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Hello everyone. We have a quiz and the
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first question is what does a current of
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represent in a standard 4 to 20 milliamp
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The 4 to 20 millia signals usually
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represents a process variable. The 4
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miampere represents 0% of process
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variable that means zero scale and 20
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full scale that means the full span of
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In this case if you see the options we
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negative value maximum process value
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minimum process value. The 4 milliampere
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represents minimum process value. That
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means zero percentage of the process
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variable or zero scale.
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What does a current of 20 miampere
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represent in a standard 4 to 20 milliamp
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loop? As discussed, the 20 m represents
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the 100%age of process variable. That
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means maximum process value.
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And we will go to next question.
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Why is 4 milliampere used as a minimum
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value in a 4 to 20 millia signal?
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What is the primary advantage of using 4
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milliampere instead of 0 milliampere?
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It is for live zero detection.
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What is the meaning of live zero?
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It is easy to identify a true zero
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reading and a practical failure.
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If you have 4 milliampere then the PLC
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identifies this 4 milliampere as true
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zero reading. Let's say due to some
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issue the cable is broken
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or your sensor is faulty then it is easy
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for the PLC to identify this faulty
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signal and we will call this as live
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In a 0 to 20 milliamp system, that means
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here the signal range is 0 to 20
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And here we're controlling a tank level.
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What will the PLC display if the tank is
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50% is full but the wire break? Let's
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say the system is normal. Then in that
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case we will receive 10 miampere signal
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to the PLC. That means it shows us
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50%age of tank level.
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The tank level the representation is 0%
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level signal. In this case the wire is
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broken. That means no current will be
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received by the PLC. That means we have
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zero milliampere at the PLC.
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As this scale range is 0 to 20 miampere,
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the zero here represents zero percentage
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of level signal. Option number D.
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Go to next question. Why are current
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voltage signals in industry?
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option number C. less noise sensitivity.
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The voltage signals are easily affected
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by the cable resistances,
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electromagnetic interferences,
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induction voltage interferences etc.
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That is why the current signal is more
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preferred and the standard in the
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industry for carrying the signals from
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the field to the control room or vice
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Next. Go to next question.
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What happens to loop current if a wire
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breaks in a 4 to 20 milliamp circuit?
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We have a sensor or transmitter and it
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is connected to a PLC
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and it is using 4 to 20 mAh loop
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The four represents minimum process
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value and 20 milliampere represents
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maximum process value
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and due to some reason the wire is
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broken right then how much current will
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it is zero the wire is broken there is
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no flow in the current option number C
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it drops to 0 milliampere
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let's go to next Question
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why are precision resistors used in 4 to
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The main reason is to convert this loop
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current into equivalent voltage signal 1
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The PLC's and DCS systems have analog
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input cards. All these cards have
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converters like analog to digital
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These converters require
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voltage signal as input. For that
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we have to convert this current signal
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into voltage signal. And then this
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voltage signal will be converted into
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equivalent digital signal and then it
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will be transformed from the analog
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input card to the CPU for further
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For that purpose, we need a simple
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standard resistor for converting
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current to voltage signal.
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What resistor value is commonly used to
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convert 4 to 20 milliamp signal into 1
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We are using standard 250 ohms resistor.
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As per the ohms law, V is equal to IIR.
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You can calculate R. R is equal to V by
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You can do this calculation. R is equal
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V by I. V means let's say 1 volt divided
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How much? It is 250 ohms.
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What does a loop current of 12
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milliampere represent in a linear
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Here we are assuming the standard
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uh current signal range which is 4 to 20
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milliamp signal. And in this standard
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range what is the 12 milliampere
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If you see you can do this calculation
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12 miampere is the uh 50% of signal if
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you see in this range 4 to 20 miampere
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signal so 12 miampere represents 50% of
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nothing but in terms of scale mid scale
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option C 50% of the scale of the
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measurement okay Next question.
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Which device converts a sensor signal
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into a standardized 4 to20 milliampere
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We have different options. Controller,
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actuator, transmitter and power supply.
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If you see the transmitter, it measures
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the uh process parameter and then
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converts that signal into equivalent 4
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to 20 milliamp signal and then sends
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this signal to the PLC system.
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So option C is the right answer.
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Thank you. I will meet you in the next
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If you have any questions or queries
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regarding these quizzes, comment us
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and we will update you accordingly.
