At 05:34 PM 6/28/2008, you wrote:
>At 04:50 PM 6/28/2008, you wrote:
>
> > > We analyzed darlingtons in EE class.  The second transistor never
> > > gets into saturation and has Vce much higher than Vce(sat); I forget
> > > the exact reasons,
> >note: for the purposes of this discription I will call the transistor
> >that handles most of the current the power transistor and the transistor
> >that connects to the input the signal transistor.
> >
> >The problem is that the signal transistor acts as an emmitter follower.
> >but it can only do that if it's collector is at a higher voltage than
> >it's base.
> >
> >Once the voltage accross the collector and emmitter of the output drops
> >below the base-emmitter voltage of the power transistor the signal
> >transistor starts acting like a diode and the only current gain is that
> >of the power transistor. Of course there is a transistion region between
> >good follower action and diode action.
> >
> >If you drive enough current into the base you can bring the output
> >voltage right down but if you do that then there was no point using a
> >darlington in the first place.
>
>Not really correct. As the base current of the "signal" transistor
>increases, it will begin to flow into the collector of the
>"power transistor"  and you'll never get the latter to have a low Vce.
>The collector current of the "signal" transistor *reverses* for high
>base current.

I did a little sim of the relevant ULN2003A schematic using generic
discrete BJT Spice models, and resistor values from the TI datasheet,
to illustrate the collector current reversal:

http://server2.hostingplex.com/~zstoretr/darlington.pdf

         The three curves are for different temperatures- -20=B0C, +70=B0C =
and
room temperature (0=B0C).   ;-)   A lot of variation with temperature, but
with a 5V PIC drive it would always be operating with negative collector
current.  Y axis is collector current, X axis is input voltage to the
driver.

The driver has a 50R load to fixed 5.00 VDC.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com




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