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M as nari/ E s cutiB ri c
ld eY
ECE 200

Fall 2008
 Homework
4
Basic DC Circuits with Diodes and Capacitors
l.
Il2
ptsl Find the minimum vohage V* needed
to keep
the diode
'on'. Assume
the diode
has a turnsn voltage of 1 V'
In order
to detemine
the minimum voltage F"*,n
required
to keep
the diode
'on', we
must determine
the condition
that must exist as we change V* so
that
the diode goes
from
,off
to
,on'.
To do that, we i$sume
that
the diode
is 'off which implies
that
the voltage
at node A with respect
to ground
is Yn <lv and
I
n
=
0
'
As z" increases
it wilL
eventually
cause
I/, to
just
barely
reach 1trl at which point the diode
is turned
'on', but the
cufreRt
.Irthrough
the diode will still be 0. So arthat
instant we have:
(l) KW,*,o*:
3V (2KO)/1
VA
*
0 where
Vn
=V, =lll
3V
tV
Thus.I,
=d=ZmA'
(2) KCL,*.^:
IrIrrn
=0where
In
=o
sothat
Ir=Ir=2m4.
(3) KWr*,,"*:
Il
A(8KO)lrVx^n=0
where
Vo=lY and
Iz=2mA'
Thus we have V
x,ou
=l
F
*
(8 K A) (zmA)
=
17 V
ln summary, diode
is 'ofP when Vx <17V
Yx
>l7Y
+ I, ) 0.
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[10
ptsl Consider
the circuit shown bolow. The diode has a turnon voltage of I V
a) D*errrrine
the stde ofthe diode
(i.e' is it 'on'_or
toff?)'
Note: Youmust
you might use.
1kf,)
vYv
Rr
I*
rl
r
,rl
Rri
v
3ko
Lsv+
+
4V
Assume
that the diode is 'on' in which case our KVLs and KCL can be written as:
(1)
KWuih"F:
4Y(lKQ)/"*(3tKO)Iz=0
(2)
KWnrn,^*:
(3K Q)Ir*Vr
*l'sV =0
where V,
=lV
(3)
KCL,*,At
IxIrIo=A
Wecanso1veEq.(2)togetI,=w=0.833mAsothatEq.(1)becomes
, *
a"*(3{Q) (0'!31
=l.smA. we can
now
solve
Eq
(3)
to give
t*Ga.'
I
o
=
I
"
 I
o
=1.5
mA* Q.833
mA
=
0.667 mA. Thus since
.r, > 0, the current
flowing
through
the diode
is in the proper direction
indicating
that our assumption of the diode
being
'on'
is correct.
b) Use the stde ofthe diode found
in part (a) to find the crur€nt
/* flowing
throughRr.
As determined
in part (a), the current
flowing
through
3.
tfa
ptsl Assuming
that theturnoil voltage
forthe diodes
inthe circuits shown below
is
0.7
v:
a. Determine
if the diode
is 'on' in each circuit.
b. Find the currerrts
Ir, Iz aild Io in each circuit'
c. Find the vohage d node X in each circuit.
3v
5V
:
Cireuit I
Circuit 2
CIRCUIT 1
(a) Assumingthatthe diode
is 'ofP the KVL and KCL equations
become:
(1) KW"u,o^*: 5Il(3KO)Ir [JffD/, *3V
=O
(2) KCL,*"r : Ir* Ir Ip
=0
* /t
=12
sinceln
=0
Solving Eq. 1 for
d
we have
(41(0)4
=(5Ir
+37)+
4=ZmA
so
that
the voltage Z" at
node X becomes
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This note was uploaded on 09/25/2008 for the course ECE 200 taught by Professor Escuti during the Spring '07 term at N.C. State.
 Spring '07
 Escuti
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