An interesting application was recently requested by Simone Engineering. Their customer has a 4-20mA current loop and wanted to install a loop powered indicator. Normally the Visipak V560 is a perfect fit to provide an LCD display in proportion to a 4-20mA signal. However, the customer needed the capability to remove the indicator without breaking or affecting the current signal. An inexpensive solution is to add a zener diode to the circuit. By placing a zener diode in parallel with the V560, the indicator may be removed without disrupting the current signal. The V560 has an input voltage drop of 3.9 volts maximum at 20mA. Therefore the zener diode selected must have a threshold greater than 3.9 volts. Since Zener diodes operate in reverse breakdown and the zener threshold is greater that 3.9 volts, the V560 tracks the 4-20mA signal when installed in the loop. When the V560 is removed from the loop, the zener threshold is exceeded and allows current to flow uninterrupted. For the typical application below using a Two-Wire Transmitter, the power supply must be of sufficient voltage to drive all components in the current loop. The general formula is expressed as

Vsmin=(Ri * 0.02) + Vt + Vz

where VSmin is the minimum power supply required, RL is the total resistance (load) in the current loop, Vt is the transmitter voltage requirement, and Vz is the zener threshold. Recommended Zener Diodes:

1. 1N750A VZmin=4.23V VZmax=5.12V
2. 1n751A VZmin=4.59V VZmax=5.61V