On a number of projects I'm building, there's a transformer between wall power and device power. In the parts lists, the transformers are usually designated by "12v secondary transformer" or "18v 4a Secondary Transformer".
Because the flow goes from wall power through the transformer and then to a rectifier, I assume that it's just converting 120V AC to whatever DC, stabilizing it a bit with the rectifier.
Is that a safe assumption? Instead of finding an actual transformer, can I just substitute, say an 18V 4a laptop power supply or 12v wall wart I have lying about, using that for my power input to the rectifier? Why or why not?
Or is there something more to a "secondary" transformer?