Fitering and regulating DC at 9 and 12 volts

[svstee]

Building a small dedicated pedal board for bass and I want to DIY the power adapters. I hacked a 12v 1a transformer based PS by flipping polaritly, adding a 1n4001 diode for polarity protection, adding a 100uf polarized cap and a 7812 regulator. Hum free and excellent, puts out 12.01 volts by my meter... close enough! 

I was looking at the schematic on Dano's site:



What are C2 and R1 doing here? I understand everything else. I don't need an LED so I omitted R2 and D2 and added regulation. 

[zyxwyvu]

What are C2 and R1 doing here? I understand everything else. I don't need an LED so I omitted R2 and D2 and added regulation. 

C2 is for high frequency filtering - electrolytic caps do not work as well at high frequencies (generally they're fine up to MHz, though), so a film or ceramic cap is added for better filtering at those frequencies.

R1, along with C1 and C2, form a low pass filter to smooth the output even more than with just capacitors. R1 also limits the current, though usually not significantly for single effects. If your effect draws more than about 10mA, it would cause some problems.

[earthtonesaudio]

All you need for a regulated DC supply is: DC adapter, regulator, and two caps.  One cap on the regulator's input, one on its output.  That's it.  Reverse polarity protection is something to put in the effect, not the adapter.


Essentially, you put your unregulated DC in the left jack in that schematic, omit every part except C1 and C2, put the regulator between them, and take the output from the right jack.

[aziltz]

hah!  R2 D2 go together in that schem...  sorry its late.   

[svstee]

hah!  R2 D2 go together in that schem...  sorry its late.   

I was thinking about that too! I wanted to see if anyone else saw it, or if I am just too much of a Star Wars geek. 

All you need for a regulated DC supply is: DC adapter, regulator, and two caps.  One cap on the regulator's input, one on its output.  That's it.  Reverse polarity protection is something to put in the effect, not the adapter.

Essentially, you put your unregulated DC in the left jack in that schematic, omit every part except C1 and C2, put the regulator between them, and take the output from the right jack.

Thanks for the info. So the non-polarized cap goes after the regulator? Any particular reason? Thanks.

Now that I think about it, if I'm building this into the adapter polarity protection is pointless. Duh... 

[earthtonesaudio]

To some extent it doesn't matter which cap goes where, but check the datasheet for the example application.  I think for a 7812 it doesn't really matter, but for some LDO regulators the series resistance of the output cap needs to be above a certain amount or the regulator will oscillate.

Generally though, part of the beauty of using a regulator is you don't need such large caps.  The regulator takes care of fluctuations so ripple is not an issue.  I think the output cap mostly just supplies current for short transients that the load may draw.