How can I remove AC current from sensor picking up inductive line current

Discussion in 'General Electronics Chat' started by BrantPeery, Nov 24, 2015.

  1. BrantPeery

    Thread Starter New Member

    Nov 24, 2015
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    I am working on a sensor that detects 120V AC power over a line and if voltage is detected the electronics turn on a light. I didn't design the sensor or the switch so I can't control that part. What is happening when I hook the sensor up to the 4 way switch in my house is the sensor is sensing an 80V no amperage signal on the off leg. I suspect that it is voltage induced into the off line in the 12/3 line because one leg always has power. This keeps my sensor from shutting off the power to the light even when the switch is not sending power to the sensor.
    How can I either remove (sink) the no amperage power off the line or block the no amperage power from getting to the sensor while allowing the high output 120v to come through when the power is on? I am hoping for a small low cost solution.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    Attach the schematic will help.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could try, say, a suitably-rated 100k resistor across the 'off' line. That might pull the phantom voltage down enough for the sensor to ignore it.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I am not sure you understand how a volt meter and ammeter work.

    A volt meter is very high resistance (10M ohms, generally) and can measure a voltage drop across a load (making a parallel circuit) without impacting the rest of the circuit. Or, it can be used to complete an open circuit (forming a series circuit).

    An ammeter, on the other hand, has near zero resistance and must be inserted into a circuit (series only). If you use it in parallel with a load, all current will flow through the zero resistance of the ammeter and blow the fuse of your meter.

    So, how did you measure current (amperage)?

    On a four-way switch, there will be hot terminals on the switch where you can measure line voltage but, since the switch is off, no current is flowing.

    80V is a strange voltage. Also, you have to make sure that your meter is,
    - Accurate. (test an outlet in your house or neighbor's house to confirm)
    - make sure it is set correctly (AC volts)
    - know what version of AC your meter reads (RMS (assumes a sine wave), True RMS (correct potential no mater what the wave snap), Peak-to-peak/2, average voltage, ...
     
  5. BrantPeery

    Thread Starter New Member

    Nov 24, 2015
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    I agree, this is a strange problem. It is hard to explain without sounding like I am babbling. So here is a schematic (kinda) of the problem. I did not diagram the 4 way switches in-between the 3 ways for simplicity. There is about 25' of romex in the circuit.
    upload_2015-11-24_16-5-25.png

    Things to note: I am using an electroteck digital multi-meter that has gotten me through many an electronics project without problem. I am testing the amperage on the lines by putting the multi-meter interrupting the circuit path. When the light is on, I get the correct amps for the light wattage. With it off, I still get volts detected on the line but no amps. Also the volts are on the 'off line' until I ground out the off line then the volts drop to 0v until I remove the line from ground. The line even though it shows ~80v shows no amps on the multi-meter and no spark on touching ground. It is like it is detecting the potential on the line, but there is no real power.
    I am ok with DC electronics and schematics, but I am very unfamiliar when it comes to AC components. For example, if I were to use a resistor, would it go between the switch and sense pin? What kinds of voltage drops would a resistor pull? Would heat be an issue? Is there something out there that makes it so the voltage has to have amps or it will not allow anything through?
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
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    The resistor would go between the "sense pin" wire and AC Neutral. If you use 10oK as suggested, heating should not be a problem as wattage ~150mW.
     
  7. BrantPeery

    Thread Starter New Member

    Nov 24, 2015
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    Thank you. After researching the resistor idea to make sure I understand the consequences of a resistor going straight from hot to neutral, I decided that I would give it a try. I tested hooking the resistor up to the sensor pin along with the other wire that is there and hooked the other end of the resistor up to neutral. Nothing popped and it did give enough load on the line to drain the phantom power. I tested the lights using all the combination of switches and it was all good.
    I disassembled the sensing unit and soldered the resistor up inside directly from the sensing pin to the neutral pin and closed everything up and am now a much happier man.
    Thanks guys!

    New diagram for anyone else who might be more visual like me :)
    House-Wire (3).png
    Monoprice z-wave in-wall on/off module 11989
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Did you connect according to the following colors?

    image.jpg
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Hopefully, you used at lease 1/4Watt resistor, which I should have said in my post.
     
  10. BrantPeery

    Thread Starter New Member

    Nov 24, 2015
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    I think it is a 1/4 watt. I don't have the original packaging for the resistors anymore but I believe that it what is. It wasn't getting warm when I tested it. :s
     
  11. asseling

    New Member

    Feb 27, 2016
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    Just thought I would post to say THANKS. Had the exact same issue with the same Monoprice Z-Wave switch. (Thank you Google for finding this thread) Added the resistor and it fixed it perfectly.
     
  12. BrantPeery

    Thread Starter New Member

    Nov 24, 2015
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    I am so glad to hear that this helped somebody else too. It took me weeks to find this solution and be brave enough to try it. I wish Monoprice would pay attention and put that resistor in for us!
     
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