Saturday, 18 October 2014

How to Regulate Power Supply Voltage in Electronic circuits Or Reduce voltage dropping

How to Regulate Voltage in  Electronic Circuits or Reduce voltage dropping in power supplies.



Generally the sole purpose of a power supply is to provide power to an electronic circuit so that we can operate that circuit within safe voltage and current tolerance/ range. For a given amount of power, there is an inverse relationship between voltage and current (ohm's law V=IR, where v is voltage across the circuit, I is current in the circuit and R is the equivalent resistance across the circuit). Whenever there is an increase in current there must be a voltage, and whenever current decreases,voltage must increase. This simple phenomenon unfortunately, has an adverse effect on power supply circuits.

When anyone connect a voltmeter to the output terminals of a power supply, the voltmeter itself draws current ( but an almost of insignificant amount ) so the meter reads very close to the voltage anyone should expect to obtain from the power supply.

However, if one connect a circuit which draws a significant amount of current from the power supply, then there will be a voltage drop occurs in perportion to the amount of current.


Depending on the nature of the circuit one is connecting to the power supply, this voltage drop
may or may not be a bad thing for most of circuit which is insensitive to voltage variation but some circuits which is designed for 12 VDC or some fixed value DC then the circuit may not function properly. In that case the power supply needs to workeven harder to make sure it delivers the desired
voltage to the circuit to function properly.

So power supply have to maintain that value and to maintain that steady voltage level regardless of the amount of current drawn from a power supply, the power supply need to incorporate a voltage regulator circuit. The voltage regulator circuitory usulaay monitors the current drawn by the load and according to that it increases or decreases the voltage accordingly to keep the voltage level fixed or constant.

So simply when a power supply  incorporates a voltage regulator circuitory  is called a regulated power supply.

Anyone can design their own voltage regulator circuit, but it is very easy to buy one of the many
available integrated circuit (IC) voltage regulators in the market to save the time. Generally voltage
regulator ICs are inexpensive and, with we only have to simply connect three pins, really very easy to incorporate into any circuit which requires a regulated power supply. In the market the most popular type of voltage regulator IC is the 78XX series, sometimes it is called as the LM78XX series too. These voltage regulators IC's is the combination of 17 transistors, three
Zener diodes, and a handful of resistors into one easy to use handy package with three pins. We also have to buy a heat sink (aluminium bar) which helps the IC to dissipate the excess power consumed by the regulator for compensation for the increases or decreases in current draw to keep the voltage at a constant level or fixed level.

About IC
The last two digits of the LM 78XX ID number printed on IC indicates the output voltage
regulated by the IC. The most
popular models are:
Model Voltage
ID no.                     Volts
7805 -------------------5
7806 -------------------6
7809 -------------------9
7810 ------------------10
7812 ------------------12
7815 ------------------15
7818 ------------------18
7824 ------------------24

The most common are the 7805 (5V) and 7812 (12) because generally every electronic circuit works on 5volt input and general use of voltage is 12 volt for operating motors, using instead of 12 volt batteries, in amplifiers etc.




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To use a 78XX voltage regulator, you just insert it in series on the positive side of the power supply
circuit and connect the ground lead to the negative side. As this shows, it’s also a good idea to place a small capacitor (normally1μF easily available ) after the regulator. You must supply a voltage regulator with about 3 V more than the regulated output voltage. Thus, for a 7805 regulator, you should give it at least 8 V so that it can regulate the voltage to 5volt. The maximum input voltage for a 7805 is 30 V. The diodes in a bridge rectifier will drop about 3 V from the transformer output, so we will need a transformer whose secondary delivers at least 11 V to produce 5V of regulated output to the circuit. Eleven-volt transformers are rare, but 12 V transformers are readily available. Thus, a 5 V regulated power supply starts with a 12 VAC transformer that delivers 12 V to the bridge rectifier, which converts the AC to DC and drops the voltage down to about 9 V and then it will deliver this voltage to the filter circuit, which smoothes out the ripples present after the rectification of the 12VAC and after filteration it passes the voltage on to the 7805 voltage regulator, which regulates the output to a fised or constant voltage of 5 Volt.

Here is an another popular voltage regulator IC is the LM317, which is an adjustable voltage regulator.

An LM317 regulator works much like a 78XX regulator, except that instead of connecting the middle
lead directly to ground, we simply connect it to a voltage divider resistance built from a pair of resistors. The value of the resistors determines the regulated output voltage.

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