how can i design oscillator with R,L,C and transistors has input voltage 100-300Vac and the output of the oscillator get coupled through transformer to create output voltage of 12VDC ,50mA,
i do not need to use any I.C.
is there is any other ideas to do this with cost less than $5.

 

1. What's purpose of this power?
2. What's the load?
3. Why you care about the cost less than $5?
4. Why don't you buy a product like 12V/100mA from market as local store or online shop?

 

No.

 

1. What's purpose of this power?
2. What's the load?
3. Why you care about the cost less than $5?
4. Why don't you buy a product like 12V/100mA from market as local store or online shop?

it is resistive load , the one available in the market from100-240Vac ,I need it for 100-300vac

 

No.

what do you mean?

 

cost less than $5

 

My approach to such things it to read about how others solved a particular problem and see whether I can apply their solutions to my case. On the internet are many schematics of a circuit similar to the one you described.
Here is a Google Image search: http://lmgtfy.com/?t=i&q=transistor-only+switching+power+supply

Most of the 5V supplies with USB connectors that I have taken apart were simple circuits like these. Be aware that having a schematic is not the same as having a design. The circuit would have to be adapted to your specifications, and in this case that will unavoidably involve a lot of testing and burned parts. You will also need an isolation transformer, preferable connected to a Varic and some basic test equipment including and oscilloscope with a X100 probe rated at 600 volts, maybe more.

If you try this only touch the test setup with one hand at a time to avoid electrocution and of course never work on the circuit while power is applies and make sure your feet are insulated from ground (earth).

Designing a power supply like the one you described that works reliably and is safe (and legal) to operate is a very big and expensive job. You might be far better off contacting an experienced power supply maker in someplace like Shenzhen rather than trying to do it yourself.

Read and learn this, especially the parts about isolation and leakage current
www.psma.com/ul_files/forums/safety/estguide2.pdf

Learn about power transformers because you will probably have to have a custom one made.
sziget.mine.nu/~danko/letoltes/Magnetics_Designing_For_SMPSs-Section1.pdf

The pdf file below contains an enlightening discussion of something similar to what you asked about. Note that you will need to add a capacitor and bleeder to the output of the rectifier unless you only want to change batteries.
iosrjournals.org/iosr-jeee/Papers/Vol10-issue3/Version-2/J010327078.pdf

 

I need it for 100-300vac

What 300V AC source do you have and what is its frequency?