Signals: 4-20mA or 0-10VDC ??? 3

[cfordyce]

Which signal type is better? What are the best qualities of each? Is one better in a noisy environment? Does one have a lower failure rate? Which is more utilized today??? Anyones opinions would be helpful. Thanks.

 

[skills]

The current loop is more immune to interference because the impedences of the reciever and transmitter is much lower. The reciever input impedence may be only 250 ohms and thusly loads the loop and signifigantly reduces errors due to noise. Voltage inputs are terribly sensative to noise because impedances run in the megaohms at times. I use both and each has it's place. For long distance, don't even consider voltage signals at this level or they will be swamped.

 

[wzhere]

I prefer 4-20mA signals. This way I know when I lose power because my amps go below 4mA.

 

[ve7brz]

4 to 20 mA has it hands down when it comes to noise immunity and/or long line lengths. You can't go wrong using it.

 

[dpc]

I agree with all who recommended 4-20 mA for field circuits. I have run a 4-20 mA circuit for 2 miles with no problems.

However, when the signal must be shared by multiple devices within a panel, it is often a good idea to convert the 4-20 mA back to a 0-10V signal for use within a single enclosure. Most PLC and DCS analog inputs are voltage-based, so this eliminates the need for resistors across the inputs and reduces the burden on the current loop.

 

[skills]

Yes, that's a good point voltage signals are easier to use with a lot of devices because of the high input impedances. The immunity consideration is less of a concern within in well laid-out cabinet or enclosure.