Current Over a distance

[Nick]

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...

 

[[email protected]]

Hi Nick,
well as i know the resistance of the media ur transfering the current
in, is the most important reson to this voltage drop.

 

[budgie]

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??

Either your V->I or I->V converters has an error. Simple to check really if you
have suitable meters.

 

[Paul Burke]

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??

Better show your circuit for V/I converter and the load. Lots of things
(simple tghings) could be wrong. 7m is no distance at all.

Paul Burke

 

[Mark]

but was it still 20mA at the other end?

that is the point of a current source,,,

the current is held constant (or tried to be held within the voltage
limits of the current source) but the voltage may vary depending upon
the resistance...

The current source will try to force 20 mA through whatever resistance
you attach to it...
Mark

 

[Roger Hamlett]

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

 

[Nick]

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...

Hi everbody,

i think i will have to check my circuit and find current drop also..

Thanks for the help...

 

[budgie]

Hi everbody,

i think i will have to check my circuit and find current drop also..

Current drop? Someone here doesn't understand what a current loop is. It's
either you or me.

 

[Nick]

Hi,

I meant to say to check the transmitted current Vs the Received
current...


With regards..

 

[Roger Hamlett]

Hi,

I meant to say to check the transmitted current Vs the Received
current...


With regards..

In which case, you were either doing something silly with the connections,
or your wires between, have a massive DC leakage....

Best Wishes