Transformerless power supply with only Live wire

[Ibob]

I want to build a circuit to switch a bulb with wireless switch and also manually with a button.
I have prepared everything except the power supply. The problem is i have no neutral in the wall where the wall switch is. So i need to draw power somehow to feed the 12v receiver and other elements, and also i need to light the bulb. Can someone help me with that?
Thanks in advance!

 

[MikeMl]

First off, hobbyists shouldn't be modifying house wiring...

You will have to hire an electrician to rewire the house to add a new oversize wall box located near the exisiting lightswitch. New box to be fed from the main panel with a live and a neutral wire, containing a standard plug-in with oversize cover plate. Box to be large enough for a 12V relay with NO contacts rated for your line voltage used to switch line voltage to a new in-wall wire running to the existing light switch to connect to the light-side of the wall-switch. This effectively puts the wall-switch in parallel with the relay contacts, so either can turn on the light. If the wall switch is on, it will have to be turned off before the remote-control can turn it off...

You will need to get an isolated lineVAC to 12VDC power supply to power your circuit. It could be a small plug-pack (wall-wart) plugged into the new outlet plug described above. The oversize cover-plate on the new box can be drilled for a suitable connector so that your circuit can control the 12V coil of the relay. In other words, your circuit should be outside the new electrical box; let the electrician hook up the relay inside the box...

 

[Ibob]

Thanks Mike, but rewiring the house is not an option here.
This power supply I am after for is not so incredible, i already have lots of PIR wall switch sensors mounted at the junction boxes which switch on/off the lights with the only Live wire in the junction box, plus there is no click sound, i.e. they use triacs for the control.
And besides, i know the risks from the transformerless power supplies.

 

[Sangoma]

What about a small capacitor bypassing the switching circuit so that the light draws a small amount of current through the capacitor, just enough to keep a rechargeable battery charged, that battery will power your receiver???

 

[Ibob]

Sangoma, thanks, but I'd like a batteryless solution as it is possible. I know dimmers that work with no battery and with only the Live wire, so I am curious how the power supply is made..

 

[MrAl]

Hello there,


A switch works by connecting one lead to another lead more or less. But once that lead is connected to the other lead, the voltage goes to zero. So there is no voltage left to power anything. At first though we do have voltage. In a 120vac system we would have 120vac which has 170v peak, and that's plenty of voltage, at least for fairly low current. But once that switch is closed the voltage goes to zero because the idea is to supply all the voltage to the bulb and not to loose any.

So the voltage goes away very completely when the switch is closed. But that's a switch. A triac has a small voltage drop. So one possibility is to use a small transformer to step that voltage up, but that might be hard to do because when the switch is off we would need to disconnect the transformer.

It's the voltage that goes away, but interestingly that's when the current starts. The switch closes or the triac turns on, the voltage goes low, but the current then goes high. Since we need power even with the switch closed, one way to get that power would be to use a current transformer.

The current transformer would tap energy from the flow of the current and supply a small light current power supply. So when current is flowing and the light bulb is on, we still have power. When the switch is off we have plenty of voltage so that's not a problem. So you'd have to wire "OR" the two power sources together. The first source is when the switch is open, the second source when the switch is closed. Rectifier diodes would perform the wired "OR" function.

 

[Sangoma]

Sangoma, thanks, but I'd like a batteryless solution as it is possible. I know dimmers that work with no battery and with only the Live wire, so I am curious how the power supply is made..

very similar way, if you have current flowing through anything, you will have a pd across it (maybe not if it is at 0K)

You have an effective neutral(for this purpose), through the load. If the load went open circuit, it would not work, but then it wouldn't light up anyway

 

[MrAl]

Hello,

Check post #6. A current transformer provides power even with little voltage drop.

 

[tcmtech]

How much power does the receiver need?

Just a few ma or less on average?

For a low power application there are a number of ways to cheat the system and get the small amount of power you need but we need to know how much or how little in needed first.

I am guessing you are on a remote switch circuit that also does not have a ground wire either?

Then there is the option of moving the whole circuit to a different part of the lighting powers wiring.

 

[MrAl]

Hello,

Do you think that the current transformer will be the best way to obtain the small power needed? The only other way is a voltage drop, which reduces power to the bulb. Not the end of the world, but that's something to think about.
Opening the connection for a very short period of time would be an option i guess but that's more complex.

 

[Ibob]

Hi MrAl,
What you're saying makes sense definitely.
May I ask if possible for some principle circuit so that I can test it.
Btw, I live in Europe and the voltage is 220-230v in the mains.
Meanwhile I will open one of these sensors I was talking about above to see if any transformer inside.

