While doing a conversion from Provox to DeltaV I came across an 120VAC isolated input that wasn't working. The input was from a PLC's output card, that while connected to the Provox system successfully modulated between 30VAC (OFF) and 120VAC (ON). When connected to the DeltaV it modulated somewhere between 80VAC (OFF) and 120VAC (ON). The DeltaV CHARM 120VAC Isolated input has a much higher input impedance than the Provox (60k Ohm vs 11k Ohm) which explained the difference, so I put an 13k Ohm resistor across the leads to get the voltage to drop low enough for the DeltaV to register the OFF.
Good idea, bad idea? Other solutions? The 1.3 watts of heat don't bother me but I could see this being a problem if it was multiple points.
Hi Travis
We have had the same problem 20 years ago with AC input on Provox IO card that was due to inductance in wiring when contact is OFF.
We have solved it by adding a capacitor, but I don't remember the nominal value.
You have to calculate this value according the AC frequency (50 or 60Hz)