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Getting power from 220VAC switch

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pcbheaven.com

New Member
Hi there. I'm thinking of getting 5V power from the switch of a 220VAC, for dimmer and staff. I did some tests with a 10 Ohm resistor in series with the switch and got up to 6V. Bigger resistors gave me more voltage.

But the light was on....

The question:
How could i get the same voltage when the light is off
I do not want to use transformer as they are BIG to fit in a household switch case...
 

Sceadwian

Banned
You can't directly derive 5V DC from 220VAC safely in that space without using a switch mode power supply or a transformer unless the current you need is VERY low. A resistor will not work, and will likely burn your house down.
 

pcbheaven.com

New Member
I will power 2 chips a couple of resistors 1-2 transistors maybe a couple of capacitors... 100mA or something. They are not much.

Also, i will get 8-9 volts from the resistor through a 7805 for example... And before that, 2 diodes (or 4) to rectify the power.

Why should this burn the house anyway?
 

Sceadwian

Banned
How are you rectifying it, 7805's don't run on AC. What you're describing so far is a linear regulator using the voltage drop across a resistor to power it, to get 100ma out you'll need 100ma in which is 22 watts of power, that's a lot of heat to be generating in a wall switch.
 
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Chippie

Member
You can use a capacitor as a reactive dropper....

Still not a good idea to derive power from the mains without a transformer...but hey what do I know? I've only been doing electronics for over 40 years...
 

Chippie

Member
The heat wil come from the resistor? Are there extra small transformes to fit in a household switch box?

Yes the heat comes form the resistor.....

Small transformers are available but what is a household switch box.?

Fusebox maybe?

Thats a term I'm not familiar with....Whats your location?
 

Sceadwian

Banned
A wall junction box Chippie. Very small cramped area, not a lot of room for parts.
 
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pcbheaven.com

New Member
Concrete houses have switches and other electric staff fit inside the wall. The diameter is 60mm and the depth is 40mm. Within this cylinder must all parts fit. That's why i ask for small transformer. But no matter what, there will be no contact with human. The sensor is a capacitance sensor.
 

Sceadwian

Banned
pcbheaven, I'm holding in my hands as we speak the power supply for an LG cell phone 100-240V 50/60hz 4.8v out .9amps. The dimensions would fit in a 40x60mm space it's about 25mm's thick. Gut the housing and I'm sure you could make it fit a smaller space than that. I have several others floating around the house that are about the same size and output from 4.8-6 volts a few over 1amp. You should be able to find something locally.
 
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tcmtech

Banned
Most Helpful Member
The built in recharging circuit for the Remington cordless shavers are tiny switching power supplies. 100 -240 VAC input. change one zener and you have 5 volt 100 ma output.
Just a thought!
I would do the small AC capacitor and zener system myself if its isolated and protected. Or if the circuit runs off a single power and one common line and not two power lines.
 

GraveYard_Killer

New Member
You can use a capacitor as a reactive dropper....

Still not a good idea to derive power from the mains without a transformer...but hey what do I know? I've only been doing electronics for over 40 years...

hello Chippie,

I am currently designing a CS5463 based power meter... I had used a reactive dropper but I am planning to change my design to a transformer... the CS5463 needs its reference tied to one of the lines in 220V AC mains (two 110V 180deg out of phase)... if I will be having a 220V AC to 12V AC transformer, is it safe/ok do this kind of configuration in order for me to have a reference in the 220V AC line?

7940-testjsh.jpg
 
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tcmtech

Banned
Most Helpful Member
It will work but its less efficient than the reactive power setup you already have.
And being the secondary is still tied to the one line you wont get away from the live parts issues.
 

Chippie

Member
I'm not too clued up when it comes to electrical supplies outside of the UK...but from what I understand, your supply to the house is 3ph? Your transformer is connected across 2 phases giving 220v?

You are now connecting 1 phase to your circuit as a reference?

If that is what is required for it to function the fine....

You must NOT ALLOW any electrical contact between the circuitry and any one coming into contact with your device!!

What you should do is build it in a non conductive enclosure with a warning lable up front.....

Does this help?
 
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