What is 4 20ma?

4-20ma is a type of signal transmission.

• A process value, such as pressure, RPM, or whatever, is converted into a current in the range of 4ma to 20ma and sent out on a twisted pair cable.
• The receiver passes the current through a resistor, converting it back to voltage, and processes it accordingly. If that resistor were 250 ohms, the voltage would be 1-5v.
• An offset of 4ma is used so that the receiver can detect an open connection.
• The ground or power reference is usually maintained at the transmitter, but sometimes it is at the receiver.
• A twisted pair cable is used so that the effect of common mode noise is minimized.
• The receiver is usually an opamp so that the effect of common mode voltage is minimized.