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Masnari/Escuti/Brickley
ECE 200 – Fall 2008 – Homework 4
Basic DC Circuits with Diodes and Capacitors
1.
[12 pts]
Find the minimum voltage V
x
needed to keep the diode ‘on’.
Assume the diode
has a turnon voltage of 1 V.
In order to determine the minimum voltage
min
X
V
required to keep the diode ‘on’, we
must determine the condition that must exist as we change
X
V
so that the diode goes from
‘off’ to ‘on’. To do that, we assume that the diode is ‘off’ which implies that the voltage
at node A with respect to ground is
V
V
A
1
<
and
0
=
D
I
. As
X
V
increases it will
eventually cause
A
V
to just barely reach
V
1
at which point the diode is turned ‘on’, but the
current
D
I
through the diode will still be 0. So at that instant we have:
(1)
0
)
2
(
3
:
1
=
−
Ω
−
−
A
loop
left
V
I
K
V
KVL
where
V
V
V
A
1
=
=
γ
Thus
mA
K
V
V
I
2
2
1
3
1
−
=
Ω
−
−
=
.
(2)
0
:
2
1
=
−
−
D
A
node
I
I
I
KCL
where
0
=
D
I
so that
mA
I
I
2
1
2
−
=
=
.
(3)
0
)
8
(
:
min
2
=
−
Ω
−
X
A
loop
right
V
I
K
V
KVL
where
mA
I
and
V
V
A
2
1
2
−
=
=
.
Thus we have
V
mA
K
V
X
17
)
2
(
)
8
(
1
min
=
−
Ω
−
=
.
In summary, diode is ‘off’ when
0
17
=
⇒
<
D
X
I
V
V
and diode is ‘on’ when
0
17
≥
⇒
≥
D
X
I
V
V
.
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View Full Document2.
[10 pts]
Consider the circuit shown below.
The diode has a turnon voltage of 1 V.
a)
Determine the state of the diode (i.e. is it ‘on’ or ‘off’?).
Note: You must
clearly show all your work including any neatly drawn equivalent circuits that
you might use.
R
1
R
2
3 k
Ω
1 k
Ω
I
x
4 V
1.5 V
Assume that the diode is ‘on’ in which case our KVLs and KCL can be written as:
(1)
0
)
3
(
)
1
(
4
:
2
=
Ω
−
Ω
−
I
K
I
K
V
KVL
X
loop
left
(2)
0
5
.
1
)
3
(
:
2
=
−
−
Ω
V
V
I
K
KVL
loop
right
γ
where
V
V
1
=
(3)
0
:
2
=
−
−
D
X
A
node
I
I
I
KCL
We can solve Eq. (2) to get
mA
K
V
V
I
833
.
0
3
5
.
1
1
2
=
Ω
+
=
so that Eq. (1) becomes
mA
K
mA
K
V
I
X
5
.
1
)
1
(
)
833
.
0
(
)
3
(
4
=
Ω
Ω
−
=
. We can now solve Eq (3) to give
mA
mA
mA
I
I
I
D
X
D
667
.
0
833
.
0
5
.
1
=
−
=
−
=
. Thus since
0
>
D
I
, 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 state of the diode found in part (a) to find the current
I
x
flowing
through R
1
.
As determined in part (a),
the current flowing through
1
R
is
mA
I
X
5
.
1
=
3.
[14 pts]
Assuming that the turnon voltage for the diodes in the circuits shown below is
0.7 V:
a.
Determine if the diode is ‘on’ in each circuit.
b.
Find the currents I
1
, I
2
and I
D
in each circuit.
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This note was uploaded on 03/10/2009 for the course ECE 200 taught by Professor Escuti during the Fall '07 term at N.C. State.
 Fall '07
 Escuti
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