I have a 12V power supply that I am using to power some lights in my garden. I want to also power (in parallel) a set of 12V LED lights which (of course) require a 12V DC supply. Is there a simple method of AC->DC conversion that I can use? I am pretty handy at electonics so I could make a simple converter given the correct guidance.
Not to get all techy on you, but to do what you want to do with a 12 vac power supply, you will need a bridge rectifier. Besides converting to 12vdc, you need to fill the 60 hz reverse polarity portion of the AC wave. This requires a voltage divider, then reassemble the two rectified polarity sections into one same, continuous polarity voltage source. Keep in mind, if the AC source is sine wave AC, not switched AC, the DC result will only be apx 70% of the AC peak to peak voltage, or 17 vdc. Think the geeks at electronics exchange may have a simple circuit they could share with you. Realistically, since the Led lights probably have such a small amp draw, you probably could pick up a 24VDC power supply for cheap money online or maybe even your local Radio Shack, and save your self a lot of hassle.
Simple rectifier is 4 diodes one from each AC line to the positive input of the LED and two from the negative output of the LED to each of the AC lines. (4) 1N4007 diodes will do just fine for this. Just wire two up together on one end and the other two the opposite way like and in and out Y and attach to each end of the diode(s) the other ends mate up from each Y to the AC lines. if it doesn't work reverse the diode connection cause you probably hooked up positive to negative and vice-versa.
google "rectifier circuit" for a diagram
It is required an 12VAC to 12VDC converter, Not a simply bridge circuit, because the 12VAC will become about 17VDC after bridge circuit at no load condition, that will damage your lighting, but at full load condition, the voltage will be draw to a very low level that will not able to light up the LED, So it is required an high efficiency PWM regulation circuit, such as an DC-DC converter design, but with AC input, you can look at here, there are a 84W 12VAC to 12VDC adapter: http://www.enersystec.com/sdp/1045530/4/pd-5076639/12433566-2210481/12VAC_to_12VDC_Converter_Power_Supply_Adapter.html