On Tue, 19 Nov 2002, Olin Lathrop wrote:

> >   For 1A load, and a darlington ( let say h21e = 100 ) will be 10mA
> > on command. It's not very huge...
>
> First, that's only the on current into the base of the darlington.  The
> resistor from the +V supply to this base needs to provide this 10mA with
> as little voltage drop as possible, because any voltage there appears
> directly accross the pass transistor.  That will therefore cause a much
> larger current when the base is pulled to near 0V to shut off darlington.
>
> Second, the darlington will have significant voltage drop even if the base
> were tied directly to the supply.  1.5V drop x 1A = 1.5 watts.  That's
> enough to require a heat sink with most packages.
>
> Let's say you spend 500mV accross the base resistor to get the 10mA base
> current.  500mV / 10mA = 50 ohms.  15V / 50 ohms = 300mA to pull the base
> to 0V to turn off the supply.  15V * 300mA = 4.5 watts just to keep the
> supply off!  At the same time the drop accross the darlington when on is
> 2V (1.5 for the two B-E junctions plus 500mV for the base resistor).  It
> will dissipate 2W when on and drop the 15V supply to a 13V supply.

  Olin dear,

  We have a romanian word : "unde dai si unde crapa !"
  more or less in english: "who you knock and who say ouch !"

  You haven't understand me ( as usual... ) The darlington is NPN, it has
*no* BE resistor, the resistor is connected from colector to base and it
has a large enough value , the colector is connected to +13V, the emiter
is connected to the load. Base of darlington is driven by an open colector
gate or just a npn to the ground. A low voltage ( zero ) on darlington base
means a zero voltage on the load.  I suggest you ( only if you have the
pleasure ) to reconsider all your computation from the next sentence :

" At the same time the drop accross the darlington when on is
  2V (1.5 for the two B-E junctions plus 500mV for the base resistor).  It
  will dissipate 2W when on and drop the 15V supply to a 13V supply. "

Vbe has nothing to do with Vce, so also with the voltage drop across the
transistor... if the electronics bases are the same in the whole world
( I'm not sure right now ... :) )
Also the Vce sat at 1A is not 1.5V only for "bulistors" ( bad-transistors
) a more resonable value is below the 0.5V

And remember, I don't want to argue, the greater fights on the list are
because of not-understandings. But I don't accept mistakes of funny
computations.

best regards,
Vasile

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