smallest way to convert AC mains to DC 12v? Is it safe to go transformerless?


When you're space restricted (like behind power boards) what would be the best way to step down ac mains (220v 50Hz for me) to say 12v?????

Also, Is it safe to make a transformerless power supply? Well, its not isolated from the mains, so........... Im not so confident..

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verence6 years ago
So, not to only criticize, (although my comment was not meant to critcize but as an advise) here a way to get a lower voltage without a transformer:

http://www.electronicecircuits.com/electronic-circuits/transformerless-power-supply/
(not my web site, not my idea, not my schematics, but looks good to me on the first glance - just did a search for 'transformerless powersupply')

This circuit will give you 25mA (as the site states) - maybe you can find another relay with lower consumption (or just try if it works). Maybe you can replace the relay with a solid state switch? You don't need the neon lamp if space is premium.

But please, please, please, play it safe! Mains voltage is not for kidding. Use an isolation transformer while testing. One hand in the pocket when the power is on - so the current can't flow in on one hand and out on the other while crossing the heart. Testing must be done only behind a residual-current device. Put the finished circuit in a casing isolated according to the regulations of your country.
I know, I sound like a nanny, but main power can be fatal and a burnt down house due to some smouldering electrical DIY art is a bad deal money wise.
pro2xy (author)  verence6 years ago
:D

Oh yeah, that hardly looks like criticism in the first place, LOL!!
OK Nanny, I promise I'll play it safe and be secure and come back alive.. Hahahah!! Nahh, just kidding.
i agree, maybe I should know a bit more about it before trying out my hands on the transformerless powersupply. Thanks for the advise.
I'll have a look at the site just for the info... :)
A Full Wave Bridge Rectifier will do the job.

I think it's safe enough to have a transformer-less power supply as long as you do it right.
A full wave bridge rectifier on 200V AC? Yes, that will give you a kind of pulsing DC. 100Hz ripple and a peak of 400V! Neither good for delicate electronics (ripple) nor good for anyone coming in contact with it - lethal voltage.

It might be safe to do it without a transformer, but for anyone who has to ask about it here, it is most probably not a good way to do it.

I don't won't to be un-nice, pranavsharma2504, but my recommendation: take a transformer and live happily, or study the matter, learn about electronics, play, break, repair and design safe electronic stuff and when you know what you are doing, design something without a transformer.
pro2xy (author)  The MadScientist6 years ago
Yeah, as i said, im personally NOT very confident about the transformerless idea, but then I also know that I never have to touch the entire circuit again...

How do I proceed to do it? I make a full wave rectifier and I get 220vdc?
Sorry, I am not to sure on that last bit I hadn't thought about it properly.

I think that a transformer is your best option, you might be able to get a 12V one out of a garage sale drill charger.
Transformerless designs are not for amateur designs really. The loss of isolation is a major danger to the uninitiated, and causes havoc with measurement equipment like oscilloscopes.

Bearing in mind how small you CAN get power transformers, what are you doing ?

Steve
pro2xy (author)  steveastrouk6 years ago
I agree with you, Im not very confident about the transformerless, personally...

I have to fit it in a space of 2*3*3 cms.. I need 12v and around 350 mA
it has to drive a relay and a uC, (basically home automation switches)
Why do you need 350mA for a load like that ? That's 5W - one heck of a relay/uC setup.

Steve
pro2xy (author)  steveastrouk6 years ago
well.....you're right...
YOu can get 0.35VA transformers, which should fit in the space available.
Steve
pro2xy (author)  steveastrouk6 years ago
Yeah!! I got it wrong, sorry...... :P I calculated the 30mA for the relay as 300mA!!!!! :P
rickharris6 years ago
wall wart
pro2xy (author)  rickharris6 years ago
I have to do it behind the power board, and im space restricted.