I am facing annoying troubles converting 24V AC to 12 V DC. The goal is to supply 1.2A current to 12V DC load. I am both limited in input power (I can only use 24V AC output of a transformer that can supply up to 15W), size and possibilities of heat dissipation (the circuit will be placed within a sealed box with almost no air circulation). I've got some help in building the converter (See picture), but the problem is that it is way too demanding on transformer power input. I've added Proteus simulation screenshot, you can clearly see the power that is taken from the transformer (V1 on the screenshot). Any suggestions on how to make it more efficient or is there any other way for me to reach my goal? Thank you.
This just screams for a switching converter, you can buy these on ebay. I suggest an LM2596 based module.
You can remove everything in your circuit that is right of C2 and replace it with this module. Then turn the pot on the module so that the output supplies 12 V and you're done. Notice how this module does not need a heatsink !
BUT like brhans comments, you're really pushing it with 15 W in and 14.4 W out. You should really be using a slightly more powerfull transformer, like 20 W or so. When using a switching converter you can also just make the input voltage slightly higher (up to what the LM2596 can handle) to increase the power.