Re: gEDA-user: round dots and square dots

[Steven Michalske <smichalske@gmail.com>]

On May 16, 2007, at 1:03 PM, DJ Delorie wrote:

> >    1) Way do the network connections on the ground line show as  
> > squares?
>
> Probably because your monitor has a limited number of pixels to use,
> so sometimes the dots show up as squares, but it's only by accident.
>
> >    2) Way can't I connect the bottom of the battery to the network?
>
> We'd need to see the .sch (preferably, with the components embedded).
>
> >    3) Can anyone explain how this type of circuit work?  I  
> > included the
> >       URL for the information source.  I don't know how the LM7805  
> > can
> >       control the TIP2955s?
>
> It looks like R4 turns the current drawn by the 7805 into a bias
> voltage for the transistors.  Thus, by drawing more or less current,
> it can bias the transistors more or less.  I.e. it's a current
> amplifier.  However, R1-3 limit the transistor's contributions to only
> a few tens of milliamps, far less than the 7805 can provide on its
> own.
>
> Ah, I see the problem.  Your circuit has 1000 ohm resistors, the  
> original
> circuit has 0.1 ohm resistors.
Agreed you cant get much current through 1KOhm.

R1-R3 are there because each transistor has it's own gain, known as  
beta or hfe .
The TIP2955 hfe ranges from 5 to 70, it depends on the currents  
involved and manufacturing variance.

The resistors allow the transistors to evenly distribute the  
current,  instead of having one have all of the load.
With out them you would burn up the transistor with the highest Beta  
till they were all burnt out.


I suggest using spice to simulate the circuit  and watching a few of  
the nodes.  use a current source as a load and have it ramp up.

When you model your transistors look at adjusting their gain to show  
why R1 to R3 are needed.

you can also increase the amount of current by adding more  
transistors, but you have to think of the amount of current the  
LM7805 can supply.  as its is pulling the current form the bases of  
the transistors.

11 amps out with a hfe of 10 (averaged from the transistors)  would  
mean that your LM7805 is drawing 1 amp.

approx 1/3 of an amp through each base,  and some through R4 and  
3.333 amps through each collector and 3.666 through each emitter

Steve

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