Capacitor choice for Keene Spyder power supply

[ArtVark]

Hi,
quick intro: I'm Ton, from Amsterdam. I work in IT, and like to mess around with guitars and effects.

I want to build a Spyder power supply as shown here: http://www.geofex.com/Article_Folders/Spyder/Spyder.htm using a WPDLXFMR-2 transformer.
However, I want to make two changes and am not 100% sure this is OK. I may want to have more than 100ma current occasionally, so I want to use a LM7809 (1A) instead of the 78L09 (100ma). I guess that as long as I don't come near the 300ma that's supposed to be the max of each 11v ac out of the trasnformer this should not be a problem, right?
Also, I need to order some 4700uf caps for another supply. I thought I might use these instead of the 220uf caps in the schematic as well, as I'm getting them anyway and some extra buffering can never hurt. But I'm wondering if the initial charging of these caps could pose a problem as I don't know how high the current will be and if the 300ma from the transfomer can handle this. Does anyone know if this could be a problem, and if there is a way to calculate these currents?

Thx!

[Steve Conner]

Hey Artvark

7809 should be fine, as long as you're careful not to overload the transformer and possibly burn it. If you leave the 7809 unheatsinked, it'll not be able to deliver 1 amp continuously without going into thermal shutdown, and this will help protect the trannie.

Full-size 78xx regulators are usually the same price and have better noise performance than the "L" series, so there's really no reason to use the "L"s unless you don't have space for the TO-220.

The 4700uF caps may be physically too big to fit in your case, also they'll work the transformer and rectifiers harder. Calculating the RMS current in a rectifier is notoriously tricky, because it depends on things that you almost certainly don't know, like transformer winding resistance, leakage reactance, diode V-I characteristics and the like.

2200uF per amp of output current is a popular rule of thumb for these small low-voltage supplies: in fact it's probably how R.G. Keen arrived at the 220uF value.

Steve

[ArtVark]

Hi Steve,
Thx for the info. I think I'll go for about 1000uf then. If I start to get a lot of hum, that's a good hint I'm drawing too much current then :-)

Ton