Transformerless 240v-12v

[lemmy70]

I need to convert 240v mians to 12 volts for a project i doing. Space is an issue and as the cuircuit is very low power >2W, and there will be very little variation I would like to use a resistor to drop the voltage and then regulate the voltage once rectified... Is this a possibility or is there a better way?

Cheers, Lemmy....

 

[dknguyen]

I need to convert 240v mians to 12 volts for a project i doing. Space is an issue and as the cuircuit is very low power >2W, and there will be very little variation I would like to use a resistor to drop the voltage and then regulate the voltage once rectified... Is this a possibility or is there a better way?

Cheers, Lemmy....

Do you mean low power <2W? Converting something as high as 240V to 12V is very inefficient when you try burning off the extra voltage as heat by dropping it across a resistor or some linear method since there is such a huge voltage difference betwen input and output (don't forget the size of the heatsink as well as the circuit). I still think a transformer, diode bridge rectifier, output capacitor, then linear regulator is the best method.

Not to mention that the voltage dropped across the resistor depends on the load current. No load current means full voltage which means something dies.

 

[ericgibbs]

I need to convert 240v mians to 12 volts for a project i doing. Space is an issue and as the cuircuit is very low power >2W, and there will be very little variation I would like to use a resistor to drop the voltage and then regulate the voltage once rectified... Is this a possibility or is there a better way?

Cheers, Lemmy....

hi Lemmy,
Search this forum for transformerless there are lots of threads, one may help.

REMEMBER: a transformerless mains power supply is NOT isolated and can give a lethal shock..

 

[mvs sarma]

I need to convert 240v maIns to 12 volts for a project i doing. Space is an issue and as the circuit is very low power >2W, and there will be very little variation I would like to use a resistor to drop the voltage and then regulate the voltage once rectified... Is this a possibility or is there a better way?

Cheers, Lemmy....

HEY LEMMY,
Basically a word of CAUTION. non -isolated power supplies are dangerous both to the designer and the user for whom it is being deigned. Such designs might land the designer into legal issues due to possible electrocution and and / or death. Not that i am threatening - but caution is needed.
please see this article on Off line power supply employing TL431

 

[bunghole]

perhaps an electronic down light transformer, they give out 12VAC, but then you could put that through a rectifier and smoothing capacitor. that's what i would do.

 

[Torben]

Hi Lemmy! Welcome to the forum.

I need to convert 240v mians to 12 volts for a project i doing. Space is an issue and as the cuircuit is very low power >2W, and there will be very little variation I would like to use a resistor to drop the voltage and then regulate the voltage once rectified... Is this a possibility or is there a better way?

Cheers, Lemmy....

Not a good idea, really, for a couple of reasons. The transformer helps isolate the circuit from the mains, which is the big (read: BIG) safety issue. Don't skip it unless you are really, really good at what you're doing. If it doesn't kill you, it could kill or injure someone else, start a fire, or something else evil.

Another reason: you want to drop 240VAC to 12VDC. The volts between 240 and 12 have to go somewhere, and that means that the power will need to be burned off by heating your resistor. You will need a really, really beefy resistor to handle that, and that's not cheap. You'd be making a room heater.

You can probably find a wall wart or power brick which can supply 12VDC at 2 amps from 240VAC. Try pawn/secondhand shops or a computer shop for old laptop power bricks. For instance, the power brick from the laptop I'm using right now can accept 100 to 240VAC from which it draws a max of 1.5A, and can supply 19VDC up to 4.7A. It runs cool and won't kill me. Maybe a local repair shop has something which will work for you. You're not totally avoiding the size/weight issue, but at least you're moving it offboard, and not risking anyone's life in the process.


Anyway, good luck!

Torben

 

[Brian Hoskins]

hi Lemmy,
REMEMBER: a transformerless mains power supply is NOT isolated and can give a lethal shock..

