Convert 12V 200mA to 5V 450mA

This will sound rude and I'm sorry, but if you need to ask this question you should not be trying to build a transformerless mains to 5V converter - unless such devices are used under carefully controlled conditions they are very dangerous.

What do you need the power supply for - perhaps someone can suggest  a better or safer solution.

MK

Rishi Sharma wrote:

 

If there is a better circuit which has a better efficiency and an output of 5V 500mA (or greater amperage), then please do suggest me that as well.

yes.

I am aware of such adapters but the thing is I am designing a circuit which requires me to connect only AC power in the system and the microcontroller operates at 3.3V. Therefore I require the entire ac to dc circuit onboard.

Is this for mass production or will you only build a few of them ?

Putting AC mains and low voltage uP stuff on the same board is potentially dangerous. For mass production the savings in cost may make it worth the expense of all the design precautions required.

For small volume production it is much better to buy a pre-certified module and keep the mains off the low voltage pcb.

MK

I am making a circuit that looks something like this. The red and and green wires are the live and neutral wires of the AC power. The yellow wire is 5V and black wire is ground. What this circuit does is that, I am controlling a relay using using a bluetooth module and to power that module I require an onboard AC to DC circuit. Is there any better way of doing this as you said that it is potentially dangerous?

For small numbers then buy a ready made remotely switchable power socket like this:

https://www.amazon.co.uk/Status-SREMSOC-Remote-Control-Socket/dp/B007Y7D8Z6/ref=sr_1_1?ie=UTF8&qid=1481454332&sr=8-1&key…

or this:

https://www.amazon.co.uk/FUQUN-Wireless-Remote-Control-Smartphone/dp/B01CZHXI0O/ref=sr_1_4?ie=UTF8&qid=1481454441&sr=8-4…

The first one is sub GHz RF control so you would need to make a Bluetooth to RF converter.

The second one is a complete wifi device.

These are the first ones I found on Amazon - I have no idea how good they are.

If what you really want to do is build a device like these for production and sale in reasonable quantities I can't advise you on E14 about how to do it - except that you will be entering a competitive market and will need significant financial and technical resources to make anything cheaper than you can just go out and buy.

If you want just to make one for the fun of it  - then I can give you some tips about how to do it safely - but it won't be very cheap.

MK

http://www.belkin.com/us/p/P-F7C027/ Looks like this except bluetooth instead?

Look at Power Integrations LinkSwitch TN family.  These ICs are made for wide range AC to DC buck converters.  PI has good app notes.

As others have commented, be careful, there's no isolation.  It's very easy to fry the USB port of your computer, vaporize a scope probe or kill yourself.  Don't cut the ground pin of the scope plug!  Get an isolation transformer for the bench.

Regarding the buck converter:  The voltage ripple can be too high when the converter is running in discontinuous mode (low load current).  Make sure the inductor value is high enough to operate at your nominal load.  Don't design for a higher current than necessary or the supply won't be stable.  The input to output voltage ratio is high, so the duty of the buck will be very low.  The minimum inductor value is critical.