A signal representation of 4-20ma is common because it does not require a lot of power - using a typical 24 volt power source, the resistance to deliver 20ma is only 1.2 KOhms and the power is only 0.48 watts - and, by offsetting the low end to 4ma, the receiver can detect an open connection.
Normally, a 4-20ma current loop is connected with a twisted pair cable, with both the current source and the current sink on one end. This way, you get good common mode rejection of noise. Using a differential amplifier with, say, a 10 ohm receiver resistor, you only need a delta voltage of 200 mV, so you can still have a substantial common mode range, typically, + or - 12 volts.
4-20ma is an industry standard for signalling in a process monitoring and control environment. 4ma is one end of the range. 20ma is the other. If the receiver sensed 0ma, it would conclude that the circuit is open, and raise an appropriate alarm.
Because 4-20ma is an industry standard.
Ohm's Law states that V=I*R where V = voltage, I = current, and R = resistance. So in this case, V = 3kOhm*20mA, or V = 3000*.020, and V = 60 volts.
There really is no standard color for this. Most that I have seen use the Black as the positive (+) and the white as negative (-) but I have also seen it the other way around. Just follow the convention of the plant or customer where you are working.
A: Since you know the current flow you need the voltage drop for this particular led at 20ma. All LED require a current and voltage to operate properly assuming a voltage drop of 2v then for a 12v source it becomes 12-2=10v 10v/.02=500 ohms in series to limit the current is required
4-20mA is used because in that way we can distinguish a ZERO value signal from a malfunction in the measuring circuit (i.e. disconnected cable).
4-20ma is an industry standard for signalling in a process monitoring and control environment. 4ma is one end of the range. 20ma is the other. If the receiver sensed 0ma, it would conclude that the circuit is open, and raise an appropriate alarm.
Because 4-20ma is an industry standard.
24
24 milli omps
Previously when transmitters were configured for 0-20mA signal it was very difficult to identify an open circuit at 0mA,hence to supersede this defect it was decided to configure the transmitter as 4-20mA signal,so that if there is open signal it can be easily identified.
In industrial engineering applications, a signal range of 4..20mA is often used to communicate a 0..100% value. The base offset of 4mA allows a distinction between the value zero, and an unavailable value (broken wire, etc). The common 0..10V signal, often supported as an alternative to the superior 4..20mA current signal, does not provide fault detection in this manner. It is possible that applications of voltage based signals exist, where the minimum voltage is 1V. These systems would also provide fault detection in a manner similar to the 4..20mA current signal.
4ma = 1v rising to 5v @ 20ma
20mA
The difference between Absolute Presure and Gauge Pressure is 14.7psi. So zero gauge pressure (0psig) = 14.7 psia. 600 psig = 614.7 psia. Calibrate the tranmitter so 4mA is obtained at 14.7 psig and 20mA is obtained at 614.7psig. The span will be 600 psi in both cases.
because 4 ma for live zero and at 20 ma no spark will generate and it is safe current this is why we use 4-20 ma and another thing is tx and cotrollers even plc and Dcs desinged to accept 4- 20 ma singal which is standard
The 4-20mA current loop standard was designed for analog instrumentation signals due to its simplicity, noise immunity, and ability to send signals over long distances without significant signal loss. It also allows for easy detection of broken wiring or sensor failure since a zero reading or out-of-range measurement implies an issue in the circuit.