# Battery charger for my UPS

#### umer27

##### New Member
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 .. if someone can explain the operation of these Power supplies Id be grateful

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

Thanks

Umer

PS , are there any safety issues with building and using such circuits.. such as lack of isolation coz there is no transformer.. ?

#### mneary

##### New Member
PC power supplies don't generally use buck converter topology, they use an isolation transformer at high frequency. Look at the back of the PC board out of a power supply and see the big wide isolation gap.

You can get typical schematics from the web, from app notes of chip vendors.

#### umer27

##### New Member
can I find an isolation transformer at the electronics store ? how pls explain how an isolation transformer works ?

you mean to tell me, that the yellow transformer is an isolation transformer ?

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#### mneary

##### New Member
The yellow transformer in the PC power supply is almost what you want. Maybe you can buy one, but I don't know where. Dive into the app notes for the chip sets and output transistors. Some of them will help you design your own transformer using the yellow transformer as a starting point.

Warning: This is not a trivial exercise. When you have a working power supply you will know far more about the subject than I do.