Action Instruments - Loop drive capability

Loop Drive Capability By: Larry Miller, Application Engineer
When designing a two-wire loop-powered system, the voltage supply selected is an important consideration. The supply must be of sufficient voltage to drive all components in the current loop. Two-wire transmitters typically require from 10 to 13 volts to operate and each device in the system will produce a voltage across its shunt resistance. To determine the minimum supply voltage needed, determine the total resistance in the current loop. Multiply the total resistance by 0.02 and add the two-wire transmitters requirement. The 0.02 represents the maximum current of 20 mA, hence the maximum voltage requirement.
	The general formula is: *V_Smin=(R_L 0.02) + V_t** where VSmin is the minimum voltage supply required, RL is the total resistance in the current loop and Vt is the transmitter voltage requirement. For example, a system using a Transpak T797-0000 is connected in series to the input of a PLC and a chart recorder.
Since the T797 requires 12 volts to operate and the input impedance of the PLC and recorder are 250W each, we come up with a minimum voltage requirement of 22 volts: (500 * .02) + 12=22. A 24 volt supply is sufficient to drive the 4-20 mA current signal through all devices in the system. When existing loop-powered systems are being expanded, determining voltage supply requirement is equally important. If the above system was being expanded to accommodate a remote loop-powered displays, would you choose the Visipak V560 or the V565? Only the V565 would achieve the desired result without changing the power supply. The V565 only requires 1 volt from the supply to operate resulting in a total voltage requirement of 23 volts. Selecting the V560, which requires 4 volts to operate, you would need a minimum supply voltage of 26 volts to ensure proper operation.

Last updated January 6, 2004