Extract power from 1 ac line - Page 1
Forum section: electronics / beginners

author: Erwin Ried
author_rank: Regular Contributor
timestamp: July 18, 2012, 07:39:35 am
content: Hi, maybe this is a silly question but anyone knows if is possible to power a small transformer with only 1 cable from the ac outlet? (I want to replace some light switches with a homemade controller, but inside them there are only 1 cable splitted to attach the light switch, no earth cable either)

author: houdini0118
author_rank: Newbie
timestamp: July 18, 2012, 10:08:54 am
content: I am not sure i under stand what you are asking but by what i think you are asking ill try to answer. You need 2 wires for ac neutral and live but the switch only needs to interrupt one of these to break the circuit and turn you device off. but if you don't all ready know this you probably shouldn't be playing with mains.

author: Erwin Ried
author_rank: Regular Contributor
timestamp: July 18, 2012, 10:29:26 am
content: Hey, thanks, yes I already know that, but my question is really if there is a possibility to get power only with one cable. Imagine a power adaptor with only 1 leg, there is a chance to do some trick to make it work? A floating ground or something like that?Maybe the question is ultra silly, but I don't need all the 220V, just a little bit to power a 5V device.

author: amyk
author_rank: Super Contributor
timestamp: July 18, 2012, 10:32:53 am
content: No. The current has to return somehow. SWER is not practical nor safe in your situation.

author: houdini0118
author_rank: Newbie
timestamp: July 18, 2012, 10:39:31 am
content: you could use a wall block to get 5v. They use single wire ac witch uses the earth as the neutral to send power to rural areas but its not very efficent and requires ground spikes and stuff like that.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 18, 2012, 11:44:35 am
content: Quote from: Erwin Ried on July 18, 2012, 07:39:35 amI think you will find that the other conductor is there,but they just don't bring it out to the switch.In other words,they only have to break one conductor to turn the light on & off.No,you can't do what you asked if your wiring is as you say.To operate a transformer off the mains from the light circuit,you would have to get access to the neutral line also.I strongly suggest that if you want to continue with this project,to consult a Licensed Electrician,who will know what is legal (& safe) .ORIf your Licensing Authority does allow homeowners to do their own Electrical work,get a copy of the Wiring Rules for your country,& follow them to the letter.

author: Simon
author_rank: Unknown
timestamp: July 18, 2012, 11:56:52 am
content: you can in theory use one conductor to get power and use "ground" as neutral but it is probably illegal, dangerous and WILL trip the RCD in the house. It won't lower your voltage anyway. You need a power adapter

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2012, 03:33:05 pm
content: Quote from: Simon on July 18, 2012, 11:56:52 amThere isn't necessary a neutral or ground wire accessible behind a light switch. The minimum required is a live wire going to the switch and a switched return going from the switch. Modern installations may provide a ground or earth wire in case of metal light switches but most installations don't seem to include a lighting ground.

author: Bloch
author_rank: Unknown
timestamp: July 18, 2012, 05:14:50 pm
content: Quote from: Erwin Ried on July 18, 2012, 07:39:35 amNot if you need to turn the light 100% off.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 18, 2012, 08:07:17 pm
content: Hmm, when the light is off you could easy steal the power needed for a small micro controller without turning the light on, and when the light is on you could do the same with let's say a series resistor. If you google X10 light switch you can see it's not impossible.Here is even a schematic:X10 Light Switch Schematic 220ws467

author: Rerouter
author_rank: Super Contributor
timestamp: July 18, 2012, 08:43:50 pm
content: 1-2 turns of the ac wire through a ferrite bead and however many of a second wire size to get 5V AC between its ends, as an inline transformer, however it would not be able to turn itself on, only off unless your devices own power supply could maintain it fow a week or more,

author: Erwin Ried
author_rank: Regular Contributor
timestamp: July 18, 2012, 09:42:49 pm
content: Quote from: IanB on July 18, 2012, 03:33:05 pmExactly, I think it is mainly because the cost, but I only see 1 red wire in every switch of my house.

author: Erwin Ried
author_rank: Regular Contributor
timestamp: July 18, 2012, 09:49:09 pm
content: Quote from: PA0PBZ on July 18, 2012, 08:07:17 pmWow, I can see it only uses 1 wirehttp://cache1.smarthome.com/images/2031_wire_big.gifbut I don't understand yet how (and scribd is not free)

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2012, 10:29:36 pm
content: Quote from: Erwin Ried on July 18, 2012, 09:49:09 pmI can't see it either, the picture is too small.However, suppose you are controlling a 60 W bulb on a 120 V supply. When the light switch is open this bulb will represent a short circuit. Therefore the full mains voltage is across the switch terminals and using an appropriate power supply you could draw a few mA to operate some low power control circuitry.When the switch is closed the bulb will draw 0.5 A from the mains. Now the bulb won't notice a small voltage drop, so if you inserted a (say) 4 ohm resistor in series with the circuit it would have 2 V across it while the bulb is on. This 2 V would represent a 2 V AC power supply that you could also draw a few mA from to power some control circuitry. (Instead of using a 4 ohm resistor you could also use a small transformer of course and get better isolation.)So that's two ways to get power from a lighting circuit, one when the light is off and the other when the light is on.(And if you try any of that stuff when you don't know what you are doing you will either kill someone and/or burn your house down. So don't try that for real. It's just theory.)

author: David_AVD
author_rank: Super Contributor
timestamp: July 18, 2012, 10:40:42 pm
content: Clipsal (and others presumably) make dimmer and timer switch mechanisms that only require two wires (power and load). They are generally only suitable for incandescent loads. I have a few of them at home.They draw a small amount of power through the load when the switch is off. When the switch is on, I think you'll find that the phase angle is never 100% and they use the remaining part for their own use.The whole electronics is built into a blob about 3mm x 10mm x 15mm from memory. Definitely only two wires. The blob, switch, load and power are all in series.