In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

You can easily purchase a 5 volt wall wart at 1000ma or less that is rated for digital electronics.  

In general, the wall wart will be smaller than anything you can build with discrete components and is likely to be much cheaper - the tranformer and all the circutry is optimized for size.  About the only exception would be a 'transformerless design', but not using a transformer creates a very serious safety hazard in the form of electrical shock.

If you must, you can hack off the AC plug ins and solder an electrical cord.  Similarly, you can eliminate most of the cord on the DC side.

The solid state relay is likely to use less than 10ma on the SX side as it usually just turns on an optoisolator's LED.  You can easily get by for less than 100ma total power supply, but it is really hard to find anything less than 500ma in a wall wart or transformer.  I suppose you could wind your own, but would it really be smaller?  Where would you get the components to do so?  That Jamco 300ma transformer is quite unusual.  Don't forget your rectifier bridge and filter capacitors.

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