Nick said:
Hi guys,
I had converted Voltage to current and then i had tried to send 0-20mA
current over a distance of 7meters and when i converted it to voltage
found a voltage drop of amlmost 0.5vDC. Why did this happen??
Please reply guys...
Thanks...
A number of possibilities:
Voltage limits of the driving device. If (for instance), you were only
using 5v, to drive your 'loop', and the op-amp involved can swing within
0.5v of the +ve rail, then the maximum resistance of the whole loop
(source resistor, target resistor, plus the wiring), can only be 225R.
Offset errors in the op-amp circuit itself.
Errors in your measurements/references used - remember the loop accuracy
will depend on the accuracy of the resistor used to measure the current
both at the source, and at the target.
Oscillation. It is very 'easy', when building a circuit to drive into a
load of this sort, to end up building something that oscillates....
A problem with the driver/target, not being properly isolated.
Done properly, a 4-20mA loop, can be excellent. If you look at the Siemens
application note, number 54, which shows how to build opto-coupled
drivers/receivers, using their IL300 opto-coupler, with the right op-amps,
and references, this comfortably gives a complete driver, which gives
better than 1% accuracy, driving loops with resistances varying from a few
ohms to several hundred ohms.
Best Wishes