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sometimes it's necessary to design a circuit that will control a larger load
than the
switch
or the output of a device can handle
in the case of a computer the maximum output
from let's say the parallel port is about twenty milliamps
which is not enough to drive a load that needs
considerably more
current to operate it
this is a configuration where we need to ensure with that the
switch like S1 here
when it's closed will turn this transistor on now the beta
for this transistor is somewhere between thirty and three hundred that's what the
datasheet said
and
if we pull one transistor out of here and put another one in we still have to
ensure that the circuit is going to work
so let's look at the math with this circuit
we have enough current here
we will have another current here and worst-case
is a beta of thirty
let's see the
maximum voltage from here
to here
across the 10k Ohm is
twenty volts minus point seven volts with a base emitter junction that's
equal to
nineteen point three volts
when S1 is closed
the amount of base current is
let's see nineteen point three volts divided by
10k Ohms is
equal to
let me figure this out
that's equal to one point nine three milliamps
one point nine three milliamps
time is the worst-case scenario so times which is the least amount to gain
times a beta
DC of a hundred
is equal to
one hundred and ninety three milliamps
which is possible
that it could flow from collector to emitter
collector to emitter
collector to emitter current
what would the voltage be across the uh... the voltage drop across Rc
so vRc
would equal to
Ic max
times
Rc and
Rc is 1k Ohm
we would have one hundred ninety three milliamps times 1k Ohm let's see
and this didn't work
yep it did work I just fat fingered my calculator so one hundred ninety
three milliamps times 1k Ohm is a hundred ninety volts
well that's kind of impossible
when you look at it because we've only got
twenty volts vcc
what the insurers
is the fact that
there is enough current
flow in this so that
Vce
is equal to zero volts
we know
from the datasheet however that
Vce sat
when we have a saturation current
through collector to emitter
uh...
that there is
a point two to
point three volt saturation voltage
from collector to emitter
that's what Vce sat is
so never assume that a transistor is going to saturate to zero volts
because it won't
max amount of current in the circuit than
would be twenty volts
minus Vce sat um
it's equal to about
nineteen-point eight volts
and nineteen point eight volts
divided by
1k Ohm
equals
nineteen point eight
milliamps