[sdiy] 4069 vco gives v+ noise?

[Czech Martin]

running a 1.5A 7805 from a 15v ac trafo or even higher
means a large power dissipation in the 7805.

A half wave rectifier also gives problems with larger currents.
Simply because during 50% of the timecharging is impossible.
A trafo without center tap is also no good idea for larger loads
in a balanced +-15V system. The losses will be minimum with
such a trafo.

Sorry to say this.

It is very hard to achive something good, when your starting
point is already flawed.

Today there are many trafos on the market which are absolutely safe.
They are encapsulated, sealed. It is very easy to fullfill the needs
of your elctrical code then. The only danger is that the mains leads
could touch the case , but I think you can avoid this with proper
wiring, metal case wired to PE and an IEC connector to mains.

But: BE VERY CAREFULL IF YOU FUMBLE WITH VOLTAGES > 40V!
Any failure could be your last in your life.



You can assume ~ 10mA per active element (transistor or op amp).
A small synth may have 20 elements, a medium perhaps 100-200.
Thus I assume 200mA (in + 15V as well as in -15V) for the tiny synth,
and 1-2 A for the medium. This assumption can be doubled for safety.
It is always good to avoid 100% loading of the PSU, better 80%.

For the 5V it really depends. Most circuits can be built without
5V. Only a microprocessor or the like can make this neccessary.
A voltage as low as 5V can come handy for LEDs, especially
if you have a lot of them. They need only ~ 1.5V-3.0V (blue).

Btw.: I have the habit to draw (positive) circuits.
I.e. circuits that draw current only from the +15V branch to GND.
LED for example. This will finally imbalance the PSU.
So, as the zoo of modules growths, one should track the current
in both branches, and try to even them out with the comming
modules (or balance the load in each module in the first place).

Positive thinking is sometimes not apropriate ;->

m.c.






-----Original Message-----
From: Karl Ekdahl [mailto:_nial_ at yahoo.com]
Sent: Mittwoch, 14. Mai 2003 00:17
To: Ren_Schmitz
Cc: music.maker at gte.net; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] 4069 vco gives v+ noise?


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?

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"
:)

/Karl

 --- René_Schmitz <uzs159 at uni-bonn.de> skrev: > Hi
Karl,
> 
> > I use the same pinning as in the schedule you sent
> but it's still 
> > giving noise (in the v+ line). The noise seems
> like a 50hz wave with 
> > some minor disturbances riding on it that seems to
> be the 
> > osciallating tone.
> 
> One quick test if it is related to line frequency is
> to
> set the scope to trigger on the line frequency. (Or
> trigger
> on the AC output of your transformer, if your scope
> has none such thing.)
> 
> > The minor noise can at high frequencies be
> eliminated trough adding a
> >  10uF cap at the v+ line but I can't reduce the
> major noise (the 
> 
> That is possible. There is likely some dirt
> introduced when the CMOS 
> gates switch.
> 
> > probable 50hz) without putting a resistor
> inbetween ground and gnd 
> > pin on the 4069. And that - as i said - makes the
> square out die.
> 
> That naturally reduces the load on the PSU. (Two of
> the six
> gates are used in class A, so they have substantial
> quiescent current.) A 100k resistor reduces the
> current that the 4069
> can draw, and also changes all the biassing. That is
> why the pulse out
> doesn't work anymore.
> 
> > The odd thing is that the noise only occurs if i
> add more modules to 
> > the same powersupply, for example a ASM lfo. The
> powersupply should 
> > be able to drive both modules (at least i think
> so) without any 
> > distrubances. It's based aroun 78xx, 79xx and
> 6800uF caps and uses 
> > the standard layout found in most powersupplies.
> 
> Hmm, 6800uF sounds like a lot. What
> transformers/rectifiers
> do you use? 6800uF would be adequate for a 3amp
> supply.
> You are likely reducing the conduction angle of the
> rectifier,
> if you don't nearly draw that much current. And
> possibly even
> destroying the rectifiers with overcurrent.
> Less is sometimes more. And that certainly goes for
> PSU caps.
> 
> All in all this sounds like the transformer has not
> enough
> margin for the drop over the regulators. (Like a 15V
> type for a 15V PSU....)
> 
> I would try to isolate the problem by using a dummy
> load on your PSU.
> Say use a lightbulb or (hi-power) resistor that
> draws the nominal
> current that the PSU is designed to handle. And then
> you'll see wether
> the regulation is ok or not.
> 
> Cheers,
>   René
> 
> 
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
> 
>  

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