Diodes

Analog Electronics

UNIT III

Diodes

UNIT I

Objective The student will use diodes, capacitors, regulators and LEDs through a rectifying system in order to build a regulated power supply.

Voltage Regulators

Topic 5

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage Regulators

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Zener diode. The Zener Diode is an special type of diode that is used in reverse

polarization, since this condition allows tokeeps a constant voltage. I the diode is biased in forward bias then it behaves as a common rectifying diode.

Applications

1) Voltage regulator in low power circuits.

2) Protection against peaks of voltage in DC supplying lines for Integrated

Circuits.

Symbol Physical component

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage Regulators

The zener diode works into an specific voltage of operation, so it is specified in its nomenclature, i.e. the diode MZPY7.5 is a zener diode of 7.5V.

Other typical values are: 3.3V, 5.1V, 6.3V, 7.5V, 9.1V, 12V, 15V, 18V, etc.

Zener diodes are widely used as voltage references and as regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage RegulatorsCharacteristic curve of zener

The zener Works in the breakdown zone, in one point which is called zener voltage (Vz). If the voltage is much higher tan the Vz voltage then an avalanche current is produced and the diode can get damaged.

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage Regulators What is a Voltage Regulator?

A voltage regulator is an electronic device which accepts an electrical variable voltage inside a predefined range and keeps the output voltage into a constant level.

The voltage regulators are Integrated Circuits (I.C.) which can regulate a voltage due to the operation of an electronic control formed by an arrangement of transistors. Basically those transistors detect a variation of voltage in the input and they do a correction in order to keep a constant level in the output.

Features of voltage regulators.• The response time and speed of regulation is fast • They are cheap.• The ranges of tension are limited to small voltages (3V to 24V)• Precision of output voltage is low (around +/- 3% to +/- 5%)

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage Regulators Typical Voltage regulator.

• One of the most popular voltage regulators is the family LM78xx, where “xx” is the output voltage. They have output voltages of 5, 6, 8, 9, 10, 12, 15, 18 and 24V, delivering a current of 1A as maximum.

• They are built with thermal protection against overcharge and short circuits, so the regulator is disconnected in case that temperature reaches 125°C.

• The LM78xx are regulators with positive output meanwhile the family LM79xx provide negative outputs.

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage Regulators

Construction of regulator

The package that contains to the regulator 78xx is type TO-220, which is same as the one for many power transistors.

It is necessary to provide to the regulator with a metal plate called Heat sink in order to obtain the 1 Amp current from it without overheating the component. Otherwise the thermal protection will be activated and the regulator won´t work.

Heat sink

UNIT 3. DiodesELECTRICAL CIRCUITS

Voltage Regulators

Calculation of the power.

The power consumption of the regulator depends of the output current and the voltaje between its terminals. For example, in case of LM7812, the output voltage is 12V, so if we apply a 20V input voltage to it and the output current is 0.5A, then we will get the power by multiplying the output current with the difference of potential between the input and output terminals:

PD = (Vint - Vout) x Iout = (20V – 12V) x 0.5A = 4W

It is very important to consider this data when designing a circuit because it will determine the power requirements for the circuit without affecting its operation, so the selleccion of regulator is a key issue when designing circuits.

UNIT 3. DiodesELECTRICAL CIRCUITS

AC-DC Conversion

Topic 6

UNIT 3. DiodesELECTRICAL CIRCUITS

AC-DC Conversion

A DC power supply is an electrical system which is used to feed a voltage to a circuit or equipment which works with direct current, providing the characteristics of voltaje, power and current for its proper operation.

• Practically all electronic equipment build a DC power supply as an input circuit in order to feed all the internal components since the most equipment works with DC.

Basic DC Power Supply

UNIT 3. DiodesELECTRICAL CIRCUITS

AC-DC Conversion

1) As a first step for the conversion of AC to DC, we get a rectified signal from a Rectifier which could be a Half wave rectifier or a Full wave rectifier.

Input OutputHalf Wave Rectifier

Input OutputFull Wave Rectifier

Rectifying

UNIT 3. DiodesELECTRICAL CIRCUITS

AC-DC Conversion

2) The capacitive filtering allows to eliminate the peaks from the continuous pulsate current from rectifying, this is because of the charge property of the capacitor. It get charged every time that peak voltage is applied so that voltage is stored and we can get a DC voltage which is almost same as the peak value from the rectifier, the only variation that remains after filtering is an AC signal added to the DC output which is called ripple voltage.

Signal without filtering Filtered signal

Filtering

Ripple voltage

UNIT 3. DiodesELECTRICAL CIRCUITS

AC-DC Conversion

3) The regulation helps to keep a constant output level beside any variation in the input voltage.

• Once the voltage is filtered it can be regulated by using a zener diode or an IC Voltage regulator.

Regulation

UNIT 3. DiodesELECTRICAL CIRCUITS

Activity:Calculate the electrical parameters requested in the following circuit

UNIT 3. DiodesELECTRICAL CIRCUITS