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Drawing power from a single live wire


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hi!
how do i draw power to power up a small signal device using a single incoming live wire? firgure below illustrates the desired connection:


live ---------------{device}---------------{load}---------------- neutral

what i want to do is to have the device to draw power from the live wire even when i do not want to switch 'on' the load(let say a halogen light) connected after the device.

thanks!

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Live and neutral are two mains wires, not just one. If the small signal device operates from your 120VAC or 230VAC mains and you want it to be powered all the time then connect it to the live and neutral wires separate from the load. Then connect the load to its switch in series also to the live and neutral wires.

What is your "device"? In your sketch it looks like it is a switch to turn on and off the load. How is it supposed to be powered?

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thanks for the quick reply.

my purpose for this is to replace an existing wall switch with some RC control switching (infrared, wifi, etc). but.. those conventional wall switches has only a live wire coming in... and an output wire going out connecting to the load. i do not have the intention of connecting any neutral wire to the device... because that would require extra wiring to the existing switch...

yeah, in my sketch it acts exactly like a switch and i would want to know if there is a way to draw power from the live wire and converting it to dc power without lighting up the load  ;D .... without the use of batteries or external power supply as it will require more space.


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thanks for the quick reply.

my purpose for this is to replace an existing wall switch with some RC control switching (infrared, wifi, etc). but.. those conventional wall switches has only a live wire coming in... and an output wire going out connecting to the load. i do not have the intention of connecting any neutral wire to the device... because that would require extra wiring to the existing switch...

yeah, in my sketch it acts exactly like a switch and i would want to know if there is a way to draw power from the live wire and converting it to dc power without lighting up the load
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hi gazza.

thanks for the article. but it still doesnt let me extract out dc voltage for small signal usage from the mains.. theres a device i have in hand now that does the specific application. but i cant see the components used as it is covered with some kind of protective layer over the critical components.

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hi gazza.

thanks for the article. but it still doesnt let me extract out dc voltage for small signal usage from the mains.. theres a device i have in hand now that does the specific application. but i cant see the components used as it is covered with some kind of protective layer over the critical components.


is it a rectifier?
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How many cables come into the j-box?  If there are two or more, then you almost certainly have a neutral wire to work with.  If there is only one, then the switch is downstream of the light that it controlls, but you may still have a ground wire available.

For inline wiring without a neutral/ground, you will have to use very little current in the "off" state so that the light will stay cold.  Do not assume that the fixture is safe to mess with in this condition- it is still active, just not glowing.

Either way you will need a switching device, and whether mecanical or solid state you will need to be certain that it will not heat up excessively.  I believe the heat is one of the contributing factors to dimmers only being available up to 5A.  With that in mind, until you find out from someone who is an authority on the subject, you should assume that 5W is the maximum that can be radiated from a standard junction box.

That being said, since the controller will need to be powered when the light is on, the load depends heavily on the voltage requirements of the controller.

For example:  I will assume that the "off" current will be supplied by a capacitor in series with the load to reactively limit the current.  The controller is connected to an optimized filter/regulator and a bridge rectifier.  Total losses through the recitfier and regulator are 2.5v and the controller needs 3V.  That's 5.5V, and it won't change when the load is turned on.  Five watts over 5.5V, that's 0.9A.

What's more, you'll have to find a way of using 900mA at 3V when the load is on, or your regulation will get more complicated.

Far from impossible, just sticky.  I hope you can shoehorn it in the box.

<edited for spelling>

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heys. i saw your reply in the thread and i might need abit more of your help if you dont mind.

ive tried using a capacitor with a series connection to the load. the circuit connection i have is as below:

Hot----------switching(triac)---------capacitor-----------25W light bulb----------neutral
            |                              |
            |                              |
            |                              |
            |------device----------|
           

i am currently simulating a switching of the load by using the combination of triac and an optocoupler to light up the load. my device now has a bridge rectifier RS205 and the i tied the DC output to an LED. After connecting the various values of capacitor to the circuit... i am able to achieve currrent going through the LED... but i cant seem to light up the load at either ON or OFF state of the triac. please advise. thanks!


I think you meant to put this here, and not in my message box.

I have no way of judging your competence from here, and this is a hazardous project.  As such I don't know how much help I will give you, and it does depend on the information available to me.  How many cables enter the junction box you wish to install the radio controlled switch in?  What are the dimensions of the device?  Power requirements?  What voltage are you working with?  How much space is available*?  On the last question, if you don't already know then I can't tell you how to find out (for liability reasons).
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