need to measure 300V AC using own design schemetic.

HI All,
I want to measure 300V from ac mains by the help of ADC0808. please tell me how can i achieve this. may i have to design a voltage divider network keeping the out put 5V for 300V input? because the input handling capacity of this ADC is i think up to 5V maximum. i want to make a DVM using at89c51. :| Can a voltage divider be a good solution for that? or i have to use some thing else.
Thank you

Yes, a voltage divider is the way to go.

are the Transformerless power supplies and adoptors and other small emergency lights that run a few leds use the same voltage divider networks and then rectify and filter the voltage to be used in the circuit? i think the for high current applications the voltage divider network can not be a good choice and resistors can not withstand with large currents up to 30A or more. then what kind of design should be used for that purpose???

not at all. The old style used simple transformers. In modern times, semiconductors have gotten cheaper and copper/iron has gotten more expensive, so simple SMPS are used in the modern AC-DC adaptors.

The resistive voltage divider is suitable when only detection and sensing of voltage is needed and no appreciable power has to be supplied to the load.

Those "transformerless power supplies and adaptors" use the same principles as adaptors using 50/60Hz transformers. The principle is the same in that they also use a transformer to step down the voltage from a high voltage and then rectify it. The difference is that mains transformers step down the low-frequency voltage directly whereas the "transformerless" types first convert mainc AC into DC, convert it to high-frequency AC and then step it down to the desired level before rectifying it with high-frequency diodes (more properly called fast-recovery diodes).

The advantage of first rectifying mains AC to DC and then converting it to high frequency is that smaller transformers can be used, thus reducing size and weight. The disadvantage is more complexity.