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Simple thing but no idea how.

Discussion in 'General Electronics Discussion' started by NavyBEE, May 20, 2016.

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  1. NavyBEE

    NavyBEE

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    May 20, 2016
    I basically want to put an LED or some kind of low consumption light on the outside of one of my bathrooms that is lit when ever the exhaust fan is on.

    Any ideas?

    Surely there is something simple that can be done.

    maybe just a lead off the cold side of a switch?

    Thanks in advance.
     
  2. Bluejets

    Bluejets

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    Most countries have a neon indicator complete with correct size resistor in a small casing which clips into a standard wall plate. As long as there is a neutral at the switch then yes, simple matter to connect to the load side of the switch and neutral.
     
    hevans1944 and (*steve*) like this.
  3. NavyBEE

    NavyBEE

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    May 20, 2016
    Ok. You might have to dumb it down a little for me.
    Are you saying that light can be installed in a standard outlet box and connect from the load and neutral side of the switch? So connect two wires to the light, connect one to the hot side of the switch and the other to the neutral or other side of the switch? Been a while for me.
     
  4. Bluejets

    Bluejets

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    Connection of the device is in parallel with the fan motor. So, one end connects to the load side of the switch and the other to the neutral. Not across the switch as you describe otherwise the neon will be on when the fan is off. Maybe someone here can provide a simple sketch. Difficult to do from my phone.
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    I have one of these neon-lamp, internally illuminated, switches configured as a SPDT two-way switch to control an upstairs light from a position on the upstairs wall. It's companion, an ordinary SPDT two-way switch without the neon lamp, is mounted on the downstairs wall near the foot of the stairs. The upstairs lamp only illuminates if the light it controls is off, but that's okay as the switch lamp is only needed to find the switch on the wall when the room is dark. Perhaps you could replace your existing fan switch with one of these neon-lamp illuminated switches, but the switch neon-lamp would only be lit if the fan is off because the lamp would find its return through the fan motor.

    The hot side of the 115 V AC power line goes to one pole of your existing SPST fan switch. The other pole of the SPST switch goes to one terminal of your exhaust fan motor. The other terminal of the fan motor connects to neutral (somehow) but that is not important. The fact that it does connect to neutral is important, but that doesn't have to occur inside the box at the fan switch... and probably doesn't.

    If a small neon-lamp with a current-limiting resistor is placed inside a translucent switch handle and internally wired across the two switch terminals (by the switch manufacturer), and a load (such as your fan motor) is presented on the switched circuit side, then when the switch is closed the load is energized (exhaust fan runs) and the light will be off, because the switch shorts out the light while allowing power to pass to the load. When the switch is open, the neon lamp and its current-limiting resistor are placed in series with the hot power line and the exhaust fan motor. This is a high-impedance connection, so the fan motor doesn't run. But the fan motor impedance is low enough to allow the neon-lamp and its current-limiting resistor to draw enough current to illuminate the lamp.

    Of course this is the exact opposite of what you asked for, but it does provide an indication of the status of the exhaust fan: light on, fan off; light off; fan on. And all you have to do is replace a switch.

    There are also switches made with a separate toggle switch and neon-lamp The current-limiting resistor is usually built-in to the lamp assembly. and this combination will work as you originally requested only if there is a neutral wire present in the original switch box. The lamp is internally wired between the load (exhaust fan) terminal on the SPST switch and the neutral wire terminal provided on the switch case..

    So, you first need to determine is if there is a neutral (white) wire in the existing box. DON'T take the white color of a wire in a switch box as being gospel! If there is only a black wire, a white wire, and possibly an un-insulated or green wire in the switch box, NONE of these wires is connected to neutral. Always test to make sure any white wire you find is really a neutral wire. The electrician who connected the switch is supposed to use black paint to identify any wire that could be "hot" but when wiring a switch to a load, but they often don't bother.

    Today, the National Electrical Code requires safety (third wire or green wire) grounds everywhere, bonded to metal handy-boxes if that is what is used to house the switch or a receptacle, and continuous back to the distribution panel in any case. I don't think the following is kosher according to Code, and therefore I cannot recommend it, but you could use that safety ground in place of a white neutral wire to provide a return path for the neon lamp. Problem is, that connection could (probably will) trip a GFI circuit breaker when the exhaust fan is turned on. The better alternative is to snake a white neutral wire from the switch box through the wall(s) and connect it to the neutral bar in the circuit-breaker power-distribution panel.
     
    Last edited: May 20, 2016
  6. Bluejets

    Bluejets

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    Suggesting the earth be used as a neutral return is a little irresponsible don' t you think?
     
