• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Battery charger for my UPS

Thread starter #1
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..



PS , are there any safety issues with building and using such circuits.. such as lack of isolation coz there is no transformer.. ?
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.
Thread starter #3
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 ?
Last edited:
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.


Most Helpful Member
SG1842 Power Switch IC

Here is a neat little IC that you may find is just what you are looking for! its a complete power switching system in one easy to use IC!
I just started playing with them. The world of HF switchers just got way simpler! :D So far I have been able to get one to run a flyback transformer out of a 300 watt computer power supply at 12 volts 20 amps (dummy load only) from a 170VDC source. All the parts are recycled from the computer power supply also. But I may be pushing the IC's PWM limit! It is rated for 8 amps switching . So 8 amps 170 volts @ 50% duty cycle stepped down to 12 volts DC should give me 56.6 amps peak in theory! But I have not pushed it to see when it shuts down. At $11.20 each from Digikey I dont want to burn it up just yet!


Last edited:


Well-Known Member
If you are making a SMPS there are so many more questions about the transformer than the current, voltage and power.

The inductance is important, as are the saturation current, the number of turns for all the windings (and there are almost always 3 or more) and the leakage inductance on each winding.

The construction is also important so that the secondary winding should always be better coupled to the primary than the auxiliary winding.

SMPS are tricky. If you're not yet familiar with what a transformer is I suggest you try a linear supply first.


Most Helpful Member
Fairchild 1M0880 / AN4105 5 Pin SMPS Driver

Here is another simple ALL IN ONE switcher IC. This one even has the switching fet built in!
1M0880 is the IC specs. AN4105 is the application circuits data. Both come from the same reference source numbers.

I came across this IC in a 46" plasma screen power supply. Its was driving a 250 watt load all by itself!



Most Helpful Member
As the "GTI go to guy" (sort of) I am wondering if your looking at this from a (DIY cost) VS (factory made cost) stand point or just to see if you can build one that works stand point.
I personaly support DIY when it also beats factory cost.
If its for self education, great!
If its to save money, great!
If its just to prove you can do it, great!
If its to save $2 worth of electricity a year then I think your a nutter that needs to do a whole house energy audit, and find where the real power waisters are.

Latest threads

EE World Online Articles