Building a transformerless battery charger

Thread Starter


Joined Feb 28, 2009
Hi all ,

I want to be able to build a battery charger for a UPS , that is based on SMPS technology. I dont want to use bulky old transformers..

I have looked at how Computer power supplies have been designed.. They essentially have a buck converter sort of topology..

Please tell me where to start..

My requirements are the circuit should take household 220V Ac , convert it to a suitable voltage for charging (13.8 V) @ 5-15 Amps..

It is my understanding that in the computer PSW, they rectify the 220 V AC, and turn it into DC voltage (300VDC ? ) then use high frequency transformers (the yellow ones) to step the voltage down in stages ..

I have good know-how of microcontrollers.. so any solution there would be a piece of cake for me..




Joined Feb 19, 2009
Look at X and Y rated capacitors, and Isolation. There is a whole lot happening that isn't immediately obvious to keep the small adapters both safe, and from "polluting" the power system.

Rather than a computer power supply, look simpler, as you only need one output. An example easier to view the "bare minimum" function and operation is an electronic DC motor speed controller, such as on a sewing machine. Essentially, as simple as a Triac based light dimmer, the majority of the components are for safety or isolation or both.