The account that uses voltaic signal is to not allow to suffer interference easily, because of the extent of the noise voltage of industrial spot the likelihood achieves several V, but the power of noise is very weak, so noise electric current is less than NA level normally, because this gives 4-20mA to transmit transmission error,internal resistance of current source of very small; tends infinity, lead resistor series connection does not affect precision in loop, because the great internal resistance of current source and constant current are outputted,can transmit hundreds meters of; on common double skeining thread accordingly, in sink we need to place a 250 ohm to be able to obtain the voltage of 0-5V to the resistor of the ground only, the noise of input electric current that the advantage of the receiver of low input impedance is NA class produces very faint voltage noise only.

 

Upper limit takes 20mA is the requirement because of explosion proof: The electric current of 20mA connects the scintilla energy that cause can’t ignition gas. The account that floor level did not take 0mA is to can detect line: When regular job won’t under 4mA, open circuit because of breakdown when transmission line, annulus road electric current falls for 0. Often take 2mA to serve as a line to call the police value. Electric flow pattern changes send implement output physical quantity changeover into 4~20mA electric current, the power source outside wanting to have necessarily is its power supply. The most typical is to change send implement need two power supply cord, add on two electric current to output a line, want to receive 4 lines in all, say make for 4 lines change send implement. Of course, voltaic output is OK as public as power source a line (public VCC or GND) , can save a line, basically make 4 lines now so change send implement say make for 3 lines change send implement. Actually everybody may notice, itself of 4-20mA electric current can send to change implement power supply, change send implement a special load is equivalent to in circuit, this kind changes send implement 2 lines are received outside needing only, be called consequently two lines are made change send implement. Floor level of level of annulus of industrial electric current is 4mA, because this is inside span limits, change send implement have 24V only normally, 4mA power supply (accordingly, efficient below light laden condition DC/DC power source (TPS54331, TPS54160) , the sensor of low power comsumption and signal catenary product, and the processor of low power comsumption (send and receive very important) to two thready 4-20mA like MSP430) . This makes the design that two lines make sensor becomes a possibility and rich challenge.

 

General need designs converter of a VI, input 0-3.3v, output 4mA-20mA, can use carry to put LM358, power supply + 12v.

 

We see imitate measure equipment why to be favored systematically transmit signal ~ with 4~20mA

 

4-20mA. DC(1-5V.DC) signal is made is international Electrotechnical Committee (the imitate signal that system of control of IEC) process uses transmits a standard. Our country also uses signal of standard of this one international to make, appearance transmits signal to use 4-20mA.DC, receive signal to use 1-5V.DC, use electric current to transmit namely, the signal system that voltage receives.

 

The signal electric current of general instrument appearance is 4-20mA, show the smallest electric current is 4mA, the biggest electric current is 20mA. Transmission signal moment, because there also is resistance on lead, if use voltage to transmit a meeting to arise to be pressed certainly inside lead,fall, the signal of that sink can produce certain error, so signal of general use electric current serves as change send implement the standard is transmitted.

 

One, what is 4~20mA.DC(1~5V.DC) signal is made?

 

4~20mA.DC(1~5V.DC) signal makes is international Electrotechnical Committee (IEC) : The process controls a system to use imitate signal level. Our country begins to use signal of standard of this one international to make from DDZ- Ⅲ dynamoelectric appearance, appearance transmits signal to use 4~20mA.DC, contact signal uses 1~5V.DC, use electric current to transmit namely, the signal system that voltage receives.

Principle of job of annulus of 4~20mA electric current:

 

 

In industrial spot, the recuperation that will finish signal with an appearance amplifier is hand-in-hand president line is transmitted, can produce the following problem: The first, as a result of transmission signal it is voltage signal, transmission line can get the interference; of noise the 2nd, the distributinging resistor of transmission line can produce voltage drop; the 3rd, the working voltage that how offers appearance amplifier in the spot also is a problem.

 

To solve afore-mentioned problems and escape the influence of relevant noise, we transmit signal with electric current, because electric current is right noise is not sensitive. The voltaic annulus of 4 ～ 20mA is to use 4mA to represent 0 signal, represent the full scale of signal with 20mA, and call the police under what the signal of 20mA of 4mA prep above uses at all sorts of breakdown.

 

2, the good point that 4~20mA.DC(1~5V.DC) signal makes?

 

Spot appearance can realize two lines to make, alleged two lines are made namely power source, laden series connection is together, have one public site, and the site changes send implement use two wire only with the signal contact before controlling room appearance and power supply. Because signal start electric current is 4mA.DC, send to change implement provided static job electricity, appearance is electric at the same time was 4mA.DC at 0 o’clock, not with machinery at 0 o’clock coincide, this kind of “ is vivid ” was helpful for identifying at 0 o’clock cut off the power and break the breakdown such as the line. And two lines are made still facilitate use safe bar, benefit at safe explosion proof.

 

Control room appearance uses signal of voltage paralell connection to transmit, same there is public end between the appearance that place belongs to a control system, facilitating measuring instrument, adjust appearance, computer, warning decice deserves to use, convenient wiring.

 

Spot appearance and the reason that control the contact signal between room appearance to use 4~20mA.DC are: Because the distance between the spot and control room is further, the resistance of join electrical wiring is greater, if use voltage signal far pass, resistor of excel electrical wiring and the cent that receive appearance to input resistor are pressed, will produce bigger error, and serve as with signal of constant current source far pass, want deferent loop not to appear only branch, the electric current in loop won’t follow electrical wiring accident and change, assured deferent precision thereby.