 

[Mr RB]

Hello,
...
Opening the connection for a very short period of time would be an option i guess but that's more complex.

I like that last suggestion. You could full wave recitify and wait for the mains to reach 10v or so and charge a cap to 10v DC, as the mains will supply plenty of current at 10v. Then switch the TRIAC on after that.

You lose the first few percent of the mains cycle, but an incandescent light bulb won't draw much current at all during those few percent.

 

[alec_t]

Any method which bypasses the switch poses the danger that the load is never fully isolated from mains voltage (even though only a small current may be drawn). Anyone changing a bulb, for example, could receive a shock.

 

[Ibob]

Can someone come up with a circuit please?

 

[MrAl]

Hi,

The circuit is simple (see below):

1. A full or maybe even half wave rectifier and resistor and series cap and filter capacitor, zener diode.
2. A current transformer also with a full wave rectifier, connected to the same filter capacitor.

So the idea is to create two unregulated power supplies, one from the mains voltage and one from the current transformer, rectify them both so you get DC, then tie them together, then maybe use a regulator IC chip to regulate the voltage and that powers everything.

Note this would mean some min value for the load like the light bulb may have to be 60 watts or better but there's room for improvement here.

MrRB:
Yeah i think 10v would be good enough and then with a decent size filter cap that should store the power over the half cycle. I did a similar trick with an RS232 port where i needed more power for the data pulses but the data pulses dont last for the entire time period so the cap could charge up during the brief rest period, and pulling power from the port continuously loads it down too much.

                    RECT1
                   --------
 H o---R1---C1----| ~    + |------+----R2----+-----+-----o  ------+
                  |        |      |          |     |              |
 N o--------------| ~    - |--+   C2         C3    DZ1            |
                   --------   |   |          |     |              |
                              +---+----------+-----+-----o GND    |
                                                                  | JUMPER
                                                                  |
                                                                  |
         T1              RECT2                                    |
        -----           --------                                  |
   o---|     |--+------| ~    + |------+-------o   ---------------+
       |     |  |      |        |      |    
       |     |  R3  +--| ~    - |--+   C4
       |     |  |   |   --------   |   |    
   o---|     |--+---+              +---+-------o GND
        -----


   The upper circuit input comes from the mains line.
   Use the JUMPER to connect the two outputs.  Connect both GND terminals together also.
   Transformer T1 is a current transformer and the primary gets connected in series with the load,
   the secondary has a resistor R3 across it's output.
   DZ1 is a zener diode.
   C1 is a high voltage cap maybe 500v and 0.1uf.  The other caps are electrolytics.

 

[Ibob]

MrAl,
Thanks a lot for the schematics.
I have several questions, due to my lower level of understanding I suppose.
1. I don't get the JUMPER idea. What it is all about actually?
2. Where do I put the switching logic? (on/off - it will be a button - manual switching, and remote controlled relay - coming from the radio switch working on 12V - I've ordered it, but still don't have it). So far I know how to use 555 and MOC + Triac to do the on/off switching.
3. Where is my bulb here? It should be here: N o----X---- ... right?
4. I am not of with the current transformers. Can you give me an idea of the parameters for such? How big these transformers are? I need to build the whole project in a way so that it will fit inside the junction box
Thanks again for helping me out.

 

[MrAl]

Hi,

1. The jumper is just drawn to show how the two power supplies (upper and lower) connect their positive outputs together. The grounds (GND) also connect together.

2. The logic gets powered from the output of the two supplies which are tied together.

3. The bulb goes between the triac and the line as usual, but also in series with that is the primary of the current transformer.

4. You can get pretty small current transformers as long as the current on the output is light. I use one of these daily that is about 1.2 inches in diameter and about 0.3 inches thick. So it's a small donut shape.

How much current does the 'radio' switch require?

 

[Ibob]

...
How much current does the 'radio' switch require?

I cannot really say, as I don't have them yet.
I've ordered these:
**broken link removed**

 

[Sangoma]

I cannot really say, as I don't have them yet.
I've ordered these:
**broken link removed**

It says less tha 6mA standby, plus operating the relay

The ones you gave the link to are momentary, which means the light only stays on when you hold the button down, I would have thought latching would have suited better, unless you only want the light on for a short time like walking along a short path.

 

[MrAl]

Hi,

Looks like 6ma. Should be ok.

I've seen ones that screw into the bulb socket, but you dont have to add a separate power supply. Couldnt you get one of those kinds?