I've never completely understood that Eric. I understand of course that the secondary side of a transformer is electrically isolated from the primary side, but if you've got a 1:1 isolating transformer you've still got high voltage and current on the secondary side - so you're still going to get a belt, and if you're touching ground with one hand and the secondary side with your other hand I guess it'd stop your heart and potentially kill you?

Why is the risk less with an isolating transformer? Is it because there's no Earth?

This has always confuzzled me.

Brian

 

[Nigel Goodwin]

Why is the risk less with an isolating transformer? Is it because there's no Earth?

This has always confuzzled me.

It's because the neutral is earthed at the substation, this means you can get a shock just by touching the live wire, through your body to earth.

Using an isolating transformer just means you can't get a shock off a single wire, unless you earth one of the connection on the output (with a scope or similar).

Having an earth free environment gives the same effect, instead of removing the earth from the mains, you remove it from yourself.

 

[Diver300]

I would agree with all the warnings about isolation etc.

If you are going to go for transformerless, there is less power loss if you use a dropper capacitor instead of a resistor, so it won't get hot.

The downsides are that the capacitors tend to be less accurate than resistors so you have to allow for a wider range of currents. Also you have to fit two diodes so that the reverse current has somewhere to go, or you can use a full bridge and have half the size of capacitor.

With either a capacitor or a resistor, the current is basically constant so that a shunt regulator is about the only way to regulate.

 

[KMoffett]

Transformerless should not be your first choice...especially if you have to ask why?.

That said, there is a Microchip App Note on them:
https://www.electro-tech-online.com/custompdfs/2008/01/00954A.pdf

But, read the WARNING at the bottom of the first page.

Ken

 

[rjvh]

i do understand the safety iseus with not using a transformer but it is also depending what the thing is what you build and how you protect it from beeing touched

i've made some of those transformer less supplys and put them in a housing with class IP 55 or higher i realy don't see a problem in that

i run sometimes in the problems if i want to use a triac that the voltage diferents from the gate is messured from the A1 conection so the A1 conection have to be conected in a way to the steering circuitry but is also a neutral or fase conection in order to switch the load

the output of the swiched contacts has the same potential hazard factor than the power contacts that feed the whole circuit in the first place but is protected from liquid and small objects because of the IP 55 class housing

other solutions for this problem are not known by me but if somebody can teach me an other way, i am only ear to it and will try it out

Robert-Jan

 

[monkeybiter]

It's because the neutral is earthed at the substation, this means you can get a shock just by touching the live wire, through your body to earth.

Using an isolating transformer just means you can't get a shock off a single wire, unless you earth one of the connection on the output (with a scope or similar).

Having an earth free environment gives the same effect, instead of removing the earth from the mains, you remove it from yourself.

thanks nigel, i just learned something i didn't realise i didn't know.

 

[Nigel Goodwin]

thanks nigel, i just learned something i didn't realise i didn't know.

At least now if you kill yourself you know why it happened!

 

[Dan East]

Use a light bulb and solar panel. Granted, a 2W 12V solar panel is around 6"x6", but it would certainly solve your isolation problem! Don't even ask about the efficiency though...

Dan

 

[Hero999]

i run sometimes in the problems if i want to use a triac that the voltage diferents from the gate is messured from the A1 conection so the A1 conection have to be conected in a way to the steering circuitry but is also a neutral or fase conection in order to switch the load

Use an opto-triac to isolate or a pulse transformer to isolate the triac from the rest of the circuit.

 

[Hero999]

Use a light bulb and solar panel. Granted, a 2W 12V solar panel is around 6"x6", but it would certainly solve your isolation problem! Don't even ask about the efficiency though...

That's more cumbersome and expensive than a mains transformer.

The best way to provide isolation is to use a mains adaptor as they're cheap and don't take up any space in your project.

 

[rjvh]

Use an opto-triac to isolate or a pulse transformer to isolate the triac from the rest of the circuit.

opto triac is not availible here but puls transformer might be availible
never thought about that solution,THANKS

Robert-Jan