introduction to diodes

7/29/2019 Introduction to Diodes

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Diodes

Diode Basics

References:

http://elen.ca/files/u2/elen146/notes/EPropsF.html

AAC: Chapter 2 -> The PN Junction, Junction Diodes AAC: Chapter 3 -> Introduction, Meter Check of a Diode, Diode RatingsQuestions:

Semiconductors and Diodes: Qs 20-32

EPropsF Animationo Cover Conductivity to Diode Rectifier

What is difference between intrinsic and extrinsic semiconductor.?

How do you turn intrinsic semiconductor into N-type semiconductor?

How do you turn extrinsic semiconductor into P-type semiconductor?

What is the effect of temperature on conductivity on a metal? On a semiconductor?

How does doping affect the valence bands and conduction bands?

N-type: P-Type

Diode immediately after creation:

Diode after time for electrons to fall into holes to create depletion region:


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Electrons diffuse across boundary and fall into holes creating depletion region. The electronsthat have fallen in to the holes make that side slightly more negative than the other side. Thischarge prevents more electrons from diffusing across, so the depletion region does not spreadacross the whole diode (at least not without an external electric field applied.

Apply a reverse bias to the diode:

Apply a forward bias to the diode:

A PN junction is a diode: Current through a diode is dependent on the voltage across it:

= 1NkT

qV

SD

D

eII q=1.6x10^-19 k=1.38x10^-23 T is about 298k 1


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o For LED, Vfwd~2Vo For Schottky diode, Vfwd is very low.

Basic Diode CircuitsReferences

AAC: Chapter 3 -> Rectifier Circuits, Clipper CircuitsQuestions


Assume Vfwd is 0.7V (and for the rest of the course, unless otherwise stated, you can makethis assumption). What is the current in the following circuits:

Complete the following diagram to create a very simple light dimmer with two settings: brightand dim.

Explain how this circuit works.

For each of the following circuits, determine what Vout will be. Some steps to follow:

o 1) Remember: If the diode is fwd biased, current will flow through it. What will Vout beif current flows through the diode?o 2) Remember: If the diode is rev biased, current will not flow through it. What will Vout

be if no current flows through the diode?o 3) Determine when the diode is forward biased and when it is reverse biased (positive

cycle or negative cycle, but dont forget when the input signal is between 0 and 0.7V,the diode will still be reverse biased)

o 4) Plot Vout accordingly


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Full Wave RectifiersReferences

AAC Semiconductors: Chapter 3 -> Rectifier CircuitsQuestions


Trace the current path through this circuit

For the Positive Half of the Cycle For the Negative Half of the Cycle

To 117 VACpower source

Load

117 V / 12 V CT

To 117 VACpower source

Load

117 V / 12 V CT

Plot the voltage across the load given the input voltage:

VLoad = (Vsrc/2) 0.7Vo In this case Vsrc is the voltage across the entire transformer on the load side of the

transformero Vload is half of Vsrc because the transformer is center tapped and ground is at the

center tapo Subtract 0.7V because of voltage drop across the diode. Only one diode drop in either

the forward or the reverse direction.

For rectifying diodes in a circuit like this, why is the Max Peak Inverse Voltage for the diode animportant parameter?

By using a Bridge Rectifier which uses 4 diodes, you can apply the full voltage swing of thesecondary (minus diode drops) to the load.


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Trace the path of the current through these two circuits:

(positive half cycle of input) (negative half cycle of input

Rload Rload

Plot the voltage across the load assume that the voltage out of the secondary is 17 Volts::


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Full Wave Bridge Rectifier Filtered

The filter will smooth out the bumps:

Capacitor charges up very quickly because R is the resistance of diodes (


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TCR

VV

L

mppripple =_

pprippleV _ is the peak to peak ripple voltage,

mV is the peak output value from the rectifier.

T is the half the period of the AC signal.

RL is the load resistance in Ohms.

C is the capacitor value in Farads.

To decrease Vripple, what would you do to the capacitance? Does a larger load cause smaller ripple, or bigger ripple? The ripple occurs at an offset of around 15V, but the ripple is probably 2 orders of magnitude

less than the offset. If you want to view the ripple on an oscilloscope, how could you ensure

that you can zoom in to view and measure the offset while at the same time not pushing thesignal way off the scope (due to the high offset).

The filter reduces most of the ripple (depending on design and load), but it would be nice in manyinstances to get rid of the ripple toouse a voltage regulator e.g. 7805, 7809 and many others.

The simplest of voltage regulators can be built from Zener diodes.

Zener DiodesReferences

AAC Semiconductors: Chapter 3 -> Zener DiodesQuestions


Characteristic curve of a diode:

VD

ID


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For positive VD, you basically have a circuit like this:

And as the VSRC is increased, I through the diode increases but VD changes very little (stays at around0.7V)

For negative VD you have a circuit with the diode in the opposite orientation:

As the voltage is increased in this circuit (VD becomes more negative), there is no current through thediode, until SRC becomes large enough to force conduction through the reverse biased PN junction.The threshold is called the reverse breakdown voltage (VBR). And voltages beyond VBR are in thebreakdown region

Two things happen in this breakdown region:

1) Diode current increases dramatically2) The reverse voltage remains relatively constant

These facts are true for both signal/rectifying diodes and zener diodes. The biggest differencebetween the two is the magnitude of the reverse breakdown voltage.

