need a +/- power supply or schematic

[dalbert02]

I was wondering if anyone had a source for a +18 -18 power supply. If not, could you draw a little schematic? I know how to build a regulated +12v supply but when I took what I thought I knew and applied it to a 25-0-25 center tapped transformer, it went poof! I need at least 1 amp current capability. Thank you kindly.
PS Any drawing would be great, even a crayon sketch on toilet paper would be appreciated. Thanks.

-dave

[Afroman]

If you are okay with poor efficiency and only want an amp, you could probably get away with an LM7818 for the positive rail and an LM7918 for the negative rail, suitably connected off your transformer with some diodes etc.
http://www.fairchildsemi.com/ds/LM/LM7818.pdf
http://www.htckorea.co.kr/en/data/LM79XX.pdf Schematics will be somewhere in the datasheet

[Daniel7866]

For the transformer wiring look here at fig. 6&18 http://www.st.com/stonline/books/pdf/docs/1707.pdf .
Place suitable heatsinks under 7818 & 7918.

Daniel

[dalbert02]

Thank you very much! I will use the 78/79 series as that is what I already have and figure 17 (from the first reply) or fig. 18 (from the second) is exactly what I needed. I see where I screwed up and now feel like an idiot. :( At least I learned something. :)
-dave

[Malc C]

I see where I screwed up and now feel like an idiot. :( At least I learned something. :)
-dave


OK Dave, spill the beans, what did you do to make it go poof ! ;)

[dalbert02]

Well, basically, I was thinking that since the center tap was ground, I would just build it like two regular power supplies with one using ground and rectify one tap as positive and the other using ground and rectify the other tap as negative. I would filter with capacitors so that I thought I would end up with +20, ground and -20. Then I would add the regulation chips. In my haste, and with no schematic (because I thought I already knew what I was doing and didn't need a schematic since what could be easier?) I inadvertently bridged one side of the transformer to the other with my diodes. Kinda stuipid and I don't know how I didn't realize what I was doing until it went poof! It worked when I was using the diodes in a half-wave rectifier configuration but then I thought I would get fancy, add more diodes, and make it full wave bridge rectifier on each side and that is where I got careless and literally bridged the two sides of the transformer. Ooops. Good thing it was only a $25 transformer.
-dave