Please help me..
how to make simple transformer to 3V up to 12V
In the class room we do this using laminated transformer cores called C-cores. They clip together to form a ring and the diameter is about 5cm. I remember that a primary of about 50 turns and a secondary of 250 turns will be about right. There is always a slight loss of voltage due to leakage so you need a few extra secondary turns as I have suggested. If you try to do it using nails or rods it does not work very well.
Remember that your supply voltage must be AC, (and your output will be AC too)
A simple transformer doesn't work for DC, although there are more complicated devices that can step up DC to higher voltage.
thanks for your help
How accurate do you need? How much power?
Lets say you are in a country that uses 110V. You might be able to buy a 110V to 30V transformer (4:1) quite easily. In which case you might be able to use it backwards (1:4) to step up 3V AC to around 11V AC. Ideally you want a 110 to 27.5V transformer to give you 12V but they might be harder to find than a 30V.
1. You might find a transformer with taps on the secondary for 3V and for 12V. Insulate the primary terminals and then use it as an autotransformer. It will not isolate the 3VAC and 12VAC terminals. https://en.wikipedia.org/wiki/Autotransformer
2. Consider using two transformers, a 3VAC and a 12VAC. Connect their high voltage primary windings together, then use the 3VAC windings as the input and the 12VAC as output.
3. Get an old transformer and remove the windings, keep the thicker secondary wire to reuse. Transformers usually have about two turns per volt, so put 24 turns as the 12VAC secondary, then add 6 turns on as the 3VAC primary. If you do not need isolation you could make an autotransformer by tapping 6 turns on the secondary as the primary.
There may be some complexities.
What frequency is the AC?
What power in watts is needed at the load?
To transform DC you need a DC to DC converter.
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