help i'm thick

[big_106gti_kev]

cananyone suggest web sites with basic info about operation of 20mA loops and twisted pair cabling????

taa

kev

 

[PLucas]

What 'basic' information do you require exactly, can you be a bit more specific please, then w might be able to help.

 

[Eric Nelson]

Not much info here, but the URL makes sense:

http://www.4-20ma.com

There's a pretty good explanation for beginners here:

http://www.sensorland.com/HowPage028.html

-Eric

 

[Tom Jenkins]

Twisted pair

Eric's sites were pretty good (especially the second one) but they didn't address twisted pair. You can obviously run the 4-20 mA loop (it is not generally referred to as just 20 mA) through any conductor. In order to minimize the effect of EMI/RFI (ElectroMagnetic Interference/Radio Frequency Interference) induced voltage the wires are twisted. This exposes both wires to the same amount of EMI/RFI, whereas parallel wires might have one with more exposure than the other.

In addition to using twisted pairs, analog signal wires are usually shielded as well. A metal foil is wrapped around the twisted pair. The metal foil intercepts the EMI/RFI before it can reach the twisted pair. The shield is connected at ONE end to ground, which then drains the EMI/RFI induced voltage and protects the signal from noise.

 

[gb_sv]

Hi,
Since you have not mentioned on what kind of basic info you are looking for on 4-20ma. Rightnow i presume that you are looking for why 4-20ma is being used instead of 0-20mA.

4 mA is used instead of 0 mA to indicate the minimum process variable in industry.This is to distinguish whether the process variable is really at it's minimum value or instrument is not functioning.That is,
if the instrument fails the reading will show 0 mA.

Regarding the twisted pair as Tom mentioned it's required to reduce the EMI/RFI on signals especially low current/voltage.There are various type of twisting are available you can visit to belden cable website.The link is,

http://www.bwcecom.belden.com/college/techpprs/wcfsbetp.HTM
or just try belden.com in google.

Bye
GB