Need a little help on circuit for 120VAC to 12VDC

[Harald Kapp]

The main ingredient is this:

Transformer that has an output of 14v - 35v AC with an output current between 100mA to 1A, depending how much power you will need. (I found a 16v 200mA transformer in a broken alarm clock.)

The voltage rating of the transformer on the input side determines for which mains voltaeg the circuit can be used. For 110V operation you obviously need a 110V/14V transformer.

 

[irishluck]

So just so I understand, if the transformer converts 110 to 14v or whatever you choose, whats the point of all the other components on a power supply?

 

[davenn]

So just so I understand, if the transformer converts 110 to 14v or whatever you choose, whats the point of all the other components on a power supply?

The actual transformer in a plug pack etc converts the AC mains voltage 120VAC, 220VAC ( whatever for your country)
to a lower AC voltage. it then needs to be rectified to DC voltage using diodes, smoothed using capacitors and if a stable DC supply is needed then there will be regulator IC's as well

here's an example of a PSU I built some years ago to power my radio transceiver ....



you can see the parts/stages I mentioned earlier

240VAC into the transformer, 18VAC out of it, into a bridge rectifier ( 4 diodes in it) smoothing capacitors
then the regulator IC then there are 3 transistors ... these are a current bypass around the regulator IC as it cannot handle the high current (~ 15Amps) I wanted

Dave