2007-10-05 02:12:57 · 5 answers · asked by shawn m 1 in Science & Mathematics ➔ Engineering
4 to 20 milliamps. Commonly used in controls because it is low amperage.
2007-10-05 02:22:21 · answer #1 · answered by ron197192064 4 · 0⤊ 0⤋
It is a standard used in control and instrumentation. With a single pair of conductors (instead of a coaxial line, or multiple pairs of wires) a transmitter (i.e. pressure, temperature, or flow gauge with a 4-20 mA output) can convey it's information on that single pair of wires in a loop from the gauge transmitter to a receiver. The receiver measures the current in the wire loop and also provides the head or excitation voltage.
If the transmitter allows 4 mA of current to flow in the loop pair of wires, that represents the lowest reading the gauge can make, and if 20 mA flows in the loop then the gauge is measuring the maximum value.
You might ask why it's not zero to 20 mA. If that were the case, the receiver could not tell the difference between an open wire loop (accidentally cut wires or disconnected gauge) or the minimum reading. Both conditions would have 0 mA flowing in the loop. Instead, the minimum current is 4 mA, because that also provides some level of noise reduction.
If the pressure, temperature, flow, etc. transducer and transmitter were a voltage output, then the signal would be very susceptible to noise at the receiver end. A current loop cuts down on system noise (for reasons I won't get into, here).
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2007-10-05 04:33:08 · answer #2 · answered by tlbs101 7 · 1⤊ 0⤋
Everyone here is slightly wrong in one way or another, especially the judgmental ones. There were a group of pot smokers/enthusiasts in Humboldt years ago that began their usage in high school. They would meet as soon as they could after school, which was 4:20pm. This evolved into a sort of holiday observance on April 20th at UC Berkeley and spread from there.
2016-05-21 07:38:58 · answer #3 · answered by Anonymous · 0⤊ 0⤋
Generally, when I see this, it is referring to the current through a serial communications port with RS232 or RS422 protocol. That expresses the amount of current in the serial link required to change logic states between the computer and the peripheral.
2007-10-05 04:04:23 · answer #4 · answered by Warren W- a Mormon engineer 6 · 0⤊ 0⤋
Yeah,what he said. 0-5v is also a standard for instrumentation.
2007-10-05 05:58:38 · answer #5 · answered by Russh 2 · 0⤊ 0⤋