4-20ma and loop powered

techbas,

1. both AI card and transmitter do not supply power to tx, so external power is mounted btw AI and tx, and it is done the way you described, to close the loop. here AI is connected to 4-20ma and return terminals.

2. this one was already described. here AI is connected to 24vdc and 4-20ma terminals.
 
> I have one analog input card where 4-20ma transmitters are connected in two
> different ways. The first one is as follows: One wire from a 24VDC power
> supply is connected to one wire of the transmitter, the second wire of the
> transmitter is connected to the input screw on the analog input card while the
> other wire from the 24vdc power supply is connected to the "ground screw right
> next to the input screw on the analog input card. This will close the loop.

> The other wiring option I have seen is that the two wires from the transmitters
> goes straight to the two screws on the analog input cards.

> can anyone explain the difference on these two wiring scheme?

>
>any help will be appreciated

BOTH OF THE ABOVE SCHEMES SUGGEST THAT THE CONTROL SYSTEM SIDE OR DCS CARD IS PASSIVE. IT BASICALLY SITS THERE AND MONITORS THE LOOP CURRENT. IT ALSO HAPPENS TO BE THE END OF THE LINE IN THAT IT IS CONNECTED TO GROUND AND SINKS THE CURRENT THROUGH THE POWER SUPPLY RETURN.

HOWEVER THE FIELD INSTRUMENTS IN BOTH EXAMPLES ARE DIFFERENT.

1. IN THE FIRST CASE THE INSTRUMENT HAS A PASSIVE OUTPUT WITH THE LOOP POWER BEING PROVIDE FROM THE PANEL 24VDC SUPPLY(THE FIELD INSTRUMENT MIGHT BE LOOP POWERED - IT ALSO MIGHT HAVE A SEPARATE SUPPLY THAT POWERS IT FROM ELSEWHERE).

2. IN THE SECOND CASE THE 4-20mA SIGNAL IS CLEARLY NOT GENERATED FROM THE CONTROL SIDE BUT BY THE INSTRUMENT SIDE WHICH I WOULD CALL AN ACTIVE OUTPUT FROM THE INSTRUMENT. IT IS LIKELY THAT THE INSTRUMENT HAS A SEPARATE POWER SUPPLY (4 WIRE SYSTEM)AND THAT THE 4-20 IS DRIVEN FROM THE INSTRUMENT LIKE IS USUALLY THE CASE WITH EM FLOWMETERS WHICH CAN HAVE A 110Vac SUPPLY AND AN ACTIVE 4-20mA OUTPUT.

HOPE THIS IS USEFUL AND MAKES SENSE

REGARDS
SL
 
how we can modify a PLC designed to connect 2 wire system AI to receive 4-20mA which is coming from a externally powered transmitter.

The PLC terminals 23(-AI), 40(+24VDC), 41(-24VDC) & G is designed to use as a 2 wire loop powered transmitter input.

But my application giving the AI value from another PLC 4-20 mA not from a 2 wire transmitter.

Here the terminals 23 & 40 already have 24VDC. so how I connect my 4-20mA to these input by modifying the internal wiring of the PLC.
 
get a signal isolator. Weidmuller make several varieties i'm familiar with, one that has 4-20 in with 4-20 out. this will keep both systems separate but able to "talk" to each other

cheers,
peter
 
A

amaresh kumar mishjra

> Why 4-20ma and not 0-20ma or 0-16ma.

We use 4- 20 ma not 0- 20 ma because if the loop wire disconnect then how you will come to know that you are getting 0 ma due to open wire or from transmitter itself.
 
> why we have to use 4 wire connection we can measure with 2 wire itself.........

There may be different cases

case-I: The power supply for the instruments is different from that of 24VDC e.g. oxygen analyzers and all other analytical instruments. Mostly they need 110-220 VAC/DC power supply. So they can never be loop powered.

Case-II: when the load of the instrument is more than that of specified by barriers or interposing devices e.g. LEL detectors. Though they are powered with 24 VDC still we have to do 4 wiring for them.

Like these there may be many other cases, which other forum members may have come across.
 
Does it matter where the 24 vdc for external supply comes from? Could I use a 24vdc from a normal mcb in my cabinet or does it have to be separately isolated or something like that?

> 1. both AI card and transmitter do not supply power to tx, so external power is mounted btw AI and tx, and it is done
> the way you described, to close the loop. here AI is connected to 4-20ma and return terminals.
 
I'm pretty new to use PLC. I have a ABB AI810-card with module TU810. Do anyone know if I can use a separate 24V supply from a miniature circuit breaker in a 24V distribution cabinet? This supply is not the same supply as the supply to the AI-card. Do I have to connect the zero from distr.-cabinet to the AI-card?

Is it not possible to distribute the 24V directly from TU810, or maybe another TU-module?
 
> I'm pretty new to use PLC. I have a ABB AI810-card with module TU810. Do anyone know if I can use a separate 24V supply
> from a miniature circuit breaker in a 24V distribution cabinet? This supply is not the same supply as the supply to the
> AI-card. Do I have to connect the zero from distr.-cabinet to the AI-card?

You should go to http://solutionsbank.abb.com, sign up, and get the manual. I see that there is a free subscription (I have the premium subscription at a cost), but have to assume that something basic like a hardware manual is available.

That said, per page 84 of document "3BSE 020 924R501" the AI810/TU810 drawing shows that there are two groups of four inputs that share the same common reference (terminal ZP). So for an externally powered device you connect the power supply common to ZP (L1+) for channels 1 through 4, or ZP (L2+) for channels 5 through 8. A 4-20ma signal is connected to terminal B1-B8, 0-20ma to C1-C8, and 0-10V signal to C1-C8. You can only have two independent external sources per AI810 modules, unless you are willing to tie the commons together.

> Is it not possible to distribute the 24V directly from TU810, or maybe another TU-module?

For 2 wire transmitters, the 24VDC loop power is distributed on the TU810 by connections to L1+/ZP and L2+/ZP. If you have a device that requires a separate 24V supply, you can use the same power connected to the TU810. However, you do of course need to consider the ramifications of a short circuit. I would probably go back to the 24V power supply distribution and get a separately fused feed. If the transmitter is 0-20ma or 0-10V, you then have to consider that a blown fuse may not be apparent since the signal will not go to a bad quality status. Those considerations are not really any different then if you used an external 24V source closer to the transmitter(s).
 
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