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Diagram 2, 3, 4 - wire transmitter working principle and related problems
Dec 12, 2018

Diagram 2, 3, 4 - wire transmitter working principle and related problems

Transmitter wiring mode

Two-wire system, three-wire system, four-wire system, refers to the output of a variety of analog dc current signal transmitter, its working principle and structure of the difference, rather than just refers to the transmitter wiring form.

So the first thing that came out was a four-wire transmitter;In other words, two wires are responsible for the supply of power, and the other two wires are responsible for the output of converted and amplified signals (such as voltage, current, etc.).But at present, many transmitters use two - wire system.

 

Diagram 2, 3, 4 - wire transmitter working principle and related problems

The figure above shows the wiring diagram of the two-wire transmitter.Two wire transducer as shown in the above, the power supply is 24 VDC, output signal for the DC4-20 ma, load resistance is 250 Ω, lowest negative line voltage of 24 v power supply, it is the public line signal.

 

Diagram 2, 3, 4 - wire transmitter working principle and related problems

The figure above shows the wiring diagram of the four-wire transmitter.Most of the power supply is AC 220V, and some are DC 24V.DC4 ~ 20 ma output signals, and the load resistance is 250 Ω, or DC0-10 ma, load resistance to 0-1.5 K Ω;Some also have mA and mV signals, but the load resistance or input resistance, depending on the output circuit form and different values.

 

Diagram 2, 3, 4 - wire transmitter working principle and related problems

The figure above shows the wiring diagram of the three-wire transmitter.The so-called three-wire system is that the positive end of the power supply USES one line, the positive end of the signal output USES one line, and the negative end of the power supply and the negative end of the signal share one line.Its power supply mostly DC24V, DC4 ~ 20 ma output signals, and the load resistance is 250 Ω or DC0-10 ma, load resistance to 0-1.5 K Ω;Some also have mA and mV signals, but the load resistance or input resistance, depending on the output circuit form and different values.

Related problems

1. What are the advantages and disadvantages of current output type and voltage output type?

In many applications controlled by microcontroller, transmitter is used to convert the signal that cannot be directly measured by microcontroller into the electrical analog signal that can be processed by microcontroller, such as current transmitter, pressure transmitter, temperature transmitter, flow transmitter and so on.Most of the early transmitters were of voltage output type, that is, the measurement signal was converted into 0-5v voltage output, which was the direct output of the operational amplifier. The signal power <0.05w was converted into digital signal by the analog/digital conversion circuit for the single-chip microcomputer to read and control.But needs in long distance transmission signal or the occasion, with large environmental eccn interference voltage output type sensor use is limited by a lot, exposed the anti-interference ability is poorer, line loss destroyed the precision shortcomings and so on and so on, the two wire type output current transducer with high anti-jamming ability has been widely applied.Voltage output type transducer anti-interference ability is poor, the destruction of the line loss, not how high precision, is sometimes superimposed on the dc voltage output have communication components, make SCM make wrong judgments, error control, serious when still can damage equipment, 0 to 5 v never far eastone, remote transmission, line drop after precision.Now a lot of adc, PLC, DCS input signal ports are made into a two-wire system current output transmitter 4-20ma, which proves the inevitable trend of voltage output transmitter is eliminated.

pressure transmitter

2.What are the six comprehensive protection functions of the two-wire current transmitter?

(1) input overload protection;

(2) output over-current limiting protection;

(3) long-term short-circuit protection of output current;

(4) two-wire port transient induced lightning and TVS suppression protection of inrush current;

(5) the overvoltage limit protection of the working power supply is no more than 35v;

(6) reverse connection protection of working power supply.

3.How to distinguish the merits and demerits of the current and voltage transmitter?

With the commonly used 0.5 accuracy of the current and voltage transmitter as an example, from the following methods to distinguish the pros and cons.