  7. hevans1944

    hevans1944 Hop - AC8NS

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    Yes it would be, but I did NOT suggest he do that. I just said it was possible. We are considering milliamperes of neon-lamp current passing to earth ground, which is bonded to neutral at the service panel, not a heavy power-consuming load. So if there is no GFI circuit-breaker supplying power to the exhaust fan, electrically it would work. And it probably won't work if there is a GFI circuit breaker providing power to the exhaust fan switch. A GFI circuit breaker detects very small differential currents between the supply (black wire) and load (white wire) and will trip off if the difference is large enough. A neon-lamp with a return to safety ground (instead of neutral) on the load side of the switch may draw enough current to trip a sensitive GFI circuit-breaker as soon as the exhaust fan is turned on.

    Many DIY home electricians have no idea of how a GFI circuit-breaker works. If their distribution panel does not have GFI circuit breakers, they may soon discover, after purchasing a lighted switch with a neutral screw terminal, that connecting between that screw and the "green wire safety ground" will allow the neon-lamp to illuminate when the switch is turned on. As you noted, this is not a good practice.

    I did suggest running a white neutral wire to the box if one is not already present, and not assuming a white wire already in the box is a neutral wire. It is common to run a pair of black and white conductors (with or without an insulated green-wire or un-insulated safety ground wire) to a wall switch with the load some distance away. The white wire in that cable is never a neutral wire, and its its ends should be painted black as a precaution so as not to identify it as such. But not every electrician carries a little bottle of black paint or black finger-nail polish to the job, so it doesn't get done that way. Some of them cannot even be bothered to wrap the end of the white wire with black electrician's tape, which is not a permanent solution because the glue on the tape will harden with age, the tape will unwrap from around the wire, and fall off to land unnoticed in the bottom of the switch box.

    Playing with electricity, especially mains power which has the full faith and credit of your local utility backing it, is a dangerous hobby for the uninformed.
     
  8. NavyBEE

    NavyBEE

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    May 20, 2016
    Its on a GFCI circuit.
    Separate light and fan switches.

    I have no idea what to do now.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Almost right. If you connect the light to the hot side of the switch, the light will be on all the time. Instead, connect the light between the load side or the switch and neutral. This adds one more connection to the load side of the switch, the other load connection being the original wire connecting to the exhaust fan motor.

    Without a neutral wire in the switch box, there is no way to have the lamp light at the switch when the exhaust fan is on. If you can gain access to the exhaust fan wiring in the wall or ceiling, you can wire a light in parallel with the motor connections. In lieu of that, you would have to run a white neutral wire from the circuit-breaker panele to the switch box containing the switch that controls your exhaust fan.

    Can you make a sketch of the wires inside the switch box, their colors, and what they are connected to?
     
  10. NavyBEE

    NavyBEE

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    May 20, 2016
    I got it. Connect on the load of the switch and the neutral.
    Flip the switch....boom! Power applied to the load side.
    Potential exists.
    Light illuminates.

    Any suggestions for a good light?

    Thanks everyone who commented.
    Its much appreciated.

    I will open up the box and see if I can figure it out.
     
  11. hevans1944

    hevans1944 Hop - AC8NS

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    And if you don't find a neutral wire inside the switch box, don't you dare use that third-wire safety ground as the return for a small neon lamp that draws insufficient current with, say, a 1/4 watt, 47 kΩ (or higher, up to about 220 kΩ), resister in series with it to trip the GFCI circuit-breaker.;) Visit the link in this post to learn more about small neon lamps and their application.

    Before I retired, we often wired neon lamps into custom panels as pilot lights to indicate a circuit had AC power applied. These were extremely cheap commercial lamps consisting of a cylindrical plastic shell with a plastic lens on one end and two insulated lead wires at the other end. They were inserted through a hole from the front of the panel and secured with a push-on flat-spring retainer. Inside each lamp assembly were a small neon lamp (like an NE-2) and a small resistor, but I know this only because I opened one up to find out what was inside. The lamps are not repairable. When the neon lamp eventually darkens from use, you just rip the retaining spring off, cut the wires, and solder in a replacement. Thinking the outer shell might serve as a holder for an LED, I dissected one of these lamps. They will work for that purpose, after removing the lamp and its attached resistor, but then panel-mounted LED holders are readily available... so why bother?

    Note that it would be possible to substitute a small bridge rectifier (to avoid 60 Hz flicker), a series current-limiting resistor, and an LED for the neon lamp and its current-limiting resistor, but you still need a "hot" and "neutral" to power either one. An LED draws a lot more current than a minimally illuminated neon lamp, so substituting an LED for neon doesn't gain any power advantage.

    So here's hoping you find a neutral wire in your switch box. Be careful around house wiring. The GFCI is not designed to save you from electrical shock. It is there to disconnect power to an appliance that has a ground fault. If that ground fault happens to be you, that could still be fatal... depending on myriad factors you have absolutely no control over, including sheer luck. Remember back to your earlier training and only work on un-powered circuits. Keep one hand in your pocket to avoid a conductive path across your chest and through your heart. Do not depend on insulated tools or gloves for protection against electrical shock. Be safe. Have fun and learn (or relearn as the case may be).
     
    Last edited: May 21, 2016
  12. Bluejets

    Bluejets

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    Oct 5, 2014
    I don't believe it.
     
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