# L.105.3 DC DIODE CIRCUITS PAGE 1 I. DC Diode Circuits 1. Why ...

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L.105.3 DC DIODE CIRCUITS PAGE 1
I. DC Diode Circuits
1. Why Study?
A. applications
i) LED display circuits—may need to design
ii) transistor biasing circuits, to be studied later—transistor contains
built-in diode
iii) diode steering and isolation circuits
iv) will also apply to AC diode circuits
B. goals
i) be able to compute the current in a series diode circuit
ii) be able to select the correct series current-limiting resistor in a
diode or LED circuit
iii) be able to compute the diode current in a parallel or series-parallel
diode and resistor circuit
iv) be able to compute dI/dV for a diode in a given circuit
v) be able to design diode steering or isolation circuits
2. How to Find I in a Series Diode Circuit
A. example circuit (DRAW and CONSTRUCT circuit of Fig. 1)

L.105.3 DC DIODE CIRCUITS PAGE 2
B. possible methods of solution
i) try to find "resistance" of diode
(a) as in series resistor circuit
(b) then add to the 100 Ω, divide total into 5 V
(c) but diode has no fixed resistance
(d) as seen during diode testing, "resistance" changes with
current, and we don't know the current
(e) characteristic curve not a straight line, as seen in previous
lab (draw Fig. 2)

(f) this method N.G.
ii) ideal diode approximation
V(V)
I(mA)
0
0
Fig. 2—Typical diode characteristic curve
0.5 1.0
100
200
300
L.105.3 DC DIODE CIRCUITS PAGE 3
(a) assume forward-biased diode is perfect short-circuit
(b) no voltage drop
(c) then current would be 5 V / 100 Ω = 50 mA
(d) check it with meter
(e) not that accurate, especially for low power supply voltages
like 5 V
(f) OK for quick estimates
(g) trouble is that diode does drop some voltage (SHOW)
iii) fixed voltage-drop approximation
(a) termed "second approximation" in text
(b) assume forward-biased diode always has 0.7-V drop, for
any current level
(c) based on characteristic curve (SHOW using curve tracer) • nearly vertical
• for whatever current level, voltage across diode is
around the same value, nominally 0.7 V
(d) so voltage across resistor is 5 - 0.7 = 4.3 V
(e) current through resistor (= current through diode) is then =
4.3 V / 100 Ω = 43 mA (check it)
(f) OK for nearly all circuits, because difference between
assumed 0.7 V and actual diode voltage drop (check it with
meter) doesn't make that much difference in calculation
(g) check again with current meter
(h) works for LEDs also—use 1.7 V for LED drop
L.105.3 DC DIODE CIRCUITS PAGE 4
(i) how is V
D
affected by temperature?
• connect voltmeter across diode
• carefully heat diode with hot air
• what happens to V
D
? (SHOW on curve tracer)
(j) example problems • Fig. 3, find I

• Fig. 4, find I

• Fig. 5, find R so that I = 20 mA

L.105.3 DC DIODE CIRCUITS PAGE 5
• Fig. 6, find I1, I2, and I3

(a) most accurate, because it uses actual characteristic curve of
diode, no assumption that V = 0.7 V
(b) but not very practical because exact characteristic curve not
normally available unless you measure it yourself
(c) used in transistor amplifier theory to illustrate operation,
later
3. dI/dV
A. meaning of (measure values, time permitting)
i) diode conducting certain current, I
ii) has certain voltage across it, V
iii) corresponds to certain point on characteristic curve (DRAW)
iv) now suppose V is increased slightly, say by increasing the supply
voltage, by an amount Δ V
v) I will increase also, by an amount Δ I
vi) you've moved up on the characteristic curve to a new point
L.105.3 DC DIODE CIRCUITS PAGE 6
vii) how much did I change in response to change in V? depends on
dI/dV
viii) defined as Δ I / Δ V, for very small changes in I and V

ix) units? A/V or mA/mV
x) corresponds to slope of curve at the operating point
(a) the more slope, the greater is dI/dV
(b) so it depends where you are on the curve (in what way?)
xi) examples
(a) where is dI/dV greatest on diode curve?
(b) least?
(c) if dI/dV is 100 mA per Volt, by how much will I increase if
V is increased by .01 V?
ΔΔΔΔI
ΔΔΔΔV
0.70
0.65
100
45
I(mA)
V(V)
ΔΔΔΔI / ΔΔΔΔV = slope of curve ≈≈≈≈ dI / dV

Fig. 7—dI / dV for the diode
L.105.3 DC DIODE CIRCUITS PAGE 7
• solution: Δ I = (dI/dV) x Δ V = 100 mA/V x .01 V
= 1 mA
(d) if I drops from 100 mA to 45 mA when V is decreased from
0.70 to 0.65 V, what is dI/dV? (Fig. 7) • solution: Δ I = 100 mA - 45 mA = 55 mA
• Δ V = 0.70 V - 0.65 V = .05 V
• dI/dV = Δ I / Δ V = 55 mA/.05 V = 1.1A/V
B. main application—transistors, which contain a diode
C. formula for dI/dV of a diode
i) dI/dV = I/25 mV
(a) I is current through diode
(b) we know that dI/dV depends on how much current is
flowing through diode
(c) works for Ge or Si or LED
(d) room temperature only—text shows how to change formula
for other temperatures (25 mV figure changes with
temperature)
(e) tells you changes only, not the absolute value of the voltage
or current
ii) examples of use
(a) find dI/dV in circuit of Fig. 1 solution: dI/dV = 45 mA / 25
mV = 1.8 mA / mV try it on test circuit by increasing power
supply voltage slightly and measuring changes in current
and voltage across diode
(b) [OPTIONAL:
L.105.3 DC DIODE CIRCUITS PAGE 8
(c) (lab problem) at what level of I will dI/dV equal 5 mA/.05
V • solution: 5 mA/.05 V = 100 mA/V = dI/dV = I/25
mV
• so I = 100 mA/V x 25 mV = 2.5 mA]
II. Diode Steering and Isolation Circuits
1. definition: using diodes to guide signals to the right place in a circuit,
or to prevent signals from interfering with one another
2. examples
A. allow only positive voltages to enter a circuit (Fig. 8)

