[sdiy] 4069 vco gives v+ noise?

[René Schmitz]

Hi Karl,

Karl Ekdahl wrote:
> Ok, the hum is the 50hz non-rectified voltage. I use a 15v AC without
> center tap, two 4700uF 40v filter caps (not 6800 as i thought) and a
> half wave rectifier.
> 
> This drives five voltage regulators (L7805CV (1,5A), GL7812, GL7815,
> L7812CV (1,5A), KA7905), two of them are 1,5A but i don't know about
> the rest. Maybe i should change all of them to be 1,5A or maybe i 
> shouldn't try to drive as many regulators from one transformer?

I think you should settle for 12V supplies with that
transformer. I guess that is the problem, the raw supply
does need to be at least 3V higher than the voltage you
want to regulate. I guess it drops below that during
the cycle.

Another option is to use an 18V transformer instead.
(And if you have to buy one, take one with 18-0-18
center tap or 2x18V secondaries, and go for fullwave.)

But before you spend time or cash I'd say measure your
raw supply to back up this theory.

Also see if the dirt shows up on the 12V and 5V lines.
(It shouldn't)

> Could anyone help me with the math here? How do i calculate how big
> filter caps i would need? The load i will drive would be 
> "the-average-load-of-a-beginners-first-homemade-small-modular" :)

My rule of thumb says: 2200uF per Amp. (But thats for fullwave, so 4700u
/ Amp for halfwave) And by that current I mean the current that the
transformer is capable of, or the maximum allowed DC thru the
regulators, which ever is smaller.

Actually I think the problem isn't caused by the caps being too big, but
rather you should redimension them when the problem is resolved.

Hope this helps,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159