 

Controlling the contact signal between room appearance to use 1~5V.DC argument is: To facilitate much stage appearance is received jointly same a signal, be helpful for wiring and make all sorts of complex control systems. If use current source to make contact signal, receive jointly when much stage appearance same when signal, their input resistor must establish ties rise, this meeting makes the greatest laden resistance changes more than the laden ability that sends appearance, and the signal that receives instrument each loses upright potential each are not identical, can introduce interference, and cannot accomplish onefold centralized power supply.

 

Use contact of voltage source signal, the voltaic signal that uses with the contact of spot appearance must be changed for voltage signal, the simplest way is: The standard resistance that sends the series connection in loop a 250Ω is expressed in electric current, it is 4~20mA.DC changeover 1~5V.DC, normally by match electric equipment to finish this one job.

 

3, why to change send implement does choice 4~20mA.DC make deferent signal?

 

1, it is the safe consideration from field application above all

 

With explosion proof safety scintilla appearance considers safe key, it is premise in order to control appearance energy, reduce the static state that maintains appearance to work normally and dynamic power comsumption to lowermost limit. Output 4~20mA.DC standard signal change send implement, voltage of its power source uses 24V.DC normally, the main reason that uses volts d.c. is the capacitor that need not use high capacity and inductor, need to consider to change only send implement the distributinging capacitance with lead of join of control room appearance and inductance, like the lead of 2mm2 its distributing capacitance controls; to be 0.4mH/km to control; to single-track inductance for 0.05μ/km greatly under detonate hydric numerical value, apparent this is very advantageous to explosion proof.

 

2, deferent signal uses source of voltage of current source excel

 

Because the distance between the spot and control room is further, when the resistance of join electrical wiring is greater, if use voltage source signal far pass, because electrical wiring resistor and the cent that receive appearance to input resistor are pressed, will produce bigger error, if use current source signal to regard as far pass, want deferent loop not to appear only branch, the electric current in loop won’t follow electrical wiring accident and change, assured deferent precision thereby.

 

3, signal is the biggest the account that electric current chooses 20mA

 

The choice of 20mA of the biggest electric current is the consideration that is based on safe, practical, power comsumption, cost. Safe scintilla appearance can use low voltage, low electricity only, 4~20mA electric current and 24V.DC are right combustible and hydric also be safety, it is 200mA to 24V.DC detonates electric current hydricly, far in 20mA above, in addition the join distance between appearance of site of production of even integrated consideration, what take the element; such as load to return bad news of have rendered great service and cost issue, to the requirement of electronic component, wait for an element for the requirement of electric power.

 

4, the account that signal start electric current chooses 4mA

 

Output those who be 4~20mA to change send implement be in the majority with two thready, two lines are made namely power source, laden series connection is together, have one public site, and the site changes send implement use two wire only with the signal contact between control room appearance and power supply. Why is start signal 0mA? This is to be based on at 2 o’clock: It is to change send implement circuit will not work without static job electric current, 4mA.DC of signal start electric current, not with machinery at 0 o’clock coincide, this kind of “ is vivid ” was helpful for identifying at 0 o’clock cut off the power and break the breakdown such as the line.

 

4, the origin of 4~20mA sensor?

 

The account that uses voltaic signal is to not allow to suffer interference easily, and infinity of current source internal resistance, lead resistor series connection does not affect precision in loop, hundreds meters can be transmitted on common double skeining thread.

 

The account that uses voltaic signal is to not allow to suffer interference easily, because of the extent of the noise voltage of industrial spot the likelihood achieves several V, but the power of noise is very weak, so noise electric current is less than NA level normally, because this gives 4-20mA to transmit transmission error,internal resistance of current source of very small; tends infinity, lead resistor series connection does not affect precision in loop, because the great internal resistance of current source and constant current are outputted,can transmit hundreds meters of; on common double skeining thread accordingly, in sink we need to place a 250 ohm to be able to obtain the voltage of 0-5V to the resistor of the ground only, the noise of input electric current that the advantage of the receiver of low input impedance is NA class produces very faint voltage noise only.

 

Upper limit takes 20mA is the requirement because of explosion proof: The electric current of 20mA connects the scintilla energy that cause can’t ignition gas. The account that floor level did not take 0mA is to can detect line: When regular job won’t under 4mA, open circuit because of breakdown when transmission line, annulus road electric current falls for 0. Often take 2mA to serve as a line to call the police value. Electric flow pattern changes send implement output physical quantity changeover into 4~20mA electric current, the power source outside wanting to have necessarily is its power supply. The most typical is to change send implement need two power supply cord, add on two electric current to output a line, want to receive 4 lines in all, say make for 4 lines change send implement. Of course, voltaic output is OK as public as power source a line is public VCC or GND, can save a line, those who say make for 3 lines change send implement. Actually everybody may notice, itself of 4-20mA electric current can send to change implement power supply. Change send implement a special load is equivalent to in circuit, special place is sending at changing implement electric current of bad news report is outputted according to sensor between 4~20mA and change. Indication appearance needs to string together only in circuit can. This kind changes send implement 2 lines are received outside needing only, be called consequently two lines are made change send implement. Floor level of level of annulus of industrial electric current is 4mA, because this wants to be inside span limits only, change send implement have 4mA power supply at least.

 

Accordingly, the signal output of 4-20mA suffers interference not easily commonly and on the safe side, so what use generally is the signal of power source output that second line makes 4-20mA on industry. But the signal for can better processing sensor, output signal that still has more other form at present: 3.33MV/V;2MV/V;0-5V; 0-10V.

 

Add a piece additionally 4 turn to 20mA the simple circuit graph of voltage signal:

 

 

The resistor that this piece of graph uses a 250 ohm changes the 4 voltaic signal to 20mA into the 1 voltage signal to 5V, use wave of filter of a RC to add a diode next (excuse me imitate circuit is bad, knowing is what meaning) receive sheet piece the AD changeover of machine cites a base.