Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to
20 mA?
§ 4-20 mA is a standard
range used to indicate measured values for any process. The reason that 4ma is
chosen instead of 0 mA is for fail safe operation.
§ For example: A
pressure instrument gives output 4mA to indicate 0 psi up to 20 mA to
indicate 100 psi or full scale. Due to any problem in instrument (i.e) broken
wire, its output reduces to 0 mA. So if range is 0-20 mA then we can
differentiate whether it is due to broken wire or due to 0 psi.
§ In a digital input,
voltage between 1-5V is considered as ON and voltage less than 1V are
considered as OFF. A/D converters and
display devices are also working in the 5V range. So 250ohm resistor across the
4-20mA circuit will give a voltage of 1-5V.
No comments:
Post a Comment