Wednesday, March 20, 2013

Linear Voltage Regulator 1- Hobby Electronics

Good morning everyone.Today I'm going to write about linear voltage regulators.

In my previous post I have showed how easy it is to get a desired DC voltage from AC outlet but there is a problem which is that DC voltage is not regulated. What that mean is the DC voltage might fluctuate with time. Even after using smoothing capacitors the voltage that we get is not fully stable. So that voltage might not be ideal for operating many equipment where it is mandatory to keep the voltage stable.

What exactly a voltage regulator does?

A voltage regulator takes in unregulated input voltage that fluctuates with time and gives off regulated output voltage that doesn't change overtime.

Capacitors are used for maximizing stability of the circuit. When this regulator is under load the capacitors will help to minimize ripple.

Types:

Now more about Three Terminal fixed voltage Positive & Negative Regulators.

Positive Regulators are used for regulating positive voltage.
Negative Regulators are used for regulating negative voltage.

Positive voltage regulator starts with 78 & followed by two digits which provide information about voltage. Like a 7805 is a positive 5V regulator, a 7812 is a positive 12V regulator.
Negative voltage regulator starts with 79 & followed by two digits for  which provide information about voltage. Like a 7905 is a negative 5V regulator.

Common Specification of 78XX series:

1. Voltage : A 7812 has a typical 12V output but it could be as low as 11.5V & high as 12.5V.

2. Output Regulation : The output voltage regulation is seen to be typically around 4mV to a maximum of 100mV(depending on output current). It means output voltage can typically vary only 4mV from rated 12V.

3. Short-Circuit Current : Amount of short circuit current is typically 0.35A, if the output is accidentally shorted.

4. Peak Current : Rated maximum current is 1A-1.5A but its peak current is around 2.2A.

5. Dropout Voltage : Typically the dropout voltage for this ICs are 2V. This is the minimum amount of voltage difference across the input-output terminal. If it drops below 2V the IC will no longer provide regulation.

Important Note :

Linear regulators can be really hot during operation as they burns a lot of power and higher the current is drawn from the regulator and higher the difference between input and output voltage bigger the loss is. So a moderately sized heat sink is needed if more than 500mA is drawn from them.

It wastes Power as follows : (Vin - Vout) * Current out

e.g. If I use a 5V regulator from a 12V unregulated source and I'm drawing around 500mA(.5A)

it will waste (12-5)*.5= 3.5W.

3.5W heat is a lot of heat for such tiny device. It will soon stop working as soon as thermal shutdown feature kicks in.

what if I use a 7V source?

(7-5)*.5 = 1W

which is much lower.

So, one more thing try to use a closer unregulated input voltage. & it IC gets hot use proper heat sink with proper thermal paste!

Where To Find :

This ICs are very familiar & can be found in most of the places.

 From left to right 7805, 7809, 7812 and 7815