(1) the reference should be stable. 4ma is the corresponding input zero reference. The reference is unstable.(that is, 3.98-4.02ma), the pressure drop on the load 250 ohms is 0.995-1.005v, the foreign IC core multi-purpose expensive gap benchmark, temperature drift coefficient per degree change of 10ppm;

(2) the total consumption current of the internal circuit is less than 4ma, and it is equal to 4.000ma after being adjusted. In addition, the consumption current of the active rectifier filter amplifier constant current circuit does not change due to the change in the input of the original edge, and foreign IC cores are supplied with constant current;

(3) when the working voltage is 24.000v, the full range of 20.000ma, the full range of 20.000ma reading will not change because of the load 0-700 ohm changes;The change shall not exceed 20.000ma0.5%;

(4) when the full range of 20.000ma, load 250 ohms, the full range of 20.000ma reading will not vary due to the working voltage of 15.000v-30.000v changes;The change shall not exceed 20.000ma0.5%;

(5) when the original side is overloaded, the output current shall not exceed 25.000ma+10%; otherwise, the 24v working power supply for the transmitter in PLC/DCS and the a/d input clamping circuit will be damaged due to excessive power consumption. In addition, the output output of the transmitter will also be damaged due to excessive power consumption, especially for the circuit without a/d input clamping;

(6) the transmitter shall not be damaged when the working voltage is connected to the reverse at 24v, and it must be protected by polarity;

(7) clamping should be carried out between two wires due to induced lightning and induced surge voltage exceeding 24v, and the transmitter should not be damaged;Generally, 1-2 TVS transient protection diodes 1.5ke in parallel between two lines can suppress the impact of positive and negative pulses with a pulse width of 20 milliseconds every 20 seconds, and the transient power is 1.5kw-3kw.

(8) whether 0.5% of the linearity of product labeling is an absolute error or a relative error, it can be clearly distinguished by the following methods:

The pressure drop on 250 ohms is 0.995-1.005v.

When the original side input 10%, the output is 5.6ma plus or minus 0.5%(5.572-5.628ma). The pressure drop on the 250 ohm load is 1.393-1.407v.

The pressure drop on 250 ohms of load 8ma plus or minus 0.5%(7.96-8.04ma) was 1.990-2.010v when the original side input 25%.

The pressure drop on 250 ohms load of 12ma plus or minus 0.5%(11.94-12.06ma) is 2.985-3.015v when the original side input is 50%.

The pressure drop on 250 ohms of load 16ma plus or minus 0.5%(15.92-16.08ma) when the original side input 75% is output is 3.980-4.020v.

The pressure drop on 250 ohms of load 20ma plus or minus 0.5%(19.90-20.10ma) was 4.975-5.025v when the original side was 100% input.

(9) the current must be limited when the input overload of the original edge:

When the input overload of the original side is greater than 125%, the voltage drop on the 25ma+10%(25.00-27.50ma) load 250 ohms is 6.250-6.875v.

(10) when the induced surge voltage exceeds 24v, there is no clamping discrimination: in the output port of two lines and an ac 50v pointer type meter head, connect two lines of ac 50v to the output port of two lines in an instant, to see if there is clamping, clamping how many volts can be seen at a glance;

(11) there is no polarity protection discrimination: with the pointer type multimeter ohm times 10k positive and negative measurement of the output port of the two lines, there is always a ohm resistance value is infinite, there is polarity protection;

(12) there is no pole output current short-circuit protection for a long time: the original input 100% or overload is greater than 125%-200%, the load 250 ohm short-circuit, measurement of short-circuit protection limit is in 25ma+10%;

(13) discrimination between industrial level and civil and commercial level: the working temperature range of industrial level is from -25 degrees to +70 degrees, and the temperature drift coefficient is 100ppm per degree change, that is, the temperature changes by 1 degree per degree, and the accuracy changes by 1/10000;

The working temperature range of civil and commercial grade is from 0 degree (or -10 degree) to +70 degree (or +50 degree), and the temperature drift coefficient is 250ppm per degree change, that is, the temperature is 1 degree per degree change, and the accuracy is 2.5 parts per 10,000;

The temperature drift coefficient of current and voltage transmitter can be verified by constant temperature chamber or high and low temperature chamber.