I have a temperature measuring instrument giving 0 - 10 Volts (30 mA) corresponding to 0 - 100 celsius. This voltage signal needs to be transfered 500 feet away on an ammeter. Is this possible by just using a series resistor? What are the drawbacks if neglecting environmental temperatue and noise pickup disturbances.
20mA current loop
Jan 26, 2009
2 Replies
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yourself.
Rich Grise, Self-Appointed Chief,
Apostrophe Police
Quite feasible, as your instrument can source 30mA. The usual problem people hit with current loops is, it is easy to make a current *sink* (ie variable resistor) but you sometimes find there's no power at either end of the loop to drive it.
Next problem people hit is voltage drops: you will have some volts dropped across the series resistor, but people tend to overlook the cable. 500 feet each way = 1000 feet = several ohms. When we used to drive 4-20mA over a kilometre or so, we needed to make sure the voltage drops were OK *at max current*, ie 20mA. (Actually many instruments use
24mA to mean "fault", so you needed to be sure the supply could give enough volts not to go non-linear at 24mA with a kilometre of cable, doubled 'cos it's round a loop, plus the drop across the variable resistor.) Include the tolerances of the supply and resistors and so on and you can hit problems. It's just Ohm's Law, simple algebra, but it's odd how many people just think of cable as a zero ohm resistor.You can test this by looking up the specs of the cable you intend using. Calculate the DC resistance of 1000 feet, put that on the output of your instrument, add a current limiting resistor such that the total output at 10V is say 10mA, and see if the output is indeed 10mA. If the instrument only has a 10V supply within it it could have trouble sourcing over 10mA.
Next problem - ground loops. If it's a simple unpowered ammeter at the far end, no problem. If you end up connecting 2 earths together, you could have an offset (that's one reason why 4-20mA loops start at 4mA) and need to get an (expensive) galvanic isolator to sort the issue, or discuss it further here.
We generally used unshielded twisted pair for current loops.
Nemo
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