Previous in Forum: RSLogix5000 Revision Problem   Next in Forum: Level Transmitter Displacer Type
Close

Comments Format:






Change Mode
Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







Add Vote
6 comments
Rate Comments: Nested
joganand
Active Contributor

Join Date: Oct 2012
Posts: 16

Why DC Supply is Preferred for Transmitter or Sensor Switches

11/09/2012 1:30 PM

Hi frnd, I am thankful to your kind replies,I have seen many places of Instrumentation that,transmitters and sensor switches are having technical specifications having voltage only DC and range from 10 to 32V,and out put range 4 to 20mA.why only DC Supply and why it having only range between 10 to 32,why not more.
Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Good Answers:

These comments received enough positive votes to make them "good answers".

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Fredski
Guru

Join Date: Feb 2011
Posts: 7025
Good Answers: 207
#1

Re: why dc supply is preferred for transmitter or sensor switches

11/09/2012 1:48 PM

let me count the whys....this could take a while we live in increasingly "digital world" you don't need a high voltage to send a SIGNAL, no need for high voltage in communications. digital controls of all shapes and sizes are here to stay...and they'll all be DC...get used to it
Register to Reply Score 1 for Good Answer
Go to Next "Almost" Good Answer
Turbotroll3
Power-User
Hobbies - HAM Radio - New Member

Join Date: Oct 2008
Location: Houston, Texas
Posts: 252
Good Answers: 5
#2

Re: Why DC Supply is Preferred for Transmitter or Sensor Switches

11/09/2012 4:01 PM

I can think of about 3 right quick.

1. Tradition, transmitters and switches have been powered by low voltage DC for a long time.

2. Noise, if alternating current is used, it must shielded to keep from interfering with the analog signals which are typically low voltage DC.

3. Convenience, different wiring practices are required for safety. Limited energy, low voltage is less likely to injure people who come in contact with the live conductors and far less likely to start a fire.

__________________
 If the software can detect, compensate, avoid, or correct an anomalous condition in the system, it is, by definition, a software problem-regardless of the root cause. In the long run, for most classes of problems, it is cheaper to fix it in the SW
Register to Reply Score 1 for Good Answer
No more "Almost" Good Answers.
Phaddy
Power-User

Join Date: Jun 2010
Posts: 344
Good Answers: 17
#3

Re: Why DC Supply is Preferred for Transmitter or Sensor Switches

11/10/2012 4:29 AM

What really matters is the 4 to 20mA. That carries your information. Ignore the input voltage range, that's just limits that allow 4 to 20mA over the circuit that you use to transfer the info.

The 4 to 20 mA is usually converted back to a voltage at the receiving end, usually by the voltage drop across a resistor. It's the resistor that determines that you need the input voltage limits specified.
Register to Reply
BIN95
Power-User
Engineering Fields - Control Engineering - Control Engineering Technical Fields - Education - Industrial Training Engineering Fields - Manufacturing Engineering - Manufacturing Training Engineering Fields - Electrical Engineering - Electrical engineering Training Engineering Fields - Instrumentation Engineering - Instrumentation Engineering United States - Member - New Member Engineering Fields - Energy Engineering - Become part of the larger group, change your world.

Join Date: Apr 2005
Location: Las Vegas, NV
Posts: 266
Good Answers: 1
#4

Re: Why DC Supply is Preferred for Transmitter or Sensor Switches

11/10/2012 12:22 PM

All great answerers so far. I might add...

  • lower cost
  • less shock hazard risk
__________________
 AB PLC Training onsite, On-Line, training software and more. BIN95.com
Register to Reply
7
jmueller
Guru

Join Date: Jul 2006
Location: Eastern Kansas USA
Posts: 1503
Good Answers: 128
#5

Re: Why DC Supply is Preferred for Transmitter or Sensor Switches

11/10/2012 10:19 PM

Joganand,

From your question, I assume you are learning the field.

