Hello, I am looking for a circuit for an automatic timer which can switch ON and OFF an A.C. of 2 tonnes every 15/30/60 minutes (user selectable). The circuit relay and components should be able to handle 10A current.

Hello Techfan

Firstly, have a look at the commercial products that are available. There are many timers from many suppliers that will do exactly what you are asking for.

If, you really want to build your own, then the 555 is not the way to go. The 555 relies on the time constant of an RC network and the only way to get to the time levels that you are talking is to use a very high value resistor and a very large electrolytic capacitor. The leakage current through the capacitor would probably prevent the circuit from working some of the time, assuming you got it to go first, and certainly the results would be very erratic to say the least. As the capacitor ages, or the temperature changes, the leakage current will also shift and get worse.

The best way to approach a long time like this is to use an oscillator and divide it down. The oscillator using components that will give it reasonable stability. You could use the 555 as the oscillator, but there are CMOS dividers that include an oscillator.
You could also use a small processor such as an 8 pin pick or AVR. This would enable you to use some inputs for time selection, whereby you regularly read the inputs and alter the divide ratio to suit.

Best regards,

[quote name='marke' date='May 10 2007, 01:22 AM' post='6038']
Hello Techfan

Follow the link. You may find your answer in this very good tutorial on 555 Timer.
http://www.uoguelph.ca/~antoon/gadgets/555/555.html