Need to do this for D1 as well.

Thanks.
Missed that.

 

I think the MC7824 is rated to 40V.
There are two resistors, Base-Collector of TIP142, should be 332//332. I think 333 is an error.

OOPS!! Right. So perhaps a different regulator instead.

 

hi,
The best I can get from that circuit is 25V at 2.5A, is that what you want.?
E
View attachment 275133

yes, 25v is enough, but i want 3a, and lowest voltage ouput is 5v

 

Here is a switching power supply that will generate much less heat.

DC Buck Converter, DROK DC to DC Step Down Power Supply Module 10V-65V to 0-60V 0-12A Adjustable Voltage Regulator

link

 

That supply will require a DC input of some voltage, and ( DC Buck Converter, DROK DC to DC Step Down Power Supply Module 10V-65V to 0-60V 0-12A Adjustable Voltage Regulator) it is only good for a max of 1.2 amps. But three of them in parallel would work.

 

Here is a switching power supply that will generate much less heat.

Looks to be well designed, with a cooling fan and a heatsink near the size of the PCB.

 

it is only good for a max of 1.2 amps

Why do you say that when it claims 12A?

 

Here is a switching power supply that will generate much less heat.

link
View attachment 275189

thanks but i want to build mine. maybe i will buy in the future, but i want to learn now.

 

hi muno,
Why do you need 3Amps, what are you powering.?
E

 

mostly laptop computers. many of them are rated 19V 3.47A, but i think 25V 3A would be fine, since my transformer is the limit. 28-0-28 120VA

 

hi muno.
Modify the Current limit and other parts of the circuit, and you can get 3A at 25V.
E

 

thanks for the responses, but im not so familiar yet with the parts, can you suggest which of them? im so sorry for im still learning electronics in college. i have lm317t here but dont know how to insert it in the circuit. i also have 2n3055 transistors,

 

hi,
Look at this image.
E

 

should i change those to lower value resistors?

 

No, use those values I have just posted

 

thanks, so i can use the whole schematic below? i think 7k resistor is not readily made available?

 

Why do you say that when it claims 12A?

It looks mire like it says 1.2 amps to me.

 

powering a small computer with a voltage that is much too high will probably cause overheating, even with the internal supply being a switcher. And even if nothing burns up right away, that excess heat will certainly promote other problems.
What sort of rectifier scheme are you using to get the 40 volts? If the transformer has a center tap not in use that can be reduced by not using a bridge rectifier.
So please give us a bit more description of the input portion, the solution might be easy.

 

powering a small computer with a voltage that is much too high will probably cause overheating, even with the internal supply being a switcher. And even if nothing burns up right away, that excess heat will certainly promote other problems.
What sort of rectifier scheme are you using to get the 40 volts? If the transformer has a center tap not in use that can be reduced by not using a bridge rectifier.
So please give us a bit more description of the input portion, the solution might be easy.

hi i will be using full wave center tapped

 

I'm wondering how important it is for you to build this circuit - I like to do stuff myself, so I get it, but had you considered using an LM317 linear regulator. They are cheap, with input voltage up to 40V and the only limitation is 1.5A maximum. There is probably a trick to run two in parallel to get the 3A you are looking for, or mimic the internal circuitry with a (darlington pair) power transistor which, with a suitable heatsink, can handle the 3.0A current with nearly a 40V voltage drop.