hi! i want to make 20v to 12v buck converter.i have a 20v laptop power supply that can output to 60w but i want to step down 12v for my amplifer which draw around 3-4 amp.Anybody have a good circuit,i been looking around and haven't find one yet.

 

There are plenty on EBay. Have you looked there?

 

i lived in a country where ebay can't deliver

 

Can you get a supply of electronic parts, like ICs or power transistors?
If not, scrap TVs and stuff can supply parts. Not getting late model bits just makes it a bit harder. You could go to linear regs and use a large heat sink.
4 x 7812 regs in parallel, each with a 0.1R series resistor on the output to help current sharing would do, as long as the heat sink is ok.

 

electronic component is not much of the problem.the problem is that nobody sell pre made circuit

 

thanks alot

 

http://www.ti.com/lit/ds/symlink/lm7800.pdf
Page 16 has one way of increasing the current of the reg.

http://www.reuk.co.uk/wordpress/electric-circuit/high-current-voltage-regulation/
Has more info.

 

are the resistor 0.1Ohm each

 

In the one circuit they use a number of 1R resistors in parallel. The 0R1 originally suggested was just a guess, and it could be made up of a number of higher values like the circuit. They are using 0R2 total and that will be ok. Too high will cause a bit of voltage sag on high currents, and too low will not share the current between the regs. What they have in the circuit will be good I think.

Oh, yes, 0R1 is another way of writing 0.1 ohms. Of the used as the "." can get lost in the printing out as it is so small.
Similarly 1K2 is 1.2K ohms.

And a number of resistors are used in parallel to share the heating.

 

if possible can u draw circuit layout.i'm not very good at electronic since i'm still learning

 

I may tomorrow. It is late here and I should be asleep.
Someone else might be able to help you earlier. If not, then until a bit later

 

thanks you for helping

 

Here is a rough circuit layout.

I find it easier to add 10uf tantalum capacitors onto the leads of the regs. Just make sure you get them the correct way around!
Then add wires and heat shrink.
The tab of the regs are connected to the center pin and if you do not want the -ve connected to your metal case/heat sink, mount the regulators with insulation kits. Google will help if you do not know how to do that. The resistors can be mounted on perf board or tag strips.
Add a diode from the output back to the input to protect the regs from damage if your amplifier capacitors are holding charge and the laptop supply is disconnected.
I hope this helps.
It is a rough power supply but will work ok.

 

thank alot.By the way the amplifier i was talking about was the audio amplifier my bad.

 

by the way .using alot of reg will waste alot of power.will there be other option

 

The power wasted is the same if you use one linear reg or a dozen. It is just spread over them and so keeps the individual heat down, And increases the current rating.
A switching regulator is a better way to go efficiency wise, but they are not so easy to build if you have little experience.
For a start, try this linear one as it is pretty cheap to make.

There is a few good electronics teaching sites around.I like this one...
http://www.learnabout-electronics.org/

And this allaboutcircuits has good stuff too....
https://www.allaboutcircuits.com/textbook/

Read up on these and it will help you understand things better.

 

this is only reg i have at the moment.i thought u could give it some improvement since this thing run too hot even with a big heatsink

 

You've already gotten suggestions how to improve the 7812 circuit - run several of them in parallel.

 

ohh thanks alot

 

Any linear regulator, such as one based on an array of 7812's, will not work. With a linear regulator, you need one amp of input current for every amp of output current. A 4 A output (post #1) requires a 4 A input at whatever voltage, and your laptop supply can deliver only 3 A (60 W / 20 V).

If that is your only source, then you will need a switching buck converter to meet your goals. It does not have to be isolated. One of the National Semiconductor (now a part of TI) "Simple Switcher" parts might work. These are the parts most often used in the low-cost buck converters available on ebay. However, you need a circuit with an overall efficiency of at least 80% (48 W / 60 W).

ak