stepping down 300v ac

I want to stepdown 300Vac 50hz to 5v(peak)ac but i have only 220v to 12vrms transformer available can someone suggest how can i use this 220v transformer with 300v. I tried using a potential divider to first divide the voltage but it doesn't works since the primary winding of transformer comes in parallel with the resistance of potential divider circuit and mess up the voltage drop any suggestions??

Are you going to draw any current from the circuit? If you are a voltage division is not a good idea. But if you use it as a reference you could just use resistors to divide the 300 Vac into 5 Vac in the first place. But this is only if you don't draw much current from it.

"I want to stepdown 300Vac 50hz to 5v(peak)", 5V peak is 3.5V RMS. 220 to 12 V is not a good place to start from. Ignoring any transformer problems, you would get 300/250 X 12 = 14.4 V when you need 3.5, so you will have 4 times as many volts as required!!! If the 3.5V is going to feed a FIXED load (er, LED perhaps?) then a simple series resistor to drop the voltage from 300 to 75V would work but it could dissipate a lot of heat. If the load is DC, then feed it via a stabilizer, but do include a series resistor to limit the input voltage at maximum current draw to 220V. Use only 90% of the transformers power rating and the stabilizer will have a lot of work to do. DO NOT use a series capacitor to drop the voltage, you will get into all sorts of resonant problems, unless you like problem solving!
Frank

Sloth said:

Are you going to draw any current from the circuit? If you are a voltage division is not a good idea. But if you use it as a reference you could just use resistors to divide the 300 Vac into 5 Vac in the first place. But this is only if you don't draw much current from it.

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Iam feeding the 5v peak into ADC of atmega32 and i think it has very high input impedance of order 100k so the circuit is going to draw very low current the reason i was using transformer was that it will seperate hv side from lv

Just use your transformer and connect a fixed and adjustable resistor in series across the secondary and adjust the adjustable to get a suitable range (say 3V out for 300 Volts on the live side).
Frank

chuckey said:

Just use your transformer and connect a fixed and adjustable resistor in series across the secondary and adjust the adjustable to get a suitable range (say 3V out for 300 Volts on the live side).
Frank

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If i directly connect 220v rated primary of transformer with 300v won't it burn out?

No , not if you draw very little power. The magnetising current will be much higher then expected, but the overall power much lower. try it and see how hot the transformer gets, or put a series resistor in of 2.2 K to reduce the overheating?
Frank