Thank you for your reply. I found sensors with 4 - 20mA output have only 2 wires (+) / (-). Power supply and current signal are carried over the same 2 wires. One side for current signal other side it need supply.Is it true? How can it work ?Originally posted by n9352527@Jun 1 2006, 06:13 AM
First of all, 4-20mA analogue sensors are not the same as digital HART sensors, albeit both of them use 4-20mA current loop.
4-20mA analogue current loop is just that, the minimum sensor value is represented by 4mA and the maximum sensor value is represented by 20mA.
HART protocol is digital one, much like fieldbus or profibus, and it operates over the same 4-20mA analogue current loop. There are simultaneous operations of analogue and digital and digital only operations also point-to-point or multi drop network within the specifications. HART is quite popular in industry, the reason is because it can operate over widely installed old 4-20mA loops.
The reason of using current instead of voltage is noise immunity and signal integrity over varying loops and environmental conditions.
Interfacing analogue 4-20mA loop is pretty straight forward, the principle is to convert the current to voltage using a precision resistor and read the resulting voltage through an ADC.
Interfacing HART sensor is a different proposition altogether. Not only you have to content with the physical encoding over the current loop, but you also have to implement the layered OSI-like protocols. Quite significant tasks. I suggest you get hold of one of those HART books, or the specs from HART Communication Foundation (HCF) (not free) if you are interested.
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The current flows on both wires, that is why it is called current loop. The host puts a predefined voltage (not critical to accuracy) on the loop, and the sensor regulates the amount of current flowing according to the measurement value by using a form of current limiter/source circuit. The excess voltage across the current source is also used by the sensor to power itself. The portion of the current used to power the sensor is added to the current resulting from the measured value and the total is controlled by the sensor to a defined accuracy.Originally posted by anthony06@Jun 2 2006, 08:46 AM
Thank you for your reply. I found sensors with 4 - 20mA output have only 2 wires (+) / (-). Power supply and current signal are carried over the same 2 wires. One side for current signal other side it need supply.Is it true? How can it work ?
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Your explanation is very usefull to me. I'm sorry...it makes me confused.Originally posted by n9352527@Jun 2 2006, 03:52 AM
The excess voltage across the current source is also used by the sensor to power itself. The portion of the current used to power the sensor is added to the current resulting from the measured value and the total is controlled by the sensor to a defined accuracy.
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