Sensor output --- voltage output or 4-20mA output is better?

[powersys]

I went to a training course few days ago. There were two presenters. One said voltage output was less sensitive to noise, but the other one said 4-20mA output was less sensitive noise. Therefore, I was quite confused. Pls advise what are the advantages of using 4-20mA compared to voltage output type of sensor? Thanks.

 

[wwfeda]

Re: Sensor output --- voltage output or 4-20mA output is bet

Current output is good if it has to travel through a long cable before being process. There isn't any voltage drop associated with currents along the cable.

If you send through voltage, you will have to deal with voltage drops and it is something you cannot control in your design.

 

[IanP]

Re: Sensor output --- voltage output or 4-20mA output is bet

... transmitting a sensor’s output as a voltage over long distances has several drawbacks. Unless very high input-impedance devices are used, transmitting voltages over long distances produces correspondingly lower voltages at the receiving end due to wiring and interconnect resistances. However, high-impedance instruments can be sensitive to noise pickup since the lengthy signal-carrying wires often run in close proximity to other electrically noisy system wiring. Shielded wires can be used to minimize noise pickup, but their high cost may be prohibitive when long distances are involved.

Sending a current over long distances produces voltage losses proportional to the wiring’s length. However, these voltage losses — also known as “loop drops”—do not reduce the 4-20mA current as long as the transmitter and loop supply can compensate for these drops. The magnitude of the current in the loop is not affected by voltage drops in the system wiring since all of the current (i.e., electrons) originating at the negative (-) terminal of the loop power supply has to return back to its positive (+) terminal—fortunately, electrons cannot easily jump out of wires!

Quotation from: https://www.datel.com/data/meters/dms-an20.pdf

Regards,
IanP

 

[powersys]

Can I say that both methods are also sensitive to noise? Thanks.

 

[IanP]

Re: Sensor output --- voltage output or 4-20mA output is bet

Technically speaking all signals are affected by noises ..
Numerically speaking, however, noise interference in current loop, because of its nature, is almost negligaible, so, if you add another crucial factor, namely distance at which a sensor can be installed, one can easily find out answers why sensors using current loops are so commonly used in industrial applications ..

Regards,
IanP

 

[Sceadwian]

You may be confused with someone saying a differential voltage output has a high noise immunity not a normal voltage output. Normal 0-5 volt (or whatever value's you use) is very succeptable to noise over any appreciable distance. Differential voltages though are not as succpetable because they run over two wires so each line is effected the same and it's the difference of their values that's important.

 

[powersys]

Re: Sensor output --- voltage output or 4-20mA output is bet

...however, noise interference in current loop, because of its nature, is almost negligaible....

Emm... I'm sorry to say that I don't really understand this point... Would you pls elaborate a bit further? Thank you very much in advance...

 

[Sceadwian]

It is very easy to cause voltage disturbances in a transmission line, virtually all electronics devices and even a decent number of mechanical ones produce a lot of RF radiation that is easily picked up as a voltage spike in a wire. Any length of wire will act as an antenna, the longer it is the more it will pick up. The same is not true of current in a transmission line. Currents are barely if at all effected by RF radation (at least on earth) because it's the current that's important not the voltage, and even a large RF signal will generate hardly any current in an antenna, especially when compared to the 4-20ma currents that a standard current loop uses. However a differential voltage signal is almost as immune to noise as a current loop is, an example is ethernet wire. It uses two wires for a transmission line, often twisted about itself, and it is the difference in voltage between those lines that the information is encoded in. RF radation will be accepted in both lines symetrically which will not effect the differential voltage. 4-20ma is still used a lot especially in industrial environments because high powered devices are often used and interference can be severe. Hands down both current loop and differential voltage pairs will beat a standard voltage out signal in noise immunity in every situation I can think of.

 

[manrayap]

Re: Sensor output --- voltage output or 4-20mA output is bet

Why 4-20 mA? Not 0-20 mA. Is it 0-3.99 mA (below 4mA) range effected by noise or not stable? Tks.

 

[IanP]

Re: Sensor output --- voltage output or 4-20mA output is bet

Other ranges are also used, but they are not as common as 4-20mA ..
Here are the other ranges: 0-50mV, 0-100mV, 0-5V (+/-), 0-10V(+/-), 0-100V, 4-20mA, 0-20mA, 0-50mA ..
The reason why 4-20mA is more popular is the "0-range" .. Just imagine you have cut wire, and you don't know about it, what do you measure: 0mA !!! , whereas with 4-20mA at "ZERO" the measurement is 4mA ..

