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Anonymous Poster

Transducer Output Why 4-20mA, Why Not 0-20mA

07/18/2010 6:33 AM

All transducer output is starting from 4mA , why it can not start from 0mA?

Please explain this

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#1

Re: Transducer Output why 4-20mA, why not 0-20mA

07/18/2010 9:31 AM

No current (0mA) is used to indicate a faulty trancducer or a broken connection. Also it makes the circuitry for adjusting the zero point simpler, as no negative bias is needed.

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#2
In reply to #1

Re: Transducer Output why 4-20mA, why not 0-20mA

07/18/2010 10:13 AM

GA. JohnDG

4-20ma also provides the possibility that the transducer electronics need not be independently powered if less than 4ma can drive the required biasing electronics.

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#3

Re: Transducer Output why 4-20mA, why not 0-20mA

07/18/2010 1:47 PM

its possible that transducer wont work 0-4 and only starts working at 4 upwards.

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#4
In reply to #3

Re: Transducer Output why 4-20mA, why not 0-20mA

07/18/2010 1:57 PM

It's not just possible, they're designed to work that way. This way, as JohnDG pointed out, if the receiver gets anything less than a 4mA draw it should be considered an invalid signal.

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#5
In reply to #3

Re: Transducer Output why 4-20mA, why not 0-20mA

07/18/2010 8:12 PM

Peter, please do a bit of research of your own before diving in and making a fool of yourself. The simplest search will give you all the information needed here - you don't need to guess.

[And if you can't work out how to follow these links[1], then I'm afraid you are a lost cause].

[1] Hint: move the pointer so that it's over the underlined bit, and click the left button.

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#6
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Re: Transducer Output why 4-20mA, why not 0-20mA

07/19/2010 12:52 AM

get stuffed

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#7
In reply to #6

Re: Transducer Output why 4-20mA, why not 0-20mA

07/19/2010 10:32 AM

Aren't you the testy one?

Got a link for that?

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#8
In reply to #7

Re: Transducer Output why 4-20mA, why not 0-20mA

07/19/2010 11:40 AM

fed up of being picked on for trying to help.

just cos i am not a big headed t*at like you

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#10
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Re: Transducer Output why 4-20mA, why not 0-20mA

07/19/2010 6:58 PM

Promise? :D

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#9

Re: Transducer Output why 4-20mA, why not 0-20mA

07/19/2010 1:52 PM

If you have a broken sensor it will give possibly zero output so how do you know if your sensor is working if it starts at zero & 4mA shows some current can pass so it is highly lightly it is working.

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#11

Re: Transducer Output Why 4-20mA, Why Not 0-20mA

07/27/2010 8:27 PM

in any transmission system, a standard signal range is desirable. In the case of 4-20mA signal transmission, 4mA is called the live zero, although 0mA may seem obvious starting point for current signal range, a live zero is commonly used for this reference for a number of reasons:

Instruments can be checked and calibrated more accurately when a signal below the zero reference can be read on test equipment.

The dynamic response of a decreasing signal is improved with live zero(4mA), thus will avoid the transmission lag.

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Anonymous Poster
#12

Re: Transducer Output Why 4-20mA, Why Not 0-20mA

11/07/2010 1:39 AM

further question: Why 4-20mA analog signal is being used in most industrial controller..Why not using of voltage or resistance? Thnx...

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#13

Re: Transducer Output Why 4-20mA, Why Not 0-20mA

11/07/2010 1:42 AM

Why 4-20mA analog signal is being used in most DCS controller, why not using voltage and resistance? Thank you

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#14
In reply to #13

Re: Transducer Output Why 4-20mA, Why Not 0-20mA

11/07/2010 6:20 AM

The wonderful thing about current is that the current in a closed loop is the same at every point in the loop, so what goes in (at the sensor end) must come out (at the receiver end). Provided that the resistance of the wiring is not too high for the voltage the sending end has available to drive the current, the signal will be independent of the wiring resistance (and variations of it). Also, because the receiver input impedance is low (typically about 100Ω) the circuit will not be sensitive to voltage pick-up (noise) in the wiring.

Using voltage, the wiring resistance will affect the received signal level (assuming a finite current is flowing). The lower the input impedance of the receiver, the higher the current must be to maintain the voltage, and the more effect the wiring resistance will have. On the other side, if you increase the receiver input impedance, there will be a higher susceptibility to noise pick-up, so there's always some kind of trade-off made to optimize performance.

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#15

Re: Transducer Output Why 4-20mA, Why Not 0-20mA

11/07/2010 8:25 AM

JohnDG thanks a lot..actually, my boss asked me bout that since I'm new in the field of instrumentation..my answer was that of KCL as what you've stated above but he told me to research more bout it..well, thanks so much..I am now more confident with my answer..regards!

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