78xx voltage regulator
Electronic circuits are very sensitive. They require a stable voltage input. Here comes into play the role of voltage regulator. In this article we will briefly discuss about 1 Ampere linear voltage regulator IC. After going through this blog, you will be able to step down dc voltage accurately by using 78XX IC series.
Where 78 implies Voltage regulator series and XX stands for the output voltage.
Here I will explain you by taking 7805 IC as example, so that you understand it in a better way.
7805 IC can handle an input of 7 Volt to 35 Volt at 1 A input.
The input voltage is applied to pin number 1 & 2 and the output is obtained from pin 2 and 3. Pin 1 is for positive input, pin 2 is for negative input and output, pin 3 is for positive output. As shown in circuit diagram above.
For smoothing DC supply, I have used a 100 micro farad 36 v capacitor at input part of the circuit and a 10 micro farad 36v capacitor on output side.
Further on the output I have added a small led for indication purpose with a 470 ohm resistor. ( 470 ohm is to protect led from over voltage and current, this value of resistor is applicable only for 7805 IC, that is for 5 volt output ).
Our 5 volt 1 ampere output is now ready.
The circuit shown above is an ideal circuit for 78XX IC series, you can simply replace 7805 IC with any other IC in 78XX series.
This type of voltage regulators are known as linear voltage regulator.
The merit of using this type of linear voltage regulator is they have a very simple circuit.
The demerit of this type of linear voltage regulator circuit is they loose a lot of power in the form of heat, that overall deceases efficiency when compared to buck converters.
The table above shows the availability of IC’s , input voltage to the IC, the output voltage of the IC and the input and output current.
Remember The input voltage should be 2v higher than the output voltage also the maximum input voltage is limited to 35v. For continuous supply max input voltage is limited to 25 V. The input and output currents are limited to 1 Ampere.
Advantages
- Easy to use
- Low cost
- Accurate output
- Few external components required.
Disadvantages
- It is a linear voltage regulator. It means it has low efficiency.
- Looses power in the form of heat.
Power loss in 7805
We have already discussed above that this type of voltage regulators are known as linear voltage regulator. They drop voltage and losses them in the form of heat.
Suppose we provide 12v 0.5A input to 7805 IC that has an output of 5v 0.5A.
So the voltage drop made by IC will be 7v.
We know,
Power (P) = Voltage ( V ) x Current ( I )
Power loss = 7 x 0.5 = 3.5 W
Input power = 12 x 0.5 = 6 W
Power output = 5 x 0.5 = 2.5 W
The 3.5 W power loss heats up the IC.
On an average there is 20° C rise in temperature per watt.
3.5 W can raise upto 3.5 x 20 = 70° C
If we take 27° C as normal temperature of IC then the IC can heat upto 27° + 70° C = 97° C. According to manufacturers the IC can tolerate upto 125° C. 97° C is still too high.
In this case we must use a good aluminium heatsink that needs to be attached properly to the back side if IC that gets heated. There is a hole on top of IC where you can pass a bolt through it into heatsink and attach them using nut.
Caution
Although 78xx voltage regulator series is a simple and cheap solution for most of the circuits. You need to be careful while using it.
Make sure the input current should not go above the current rating of IC. It can easily blow up.
Connect input and output pins of IC carefully. Reverse connection can damage the IC.
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Author
Akas Sharma
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