Load Driving Capability


Thread Starter


What does it mean when the datasheet of the transmitter specifies that "Load driving capability at 24 V is maximum 600 ohms"?
You haven't stated in what context the transmitter operates - is the 24v sourced internally or does it have to be supplied externally?

In simplistic terms you could assume the maximum drive current is around 40ma, mindful this does not include AC current.
I am assuming the data sheet is for a 4-20 ma transmitter. The 600 ohm limit is typical for these. What it means is that the total loop impedance (including the resistance of the interconnecting wiring) should not exceed 600 ohms. Most devices that this would be connected to (analog inputs of indicators, control panels, etc.) would have an input impedance of 250 ohms, which produces 1-5 volt dc to A to D converters.
Yes, I am referring to a 4-20 mA transmitter powered by a barrier. Now, lets say the voltage given to the transmitter is 24 V. What will happen if the total loop resistance (wire+barrier+DCS input) exceeds 600 ohms? Will the current start to drop proportionately? Or will the transmitter stop functioning and give some error? (Assuming the physical parameter that is being sensed is constant).
It will clamp the current output that requires above 24 volts to be driven. Remember that such a transmitter is ideally a current source: regulated current, no matter the load, voltage may reach up to infinity. In the real world, the voltage driving such a circuit has to be a real world power supply, in this case 24V. In other words, the current source can be considered as such if the load is below 600 ohms.
I ran into a two wire level transmitter that had really high lift-off voltage, around 19V if I recall correctly. The wire run was over half a mile, not heavy gauge copper conductors.

It would run fine until it reached the point at which the power supply could no longer drive the loop resistance, and this particular device would reset and re-initialize, and then reset and re-initialize over and over until the the level dropped. Boosting the supply voltage several volts eliminated the resets.