  1. An output of 4-20mA is a continuous DC signal type, as compared to a digital signal or an AC signal. If it were an AC signal, the current would be going from zero to a peak to zero to a negative peak to zero, etc. which would be very hard to measure with the desired precision. Also, if it were an AC signal you would have to worry about capacitive and inductive effects in the signal cable--these would introduce uncertainties and errors into the measurement. A DC signal typically has a much slower rate of change, so it would be unaffected by the cable's inherent capacitance.
  2. Typically, the instrument does not supply its own power, so the signal cable has to supply the power to the instrument as well as receive the signal back from it. Again, to avoid problems with capacitance, inductance, and AC noise the signal reliability requires that the power on the cable be DC. Now, depending on the type of equipment and the measurement being made, this may require a separate DC voltage with a significant power draw (more than a few mA) and a 3 or 4-wire cable (if 3-wire, then the power may be on wires 1 & 2, with the signal being on 2 & 3).
  3. To avoid problems with "dangerous" voltages and code rules, the upper limit for excitation voltage is 50VDC. In practice, the instrument manufacturers have standardized on a lower limit of 32VDC as the maximum.
  4. Various signal ranges are frequently used, such as 0-20mA, 4-20mA, 0-10V, 0-5V, 1-5V, 0-40mA, and others! Any of these ranges that start at 0 do not allow you to detect an open circuit. The 4-20mA and its related 1-5V (achieved with a 250Ω precision resistor) are the most common, but some transmitters will allow you to select between 2 or more options.
  5. If you apply Ohm's law you can see that as the circuit resistance increases, the minimum excitation voltage increases. This imposes restrictions on the number of receiving devices the signal can be passed through in series, or on the cable gauge or the cable length. That is when your work as an instrumentation technician gets "interesting".

These are a few points for you to work with. --JMM
Register to Reply Good Answer (Score 7)
No more Good Answers. Go to first "Almost" Good Answer
Phys
Guru
Engineering Fields - Instrumentation Engineering - EE from the the Wilds of Pa.

Join Date: Feb 2006
Location: middle of nowhere, Pennsylvania
Posts: 2603
Good Answers: 63
#6

Re: Why DC Supply is Preferred for Transmitter or Sensor Switches

11/12/2012 8:24 AM

Yes - everyone else has nailed the reasons.

Do be aware that there are still a number of instruments out there that, while the signal is working in the low DC volts range, the power to run the instrument may still be 120VAC to 230VAC. (why? - I don't know - makes no sense to me) In particular, at my work, we install several flow meters each year here, and even a level transmitter or two, on our custom built process skids where the customer wants an AC voltage driving the transmitter and display. (loop powered instruments obviously are not included - only devices requiring power above what is required for signal transmission)

__________________
 Remember when reading my post: (-1)^½ m (2)^½
Register to Reply Off Topic (Score 5)
Register to Reply
Have a Different Question?
6 comments

Good Answers:

These comments received enough positive votes to make them "good answers".

"Almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, vote them!
Copy to Clipboard

Users who posted comments:

BIN95 (1); Fredski (1); jmueller (1); Phaddy (1); Phys (1); Turbotroll3 (1)

Previous in Forum: RSLogix5000 Revision Problem   Next in Forum: Level Transmitter Displacer Type

Advertisement

Got Something to Say?
Ask a Question
Start a Discussion

Search this Forum
Instrumentation:
Go

CR4 Sections
Aerospace
BioMech & BioMed
Chemical & Material Science
Civil Engineering
Communications & Electronics
Education and Engineering Careers
Electrical Engineering
General Discussion
Instrumentation
Manufacturing
Mechanical Engineering
New Technologies & Research
Software & Programming
Sustainable Engineering
Transportation

Site Directories
Forum Directory
Blog Directory
User Group Directory
RSS Feeds

CR4 Rules of Conduct
CR4 FAQ
Site Glossary

Who's Online (86 right now)
Show Members
Advertisement