The layout of the 24 LEDs will be:
This should give me more than 50 PAR in the center of the tank, and pretty close to it around the edges. The yellow circles are the coverage from the center 10 degree half angle of the light - no lenses used. But, the next 10 degrees should theoretically be even higher intensity, due to the focusing effect of the air-water interface.
To power these LEDs I needed to decide how to connect them. If they are all in series, the total voltage required would be about 96 volts, too much for cheap DC converters. So, I decided to split them into two strings of 12 in series, with the strings in parallel. That requires a 48 volt power source with up to 1.4 amps of current. I lucked out because ebay now has just such a DC converter on sale,
http://tinyurl.com/cpk3a8 I offered $12 for one, and the offer was accepted. With tax and shipping, that comes to only $23.82, much less than if I were to use "buckpucks", where I would still need a DC supply. But, this also means I need to use current limiting resistors. Here is how I decided what to use:
The two sets of resistors plus the toggle switch lets me toggle between about 700 and 350 mA of current. The numerous resistors are used first to handle the power that has to be dissipated, and because of ebay again! I found these:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=180334106900 which are 3 watt 1% 15 ohm resistors, perfect for this use. I plan to put the resistors and switch in a small metal box.