Question about voltage regulators (7812)

[fuzzmonger]

 STUPID QUESTION AHOY!

I'm putting together a 12v valve preamp pedal that requires a regulated power supply to prevent noise issues. I planned on putting a 7812 into to give a little more flexibillity regarding voltage / cheapness of power supplies. Strange thing is, 12v regulated suppliies appear to be the cheapest available (due to their use in CCTV systems) so I was wondering if plugging a regulated supply into a pedal with its own regulator would cause any weirdness to occur. I'd still like to have a regulator in the pedal for posterity should I need to borrow someone else's supply at a gig for any reason / I'd need a quick and dirty replacement PSU from a high street store.

Bring on the 'well, duh' comments and patronising looks

-fuzzmonger

[FiveseveN]

Nothing weird should happen if you follow the application notes. Sometimes designs actually employ (simple) pre-regulation for various reasons. Just remember that the 78xxs have a dropout voltage of 2 V so you need a 14 V (or better and more commonly 15 V) PSU, whether regulated or not.

[fuzzmonger]

Cheers for the info. My question was specifically pertaining to this circuit

http://www.diystompboxes.com/smfforum/index.php?topic=77190.0

The original poster says a 400mA unregulated 12v supply will work with this which confused me further regarding the dropout voltages you mentioned. Either way, I'm starting to lean towards just putting a bypass switch on the whole regulator thing, just to put my simple mind at ease.

One other thing was the the part about putting a capacitor 'across the inputs'. I'm, regrettably, a solder-by-numbers kinda guy (can read a schematic, design a layout, make a reliable board) and my general electronics knowledge is crappy - could you tell me what exactly this means; what goes where etc?

Many thanks,
-Fuzzmonger

[R.G.]

The original poster says a 400mA unregulated 12v supply will work with this which confused me further regarding the dropout voltages you mentioned. Either way, I'm starting to lean towards just putting a bypass switch on the whole regulator thing, just to put my simple mind at ease.

12V unregulated is not the same thing as 12V regulated, as mentioned.

Unregulated adapters generally say something like "12V @ 400ma". They mean that literally. It is *at least* 12V specifically AT 400ma. At lower loading, the voltage rises. With no load, it's often 15% - 20% higher, sometimes more.

And "unregulated" may be a code word for "unfiltered" too. When you take an AC voltage and use diodes to make DC out of it, you do not get smooth, steady DC like from a battery. You get pulses of voltage all going in the same direction, but dipping down between the pulses. This sounds unbelievably bad if you try to power a pedal that expects smooth, steady, battery-like DC. So you have to smooth out the pulses to a smoother level, then regulate to a single DC level. A big capacitor - or more than one! - can smooth out the pulses, which may be 15-17V "DC" out of a nominally 12V@400ma supply. Once this is smoothed out, you can use a 7812 regulator to knock off everything above 12Vdc. The 7812 uses up at least 2V above its 12V output to do this. So if the incoming smoothed voltage goes no lower than 14Vdc at any instant, you get good, smooth battery-like 12Vdc out.