B. battery backup (Fig. 9)—battery kicks in only when normal power supply
fails

L.105.3 DC DIODE CIRCUITS PAGE 9
C. OR gate / isolation circuit
i) either one of two inputs going high can trigger a circuit, but the
two inputs can't affect one another
ii) example: trailer rear lights (Fig. 10, DRAW in stages)
(a) function • a trailer has a single rear lamp on each side (left and
right) that must function as both a brake light and a
turn signal light
• when you step on the brake pedal, both left and right
lights should turn on
• but when you turn on the turn signal, only one side
should flash
• i.e., a lamp can be turned on by either the brake
pedal OR the turn signal, and hence the name
(b) circuit and operation • since we want both lamps to light when we hit the
brakes, we might at first try to wire these lamps in
parallel
• but that will cause a problem when we activate the
turn signal (why?)
• so we connect the bulbs together, but through
diodes
• in this way, when the turn signal is on, powering the
left lamp, for example, there is no path for electrons
through the right lamp and back through the turn
signal switch (why not?)

L.105.3 DC DIODE CIRCUITS PAGE 10

III. Introduction to the Circuit Maker computer program
1. a program that can analyze electronic circuits that you draw on the computer
screen
2. it can help you design circuits because you can vary the circuit component values
and immediately see the effects
3. Circuit Maker is typical of the type of circuit analysis and design tools that today’s
engineers use every day
4. example of how to use Circuit Maker—circuit of Fig. 1 (SHOW the steps)
A. start the Circuit Maker program
B. on the blank sheet that appears, start drawing your circuit by, first, clicking
the “devices” button on top of the screen
C. under “hotkeys1,” select “+V 5V” for your 5-V power supply voltage
D. click on the computer screen where you want it to go
E. now under “hotkeys2,” select “resistor 1k” and again place it on the screen
someplace under the +5-V terminal
L.105.3 DC DIODE CIRCUITS PAGE 11
F. since you really want a 100-Ω resistor and not a 1 k, you next change the
resistor value by right-clicking on the resistor and selecting from the menu
that appears “Edit Device Data”
G. change the “label-value” from 1k to 100 and click “OK”
H. now to get the diode in the circuit, go back to “hotkeys1” and select “diode
DIODE” from the list
I. click on the screen to the right of the resistor
J. finally, from “hotkeys1,” select the ground symbol and add it on the far
K. to complete your schematic, you wire the components together in either of
two ways
i) click on one of the components, say the resistor, and drag it to the
left until one of its leads touches the +V terminal—it will
automatically bond to it when you release the mouse button
ii) or you can select the wiring tool by using the “+” button on the top
of the screen
(a) click on the “+” button
(b) now place the “+” cursor or symbol over the point where
you want to start the wire you’re planning to run and left-
click the mouse once
(c) you now will see a blue line representing your wire, which
you can drag to the end point—note that you can turn at
right angles by clicking the mouse once and then keep going
until you get to the desired end point of the wire, where you
click twice to end the wire
(d) hit “ESCAPE” any time in the process to erase the line and
start over
L. with the circuit drawn, you can now have the computer simulate it to find
out how it will perform
L.105.3 DC DIODE CIRCUITS PAGE 12
i) hit the “Simulation” button on top of the screen
ii) make sure the first option on the drop down list—“analog” is
checked (not “digital”)
iii) then select “Run” from the same list
iv) you will see a multimeter appear on the screen, which you can use
to make circuit measurements, just as if you were in the lab
v) your cursor will turn into a meter probe that you can touch to any
point in the circuit to make a measurement—examples:
(a) to measure the voltage across the diode, touch the probe to
the anode of the diode, making sure the probe has a “V”
inside it indicating voltage, then click and read the value on
the multimeter box
(b) to measure the current through the resistor, move the meter
probe along the resistor until the letter “I” appears inside it,
indicating current, and click at that point
(c) to measure the power being dissipated by the diode, again
move the probe along the diode until a “P” appears and then
click the mouse for a reading
M. to end the simulation, hit the “STOP” button, and now you can modify the
circuit if you like and test it again
IV. MATERIALS
1. 2 silicon diodes
2. small lamp
3. digital multimeter
4. LED
5. DC power supply
6. dedicated ammeter
L.105.3 DC DIODE CIRCUITS PAGE 13
7. 100-Ω resistor
8. heat gun
9. curve tracer set up to measure characteristic curves of diodes: LED, silicon diode,
Ge diode
10. Circuit Maker, student edition, on PC
11. oscilloscope and 2 BNC-BNC cables
12. oscilloscope setup:
A. x-y horizontal mode
B. source or x CH 2
C. vertical mode CH1, DC input coupling
D. connect CH1 to I, CH2 to V
13. curve tracer setup
A. set collector R to 1 kΩ
B. center oscilloscope beam vertically
C. move oscilloscope beam horizontally to left edge of screen
D. set I to 10 mA/V
E. set polarity switch to +
F. set collector voltage to full scale