Stray voltage on 115V discrete input.

Have 48V AC on a discrete input coming from a valve indicating switch that is a open contact. The input wiring is #12 wire traveling over 1 mile undergound "duck bank" picking up stray voltages giving a false input to PLC. The voltage is measured with a digital DVM, if a wiggy is used the voltage is zero cause of the lower impedance of the wiggy.

What would be a good solution to this problem without having to re-wire? I was thinking of a interposing 115V coil relay and use contact as input to the PLC.

I was thinking of a snubber on the input to ground to negate the stray 48V.

Looking for suggestions.

Thanks for your time,
Frank.

kbradio said:

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I was thinking of a interposing 115V coil relay and use contact as input to the PLC.

Looking for suggestions.

Thanks for your time,
Frank.

Click to expand...

I would use he above solution.

Thanks for the response, exactly what I'm going to do. Install a slim 1 NO contact Din-rail mount 120VAC coil relay.

Connect a 2K ohm 1/4 watt resistor from the PLC input to common.

mendonsy said:

Connect a 2K ohm 1/4 watt resistor from the PLC input to common.

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Yes. I've used pull down resistors successfully in this situation. I'd use the resistor to common even if I also used an interposing relay. The induced voltage could vary enough to blink a relay with such a long run of wire. An appropriate snubber circuit should work just as well as (if not better than) a resistor, though.

I just noticed the 1/4 watt spec. That won't do. You'll need a large, ceramic resistor because you'll dissipate some heat. I'd probably use a 5 watt.

A R/C based snubber should do it. Temporarily put a .1 UFD cap across input. If that does it, the snubber will work. Relay will too.
2K is too low. 10K, 2 Watt minimum, prefer 3-5 Watt.
120/2000 = .06 Amps. .06 x 120 = 7.2 Watts.
120/10000 = .012 Amps. .012 x 120 = 1.44 Watts.

sometimes it is worth while using an old analogue multimeter or an old Square D voltage tester to confirm that it is induction. Although at 1.6 Kms I would not doubt your findings.
the interposing relay (6mm) type may have the same issue as it may energise then hold in with the residual 48V.
so I would advise you use a larger relay or definitely add the resistor as suggested

When this problem is reversed, i.e. I have induced AC voltage driving an LED lamp, What is a solution for that?

Answered my own question, It works just as well. 10k 1/2 watt works fine and doesn't get warm.