Questions regarding 4 to 20 MA current signals
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Why are 4-20ma current signals commonly used for instrument output signals, rather than 0-20ma current signals? This post was last edited by baodingzhai on 2009-3-30 at 20:56.]
It is impossible to determine whether it is no signal or 0 ma for 0-20 ma
What was said upstairs is correct; at 0 mA, it’s difficult to determine whether the instrument’s output is indeed at zero or if there is a fault with the instrument； However, it’s different at 4mA: an output of 4mA indicates the instrument’s zero position, while 0mA indicates that the instrument is malfunctioning.
Early instruments used 0-10mA for signaling, but in actual operation, when a fault occurred in the instrument, the current would be 0, making it impossible to determine whether there was indeed a fault or simply the instrument’s normal zero level. Therefore, the control current for these instruments was later changed to 4-20mA, which prevented false zero readings and eliminated potential problems.
If 0 to 20 milliamps are used, it is impossible to determine whether there is a fault with the instrument or whether it is outputting a value of 0.
This is the difference between type-3 tables and type-2 tables. 4-20mA 0-10mA
Since it is a two-wire transmission system in which the supply current to the transducer unit is used as the signal, and the transducer unit requires a certain amount of static operating current, the lower limit of the signal cannot be set at 0 mA. According to IEC standards, the lower limit for current signals is 4 mA, which is what is referred to as a \"live\" zero point. A zero point of 4mA for the instrument is safer than 0mA, as it makes it easier to determine whether the instrument is powered off or is indicating the minimum value.
Since two-wire instruments share both the signal line and the power line, when using 0-20mA, the transmitter cannot operate when the current is 0mA.