Bipolar PSU - which transformer?

[axg20202]

I'm planning on building a bipolar PSU using the instructions and layout on GGG. The trouble is, it's not totally clear what kind of transformer to use. I'm guessing this is because many are suitable. I need +/- 15V, so I know it needs a secondary that is at least 18V (AC not DC, right?). But which kind is suitable? If a transformer has two secondaries of 9V each, will this work if I connect them together or do I need a xformer with at least one 18V secondary?

I am in the UK (220-240v primary), so if anyone has a good source for a model that is suitable I'd really appreciate it.

[R.G.]

Read "Power Supplies Basics" at GEO, http://www.geofex.com

[axg20202]

Yep, I already read that RG. The GGG layout uses what you describe as the more crude method to generate raw DC to supply regulators for a bipolar supply. What I can't decide is whether to buy a 9V + 9V dual secondary transformer and sum their outputs (is this simply done by connecting the two 9V terminals and then going straight into the rectifier diodes?) or get a transformer with a single 18V secondary.

I would have thought that the 9V + 9V option would be cheaper and come in a smaller package for a given total VA rating versus a larger, more expensive package for the same VA rating with the single 18V secondary. Any ideas on the ideal minimum spec for what I want to achieve as I really want to resist doi ng my usual overspecing job - I need the works to be as small as possible while being capable of turning out a good 300 mA or more if possible?

Cheers.

[axg20202]

...also....the build instructions recommend using heatsinks but gives no guidance on this whatsoever. Assuming I use T0-220 package 7815 and 7915 regulators, just how substantial do these heatsinks have to be? Are we talking the little skinny ones with tiny fins with a base plate the same size as the tab on the regulator (available as clip on types from some suppliers)...

http://www.maplin.co.uk/module.aspx?ModuleNo=2583&doy=search

.......or do I need some that are great big lumps?

[the_random_hero]

...also....the build instructions recommend using heatsinks but gives no guidance on this whatsoever. Assuming I use T0-220 package 7815 and 7915 regulators, just how substantial do these heatsinks have to be? Are we talking the little skinny ones with tiny fins with a base plate the same size as the tab on the regulator (available as clip on types from some suppliers)...

http://www.maplin.co.uk/module.aspx?ModuleNo=2583&doy=search

.......or do I need some that are great big lumps?

It all depends on what sort of current you're drawing out of the regulators. The larger the current draw, the larger the heatsink. I'm just using a small screw-on aluminium type with some heat paste and it does get fairly warm when I'm drawing around 500mA, but not ridiculously hot. If I ever wanted to draw more current, I'd just go up to a slightly larger heatsink.

[axg20202]

Thanks. I'll just have to try some options. The fact that the heat sink gets warm/hot shows that it's working I suppose, so as long as they aren't too hot to touch, then all should be well.

Would still appreciate some tips on the transformer though.....anyone?