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Saturday, 28 June 2014

Simple Tips for Efficient use of 78xx Linear Voltage Regulators

by realfinetime  |  in Voltage Regulator at  20:05

78xx is a series of linear voltage regulator integrated circuits. These integrated circuits are commonly used in circuits that require a regulated power supply. It is very cheap and is easy to use. These two factors make them special in voltage regulators. 78xx series have a complementary negative voltage regulator series and is designated by 79xx. In these series of ICs, xx indicates the output voltage from the regulator ( Eg: 7805 have an output voltage of 5V, 7812 have an output voltage of 12V ). Combination of 78xx and 79xx series is used to create positive and negative voltage in the same circuit.

Features of 78xx regulators.

1. Three terminal regulators.
2. Internal Short-Circuit Current Limiting.
3. Output Current up to 1.5 A.
4. Output Transistor Safe-Area Compensation.
5. Internal Thermal-Overload Protection
6. High Power-Dissipation Capability

Pin out diagram of 78xx Series Integrated Circuits.

Following table shows the part number, output voltage and input voltage range of 78xx series voltage regulators.

Circuit diagram to build a fixed-output regulator using 78xx is given below.
Capacitors should be included in the input and output side of the voltage regulator circuit. Because all characteristics are measured with a 0.33 µF capacitor across the input and a 0.1 µF capacitor across the output.


Advantages of 78xx Series

1. Don't require any external device to supply constant, regulated power. It is very cheap and is very easy to use.
2. They have protection against drawing too much power, overheating and short circuits. Current limiting features of 78xx not only protect the regulator, but also protect the other parts of circuit in some situations.

Disadvantages of 78xx Series

1. Input voltage must always greater than the output voltage.That is, for 5V output from 7805, input voltage must be greater than 7.5V. So these devices cannot be powered from certain power sources. That is 7805 cannot be powered from a 6V battery.

2. Input current required and the output current of 78xx series is same. But the output voltage is always less than the input voltage which means some amount of power is dissipated as heat inside the regulator. So proper heat sinking mechanism must be implemented. This loss of power also decreases the efficiency of regulator. This inefficiency become more significant if the input voltage is very higher than the output voltage.( Eg: Powering 7805 using 24V supply )

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IMPORTANT NOTICE

All the circuits, published in this blog is only after testing and getting proper results in my private lab. When you try these circuits, you should check the supply voltage, polarity of components, presence of childrens nearby and shorts in the circuits. This website will not be responsible for any harm happened to you or your components caused by your carelessness.

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