[Arm-netbook] Use of 2N3904

[jm]

On Fri, 2012-11-09 at 20:49 +0000, luke.leighton wrote:
> On Thu, Nov 8, 2012 at 11:08 PM, jm <joem at martindale-electric.co.uk> wrote:
> > Hi,
> >
> > I'm helping out with design checking
> > http://rhombus-tech.net/community_ideas/kde_tablet/news/
> >
> > Can anyone tell me the price advantage of 2N3904 over a 2N7002 (Far East
> > pricing).
> >
> > The changes as example to section with Q3:
> > 1. replace 2N3904 with 2N7002
> > 2. remove C160
> > 3. remove R58
> > 4. Change R56 to 100K
> > 5. Change R61 to 10K
> 
>  ahh, you're a star.  ok i take it that's for the NRESET transistor as well?

Yes it does for NRESET.
If the lines are rarely used (as is normal for reset signals),
then they can be 33k with no adverse effects.

> > would make the circuit work equivalently and less prone
> > to effects of touching and hand grease finding itself
> > on the PCB and adversely affecting circuit.
> 
>  superb.  thanks joe.  wouldn't occur to me, that :)
> 
>  ... would you be willing to look at the audio (transconductance amp)
> and the mosfet h-bridge one as well?

To be quick with circuit + PCB analysis, the following can help you and
me:

1. Leave links to datasheets to important chips below diagram.
   Keep those datasheets in one directory so that once that
   place is found, we can find all the related datasheets quickly.
   STM32F103 for example. The inductor. S6M1, LM13700, RT8020,
LC5730,etc etc

2. When drawing circuits, draw them as you might find them in a text
book. When orientations and positions of components change, its hard
work locating mistakes quickly.

3. In complex systems like computer boards, use drawings of chips that
most resemble physical pin layout. It helps plan final layout, and with
debugging. U34 for example is prone to pick up so its good to know where
to place the mic in components and you may as plan from the schematic by
drawing the shape of U34 and its pins accurately. Its very important
that all the chips and numerous wires and positions of components are
subtly planned from schematic so there is less work in the final PCB
layout. Another example, the inductors for U28 pins 2 and 8 can't sit
next to each other in case they
interfere with each other. Knowing the pin positions accurately
to the real chip would allow us to plan where the inductor is positioned
in the schematic to hint at where it should be in the final layout of
PCB.



> 
> l.
> 
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