bother replying,I don't need lectures.

Asking a question implies you do not have the answer, its not common to answer questions for students arbitrarily. I think you may have gotten many responses because of others, like myself, that showed interest but no direct knowledge on the "WHY".

When its obvious that you are a student the replies will correspond, they will offer suggestions, advice etc but WILL NOT do your assignment for you.

Only way to avoid the lectures is not post.

 

[rogerhollingsworth]

4-20mA

Early on voltage signals were used but were replaced with a mA signal because the mA signal was more immune to noise than the voltage signal. The 4-20mA was accepted as a standard due to it being a robust signal and less than 4mA was a lost signalas well as loop power. It is suggested that you convert all your mV signals to mA signals as early in the system as possible. mV signals are very susceptible to noise where as mA is less sensitive, mV signals need the special, expensive TC wire, mA does not.
I have my thoughts as to why 0/10v and -10v/+10v but need to find a info source. If you find it before I do, post it.

Intended to help not impress.
Roger

 

[rsdoran]

in due time Roger you may understand.

 

[rogerhollingsworth]

Due time

I do understand, just poking fun, not serious.

If I'm poking too hard for a thin skin let me know, I'll stop.

Roger

 

[[email protected]]

Personally I don't like the 0 - 10 vdc standard, firsly an EMF signal is affected by line resistance, and secondly a "Dead" zero makes it difficult to detect broken wires/loose connections. The 4 - 20 mA standard is not affected by line resistance, and broken loop detection is easy.

 

[russrmartin]

Look harder

Mr Mc Caffrey,

You say that you posted only after searching the internet. I do not know how you search the internet, but perhaps before learning about electronics and the like, you should learn how to do a simple search on the internet. After reading the posts in your thread, I promptly went to a popular website known as yahoo.com. A search that involved typing 0-10 volt vs 4-20 milliamp and clicking search revealed that the third hit was, infact, a webpage with a tutorial on several electronic circuits. 2 of which apply to your question. Not only does it list the benefits and drawbacks of both, but also a simple explanation of how both work. I suggest you try this search and read the paragraphs on both circuits. If you insist that you tried looking on the internet, I think you need some assistance more with learning HOW TO FIND the answer to your questions, rather than the answer to your question itself.

Regards,

Russ

 

[Alan Case]

One advantage of 0-10 volts is that you can drive unlimited VFDs (within reason) from an analog PLC output card whereas 4-20 ma because it needs to be a series connection can sometimes only drive 1 VFD, maybe 2 but rarely 3. Regards Alan Case

 

[jvdcande]

Russ,

if you mean the one with abifar in the URL, it's indeed a good one!

Kind regards,

Jean Pierre Vandecandelaere

 

[russrmartin]

Objection to Alan

Alan,

I am wondering exactly why you cannot use 4-20mA for multiple outputs. I have personally done this with no problems so far. The installation was done over 2 years ago now. What we did was replace 4 large mercury relays with new solid state variable output relays. We wired all 4 of these with a 4-20 and wired them in series. The 4-20 comes from an analog output card from the PLC. You seemed to imply that this cannot be done at all. While I'm sure there are limitations, it would be my understanding that while the voltage measured in different places in this circuit would vary with multiple VFD's or whatever wired in series, the current provided should measure the same throughout the circuit. As I said, we have 4 "loads" wired in series without a problem. While I can see that trying to run 10 or 15 loads in series may cause problems, I find it hard to believe that a 0-10vdc signal will not have complications running this many "loads" as well. If not for the number of loads alone, the distance the signal would travel to cover that many loads would cause the 0-10 to be susceptable, due to resistance in the wire over a possible great distance. Can anyone verify that we are not just getting lucky here, or should I expect problems in the future when our luck runs out?

Regards

Russ

 

[Greg Gauper]

Technically, it can be done but ya gotta watch out for a couple of things.

1) What you have is one big series loop. Each 4-20mA input will have some resistance, typically 250 ohm. So if you have 4 inputs in series,

4 * 250 ohm = 1000 ohms.

20mA * 1000 ohms = 20V. The device itself will requires some minimum voltage to operate. You will have some additional resistance due to the length of the wires between runs. If you are using a 24vdc power supply, you will be marginal. Ususally the sensor will have a data sheet for calculating the minimum required power supply volltage. It usually just a matter of picking the standard size power suplly that will do the job. If the power supply is adjustable by design, you can sometimes get by by cranking up the voltage i.e. a 24vdc power supply might list that it can operate from 18 to 30vdc. That's the easy part. Note, I would recommend a single, dedicated power supply for your transducers if your choose to go this route. Make sure you document your electrical drawings to show the adjustment setting!

2) You can connect the devices in series as long as each input is isolated from each other. Some 4-20mA input modules have '-' side of the current input tied to common i.e. they convert to a straight single ended voltage in the range of 1-5V. If all of your panels use this style of input, and they have one or more commons tied together, then only the first input in your loop will work.

If your input modules are true differential style, you might be okay. There might be issues with the maximum common mode voltage permitted (maximum difference in voltage of either input with respect to the modules 'common') by your input module and your power supply. The best solution is to use isolation modules at each drop, or at least in 3 out of the 4 drops. Weidmuller, Phoenix Contact, Murr Elektronic, Entrelec, etc all make modules that are designed for this function. They can provide isolation and convert from one type of signal (4-20mA, 0-5V, 0-10V, etc) to another.

 

[rogerhollingsworth]

I know that is true with Allen-Bradley IFE modules. They use a single converter and multiplex the channels. Isolated modules are better.