For zener diodes, the VBR can range from 1.8V to several hundred volts, it is also usually called thezener voltage (VZ)

While for signal diodes, the VBR is usually over 100V.

Because the Zener voltage is fairly constant, a Zener diode can be used for voltage regulation This circuit will maintain a mostly constant Vz across the load.

There are a few important features of a zener diode that need to be known when designing avoltage regulator using one:

o VZ : The voltage at which the load will be regulatedo IZK : The zener knee current. This is the minimum amount of current that must flow

through the zener diode to turn it on so that it can regulate at the VZ.o

IZM : Maximum zener current. The maximum current the diode can safely handle. This isrelated to the power dissipation capabilities of the zener diode.e.g. If the diode can dissipate 500mW of power and VZ is 5.1V. Then IZM = 0.5/5.1 =98mA.


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Examples


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Light Emitting DiodesReference

o AAC Semiconductors: Chapter 3-> Special Purpose DiodesQuestions

o Semiconductors and Diodes: Qs 91-95


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Diode Circuit Analysis and Troubleshooting Questions

1. In the circuit below, what is the current through the diode:

2. Find the voltage across the R and the current through it in the following circuit:

3. For the circuit below, calculate the value of Rseries that you should use. ILoad is 100mA IZM is 100mA,IZK is 1mA.

4. After you determine the Rseries to use in the circuit above, calculate the power dissipation for each ofthe components in the circuit.

5. Redraw the circuit from the question above, and show how you would connect an LED to the circuit toindicate that the battery is on. Assume a forward voltage of 2V for the LED and design for a 20mA

current through the LED.

6. For the circuits shown below, indicate the voltage between the terminals A and B, and indicate thecurrent through each of the diodes:


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7. The diode shown below was tested with an ohmmeter. The table next to the diode shows the results ofthe test. Which end of the diode is the cathode?

Ohmmeter + Lead Ohmmeter - Lead Resistance

A B Infinite

B A 600

8.

9. Find ID and Vout for this circuit:


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10. In the circuit shown below, you measure a voltage of 0 volts at the junction of R1 and R2. Name somepossible troubles in the circuit.

11. On the graph of the input voltage (dotted line) sketch the output voltage V for the circuit shown

12. The BZX79C6V8 zener diode, used in the circuit below has a zener voltage of 6.8V, and zener

current (knee current) of 5 mA, and a maximum current of 75 mA. If Vsource is 24volts, andRseries is 200 . What is the allowable range of Rload? (6)

Rload

Rseries

Vsource

13. Sketch one complete cycle of the output waveform, with respect to ground for each of thefollowing circuits


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1. Identify the voltages that are supposed to appear between the listed test points:

Fuse

- -

TP1

TP3TP4

TP5

TP6TP7TP8

DC voltage output

TP9

TP10

TP2

120 VACpower input

1. VTP1TP2 =2. VTP1TP3 =3. VTP2TP3 =


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4. VTP4TP5 =5. VTP5TP6 =6. VTP7TP8 =7. VTP9TP10 =

Assume that the power transformer has a step-down ratio of 9.5:1.

(02008)

14. Identify the voltages that are supposed to appear between the listed test points

Fuse

- -

TP1

TP3TP4

TP5

TP6TP7TP8

DC voltage output

TP9

TP10

TP2

120 VACpower input

1. VTP1TP2 =2. VTP1TP3 =3. VTP2TP3 =4. VTP4TP5 =5. VTP5TP6 =6. VTP7TP8 =7. VTP9TP10 =

Assume that the power transformer has a step-down ratio of 9.5:1.

(02008)

15. A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltage issupposed to be 15 volts DC:


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Fuse

- -

TP1

TP3TP4

TP5

TP6TP7TP8

DC voltage output

TP9

TP10

TP2

1. VTP9TP10 = 0 volts DC2. VTP8TP7 = 0 volts DC3. VTP8TP5 = 0 volts DC

4. VTP6TP7 = 0 volts DC5. VTP4TP5 = 0 volts AC6. VTP1TP3 = 0 volts AC7. VTP1TP2 = 116 volts AC

Based on these measurements, what do you suspect has failed in this supply circuit? Explain your

answer. Also, critique this technicians troubleshooting technique and make your own suggestions for a more

efficient pattern of steps.(00795)

16. A technician is troubleshooting a power supply circuit with no DC output voltage. The output voltage issupposed to be 15 volts DC:

Fuse

- -

TP1

TP3TP4

TP5

TP6TP7TP8

DC voltage output

TP9

TP10

TP2

The technician begins making voltage measurements between some of the test points (TP) on the circuit board.

What follows is a sequential record of her measurements:

1. VTP1TP2 = 118 volts AC2. VTP3TP2 = 0 volts AC


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3. VTP1TP3 = 118 volts AC4. VTP4TP5 = 0.5 volts AC5. VTP7TP8 = 1.1 volts DC6. VTP9TP10 = 1.1 volts DC

Based on these measurements, what do you suspect has failed in this supply circuit? Explain your

answer. Also, critique this technicians troubleshooting technique and make your own suggestions for a more

efficient pattern of steps.

(00796)

introduction to diodes

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