Current signal 4-20mA?
- Thread starter Gromit
- Start date
mgvol
Member
Always use shielded cable, regardless of distance. Yes, shielded, twisted pair cable. Distance is going to be a function of voltage drop (your 4-20mA loop is powered by a 24 vdc supply for instance). You have to consider the gauge of the wire and the voltage drop over the length of the wire.
mgvol
Member
What are you calling typical votage drop and gauge? You need to select a cable and see what the resistance is (typically listed in ohms/foot or ohms/1000 foot) then calculate the votage drop for the length of cable you need to use. If it is insufficient, go up in wire gauge until you find one that works. No one can calculate that for you.
Mickey
Lifetime Supporting Member
Typical Transmitter Spec. Power vs Load
Sparkz
Member
Sorry guys, voltage drop over a 20mA loop cable is very small!
Typical resistance of copper wiring is 0.0175 Ohms.mm²/m (at 15°C).
To give an example: 50m of 0.25mm² 2-wire cable has a total resistance of only
2(# of conduits) x 50(length) x 0.0175 / 0.25(gauge) = 7 Ohms
A 20mA signal would only result in a 140mV voltage drop! This would have no effect whatsoever since the typical resistance of an analog input is about 250 Ohms.
But voltage drop is not the real reason why you should avoid running any analog signal over extended cables: Noise pickup is a lot worse! Best results can be achieved by using cables with a full copper shielding, like Profibus cable, in stead of weaved shielding cable, like LIYCY. No need to tell you to stay away from power lines!
Typical resistance of copper wiring is 0.0175 Ohms.mm²/m (at 15°C).
To give an example: 50m of 0.25mm² 2-wire cable has a total resistance of only
2(# of conduits) x 50(length) x 0.0175 / 0.25(gauge) = 7 Ohms
A 20mA signal would only result in a 140mV voltage drop! This would have no effect whatsoever since the typical resistance of an analog input is about 250 Ohms.
But voltage drop is not the real reason why you should avoid running any analog signal over extended cables: Noise pickup is a lot worse! Best results can be achieved by using cables with a full copper shielding, like Profibus cable, in stead of weaved shielding cable, like LIYCY. No need to tell you to stay away from power lines!
Sound advice Sparkz.
Noise is also the reason why we always advise to use FTP (foiled twisted pair) cabling for industrial ethernet in stead of STP (shielded twisted pair). Profibus cable has a second advantage: it is of a rather large gauge, so there certainly is no voltage drop issue at all.
Kind regards,
Noise is also the reason why we always advise to use FTP (foiled twisted pair) cabling for industrial ethernet in stead of STP (shielded twisted pair). Profibus cable has a second advantage: it is of a rather large gauge, so there certainly is no voltage drop issue at all.
Kind regards,
John Hawkins
Member
copper braid or aluminum shielding is only good for electrostatic interference. for electromagnetic interference metal pipe is the only answer
Voltage drop calculator:
http://www.csgnetwork.com/voltagedropcalc.html
I don't like the "Conditions" field, but it doesn't make a huge difference anyway.
This one gives you wire size needed for a max 3% drop.
http://www.elec-toolbox.com/calculators/voltdrop.htm
But as they said, interferance would be a bigger problem in 4-20mA signal anyway. Make sure the shield is properly grounded.
http://www.csgnetwork.com/voltagedropcalc.html
I don't like the "Conditions" field, but it doesn't make a huge difference anyway.
This one gives you wire size needed for a max 3% drop.
http://www.elec-toolbox.com/calculators/voltdrop.htm
But as they said, interferance would be a bigger problem in 4-20mA signal anyway. Make sure the shield is properly grounded.
HansAndersen
Member
Gromit said:
You may ought to use twisted-pair non-shielded cable by 100 m.But environment has a lot of interference or noise you may have to use shielded cable.In this case There may be some situation that you wont be able to use non-shielded cable.
HansAndersen
Member
Gromit said:
If you just will carry 4-20 mA signal.I think any shielded twisted pair cable will be enough for you.
Last edited:
Kataeb
Member
do you have a voltage drop calculator in european units? (meters, mm2,...)
jedft said:
The output electronics of 4-20 ma devices are usually constant current generators. Their function is to maintain a constant current under all loading conditions. (that is if a signal is 12 ma then the constant current generator will maintain that current under all load conditions.) This makes makes the signal very resistance to noise pickup.
Process transmitters are designed to drive a 20 ma signal into 1000 ohms load with a 24 vdc supply. The usual input resistance of the receiving device is 250 ohms. Therefore you have 750 ohms of suplus capacity. With the low resistance of copper wiring you can drive a 4-20 ma signal 10,000 feet.
Normally 16 awg twisted/shielded pairs are used for 4-20 ma signals. For long runs steel conduit should also be used. For signal distances under a 1000 ft no special precautions needed.
Process transmitters are designed to drive a 20 ma signal into 1000 ohms load with a 24 vdc supply. The usual input resistance of the receiving device is 250 ohms. Therefore you have 750 ohms of suplus capacity. With the low resistance of copper wiring you can drive a 4-20 ma signal 10,000 feet.
Normally 16 awg twisted/shielded pairs are used for 4-20 ma signals. For long runs steel conduit should also be used. For signal distances under a 1000 ft no special precautions needed.
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