How to tell current drawn from a schematic

[asteinhorn]

Hi all,

I am relatively new to the EE field, and while looking at the schematic for a VCO, I had the following question; if i wanted to build a bipolar power supply using a transformer for the VCO, what current should said transformer be able to draw? More specifically, how can I tell the maximum current the VCO will draw by looking at the schematic? I got the schematic from birthofasynth.com, and I will post a link to it below:

http://goo.gl/88KuQ

Any comments/help would be greatly appreciated. Not to be picky, but I would prefer an explanation to an answer, that I may be able to learn and use the knowledge in future projects.

Thanks!

[SeanB]

Largest current will be drawn by the buffer, as it has the potential to be required to supply 30mA into the pot. The rest add up to about the same, so you need a power supply that can supply +-15V at 100mA per rail.

Simplest is to build a power supply using a 7815 and 7915, to give a power rail of 15V, and use a transformer able to supply 15VAC at around 0.5A, with 2 secondaries, or a 30VAC unit with a centre tap. A 100V 1A or 2A bridge rectifier ( always use a bridge rectifier rated at more than double the current drawn as a good rule of thumb, or a 1A unit if the current is less than 100mA) and a capacitor of between 1000uF and 4700uF at 35V ( 1000uF gives a ripple of around 1V, and a bigger value will reduce this a little, but you have to look at peak current through the bridge diodes as too big a value can exceed the repetitive current rating of the nominal 1A Dc diode),  will then serve well as a reservoir for each rail. Unregulated input to the regulator will be in the region of 20-21V, so a small clip on TO220 heatsink ( the regulators will be dissipating around 0.6W each, so a clip on will allow you to dissipate heat up to the full 300mA the transformer will be able to safely supply) will perform adequately for each regulator. They will barely get warm, and you will have the ability to drive at least 2 of these VCO's, and have a few mA available if you want to add extra buffer amplifiers to drive long cables.