Larry said:
I imagine you are right. But I've seen and
wired some pretty big relays.
To clarify, yes I did mean a relay that is rated for a 2A load.
Well, saturation is kind of a funny thing for the
Darlington transistor pair. The input transistor
can be readily saturated but the output transistor
is generally not saturated, having its base drive
modified such that its base and collector voltages
are about equal. So, using the term "saturation"
loosely to mean "as on as can be", there is no
reason the HCT device cannot cause that to
occur in the Darlington. But for the more precise
definition, requiring an excess of carriers in the
base region and a forward biased collector-base
junction, the output transistor certainly cannot be
brought to that state.
The reason I thought this might matter to the OP
is that Darlingtons often present a dissipation issue
in switching application due to their drop while in
the on state and that drop can be significant with
respect to the load voltage as well.
I've been looking at that data sheet and I don't really understand a lot
of what it is saying. On page 4 it has the chart with all the values
and refers you to diagrams on page 5. Most of the diagrams say "Open"
by the output of the clamping diode. Doesn't that mean the Darlington
is off?
One line says Vce and has 1V typical for a 200ma load. Does that mean
that 1 volt will be dropped by the Darlington? If so I will need to
make a significant change to my voltage to drive my high intensity
LED's. For the 12V relay, that will probably have something more like
13.8 volts, it probably won't make any difference though.
I don't understand what the Vi(on) numbers are trying to tell me. For
the SN75469 in the MAX column they vary from 5V to 8V depending on the
collector current. What exactly does that mean? Is that the maximum
voltage the input line can handle? What about the minimum voltage
needed to turn the Darlington on?
Then the Vf for the clamp diode is 1.7V .... does that mean there will
be 1.7V drop from my supply?
Ii(off) seems obvious and Ii seems to be well with in what my flip flop
can provide. I assume Ir is the amount of revers current that will flow
for inductive kick back from a relay?
Finally what about Icex? Is that the minimum amount of current the
collector needs to stay on?
Thanks for everyones help. I really need to get a good book. The
problem is most books about things I am interested in are either way too
slow paced, or over my head. I learn this kind of thing very quickly
and often find beginner type books very slow paced.
--
Chris W
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