Why we don’t use 2-20mA, 1-20mA, 3-20mA or 10-50mA signal instead of 4-20mA ?
- Generally, in a transistor some amount of voltage is required for turning it on.
- This voltage is the cut in voltage. Up to this voltage it exists a nonlinearity in its characteristics. Beyond this cut in voltage the characteristics of transistor is linear.
- This nonlinear region lays between 0-4mA, beyond 4ma it linear. That’s why 4-20 mA range is used.
Due to some following reasons 4-20mA signal is used,
- 4-20mA has an inherent ‘live zero’. Zero engineering unit is 4mA not 0.0mA indicates an open circuit, fault in the line which is not valid.
- 4-20 mA has little, if any, signal loss in its circuit; current regulation compensates for minor resistances in terminal boxes, whereas voltage drop across resistances for voltage signals creates an error.
- S 4-20 mA signal is less susceptible to noise pickup than a voltage circuit. So it is free from interference from other signals.
- 4-20 mA works at a relatively long distance(>1 Km ) boosting the power supply voltage will drive the signal further.