Regards,
IanP

 

[VVV]

Re: Sensor output --- voltage output or 4-20mA output is bet

Noise and long distance between the sensor and the measurement A/D are the reasons that led to the development of the current loop.

Noise in industrial environments is always present and always strong.
To reduce the amount of noise, it is desirable to have a low impedance across the input of your A/D.
But a low input impedance will introduce large errors in the measurement if sensor outputs a voltage and the distance between the sensor and the A/D is long, which is often the case in idustrial environment. The problem occurs because the connecting wires have resistance, which coupled to a low impedance input creates losses and hence errors. Plus, various sensors can be at various distances, so it is impossible to compensate for these errors based on the known distance.

The soulution is of course a current loop. The input resistance is small, to keep the noise low. Then, by regulating the current, the voltage drop across the connecting wires does not matter. In the end you measure the voltage across the low input resistance (typical values for this resistance are 100 ohms to 1kohm; 250 ohm is very popular, since it produces a 1-5V signal across it, easy to measure). V=R*I. Since R is constant, V depends only on the current.

The 4mA lower limit is useful in detecting faulty wires: if the wires are broken, the current is zero and so is the voltage, so you know something is wrong.
If the wires are shorted, the current through the resistor will be almost zero, the voltage will be almost zero so again the fault is detected.

 

[powersys]

Re: Sensor output --- voltage output or 4-20mA output is bet

Noise and long distance between the sensor and the measurement A/D are the reasons that led to the development of the current loop.

Noise in industrial environments is always present and always strong.
To reduce the amount of noise, it is desirable to have a low impedance across the input of your A/D.
But a low input impedance will introduce large errors in the measurement if sensor outputs a voltage and the distance between the sensor and the A/D is long, which is often the case in idustrial environment. The problem occurs because the connecting wires have resistance, which coupled to a low impedance input creates losses and hence errors. Plus, various sensors can be at various distances, so it is impossible to compensate for these errors based on the known distance.

The soulution is of course a current loop. The input resistance is small, to keep the noise low. Then, by regulating the current, the voltage drop across the connecting wires does not matter. In the end you measure the voltage across the low input resistance (typical values for this resistance are 100 ohms to 1kohm; 250 ohm is very popular, since it produces a 1-5V signal across it, easy to measure). V=R*I. Since R is constant, V depends only on the current.

The 4mA lower limit is useful in detecting faulty wires: if the wires are broken, the current is zero and so is the voltage, so you know something is wrong.
If the wires are shorted, the current through the resistor will be almost zero, the voltage will be almost zero so again the fault is detected.

I don't understand the sentence highlighted in RED color above. Would u pls advise how a low impedance across the input of an A/D will reduce the amount of noise? Thanks.

 

[VVV]

Re: Sensor output --- voltage output or 4-20mA output is bet

Voltages induced in the wiring produce current that flows through the input resistance and produces a voltage across it. If the resistance is lower, the voltage developped is lower. Threfore, a lower resistance will reduce the noise, because the noise has to have a much higher energy to produce a voltage.

Imagine in the extreme case that the resistor is zero. No voltage can be developped across a zro ohm resistor. Increasing the resistance will make it easier and easier to develop a significant voltage across it.

A simple test you can do with a scope: touch the probe tip with your finger and note the noise amplitude. This is the noise induced when the input impedance is the probe's input impedance (typically 1MΩ for a 1:1 probe and 10MΩ for a 10:1 probe).
Then connect lower and lower resistors between the probe's tip and ground and note how much less noise there is, the lower the resistance.

If you do not have a scope, try with your audio amplifier: touch the input with your finger, without any resistor from the input to ground and note the hum intensity, then connect some resistance and see how the noise decreases.

 

[antedeluvian]

Re: Sensor output --- voltage output or 4-20mA output is bet

The 4mA lower limit is useful in detecting faulty wires: if the wires are broken, the current is zero and so is the voltage, so you know something is wrong.

It beats me that no one has mentioned that many 4-20mA transmitters are "loop powered". The 4mA is used to power the electronics and the transducer.

-Aubrey