Analog card ?

Hi all members!!!!!....

Analog card normally can be setting either current input or voltage input or current output or voltage output...How to decide in our system need to use current signal or voltage signal? There have inquiry by my friend, why in analog signal, 4~20mA as analog current need to decide..why not another range? But I cannot answer his question...Who know the answer, can share with me?...

My portion of knowledge on the subject

4-20ma seems to be the best and is highly recommended unless cost or equipment is an issue.

Isn't affected as much by noise or voltage drops as is a voltage based system.

The 4 as a minimum I believe had something to do w/ the quality checking of the signal. This way, a disconnected 0 could be intrepeted.

Read this article from wikipedia.....


http://en.wikipedia.org/wiki/4-20mA

zai_jnr said:

Hi all members!!!!!....

Analog card normally can be setting either current input or voltage input or current output or voltage output...How to decide in our system need to use current signal or voltage signal? There have inquiry by my friend, why in analog signal, 4~20mA as analog current need to decide..why not another range? But I cannot answer his question...Who know the answer, can share with me?...

Click to expand...

Definitely go current. Much less problems with noise and impedance. I have never seen a reason to go voltage, aside from satisfying legacy equipment.

4-20mA was chosen because it is easy to convert the 1-5V standard with a 250 Ohm resistor.

the low range value (4) makes it possible to distinguish an open circuit from a zero reading.

4 to 20 mA gained favour because it could travel large distances without losing current to the resistance of the wiring used, and was found to also be able to resist noise pickup that can occur with voltage signals.

Now input cards can be either voltage or current input, because the first thing done on most 4 to 20 mA input cards is that that signal is dropped across an internal 250 ohm resistor to produce a 1 to 5 volt signal that the card uses, or alternately a 500 ohm resistor to produce 2 to 10 volts.

Output cards normally have to be selected for, either voltage out, or current out, if they are available on the same card they normally use different terminals.

Very interesting thread here:
http://www.eng-tips.com/viewthread.cfm?qid=235142&page=1

with an advocate for voltage signals in very high noise environments.

Note the posts with a red star, further down.

The main reason for 4 mA and not zero is to power the electronics e.g. a pressure transmitter may have a strain gauge, an amplifier and an output stage. A typical transmitter uses 11 V and about 3 ma to power this. Sure anything less than 4 mA could be fault but it can also mean the pressure is below the zero range. 0 mA would indicate a wire has dropped off.
BTW if ever you use a Temperature transmitter to control a heating application it's a good idea to disable the heater if the current falls below 3 mA. I have seen oil bin a lube tank boiling because someone turned the instrument power supply off.
Regard
Roy

Roy Matson said:

The main reason for 4 mA and not zero is to power the electronics

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The power for loop powered transmitters is sourced from a 24VDC bias. It doesnt have anything to do with the 4-20mA current loop signal.

zankorel said:

The power for loop powered transmitters is sourced from a 24VDC bias. It doesnt have anything to do with the 4-20mA current loop signal.

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So if you were to use a 0-20 mA signal and you wanted to have a loop powered (2 wire) transmitter that required 2 mA for its electronics to function, how do you handle the 2 mA? Do you assume that it is a reading of 10% full scale? Sure the power for a loop powered transmitter is sourced from a 24VDC (or whatever) bias, but the 4-20 mA range allows a transmitter to actally use this power without screwing up the reading. In that sense, it has everything to do with the 4-20 mA current loop signal (emphasis on the 4.)

Brian

One advantage to using voltage is troubleshooting. Any technician can put his meter leads on a voltage signal and quickly determine what voltage is present.

In order to troubleshoot a 4-20mA signal, the loop must be broken and the meter leads placed in series. Also, one of the meter leads will have to be moved to the mA position.

If your maintenance technicians are informed, then it is not a big deal, but if they are not trained or familiar with current loops, they will have no clue what to look for, and how to look for it.

Aside from ease of troubleshooting, I too, prefer current loops.

Paul

OkiePC said:

One advantage to using voltage is troubleshooting. Any technician can put his meter leads on a voltage signal and quickly determine what voltage is present.

In order to troubleshoot a 4-20mA signal, the loop must be broken and the meter leads placed in series. Also, one of the meter leads will have to be moved to the mA position.
Paul

Click to expand...

I just ran across this meter the other day. Looks like it might be handy for the right person. I don't do enough 4-20 mA troubleshooting to need it.

Brian

OkiePC said:

In order to troubleshoot a 4-20mA signal, the loop must be broken and the meter leads placed in series. Also, one of the meter leads will have to be moved to the mA position.

Click to expand...

For this reason, I always include fused links in any analog circuits.