making a dc voltage supply for your circuit

SMPS
The circuits we make or ICs usually work on dc voltages like 5v,12v,9v,6v. We can use dc batteries for this purpose. However for uninterrupted power supply one should go for an adapter or SMPS.
We have lot of obsolete computer systems these days. You can utilize SMPS of such abandoned systems as power supply of your circuits.

Here is an image of SMPS ATX Power Connector.

You need to perform one Job before u can use it after removing from the CPU.
Short the pin that is green wire to any adjacent pin of black wire. This will pull the green wire's +5V signal low. This process makes the power supply turn on once plugged to AC main.

Note: You can utilize this feature to switch off/on the SMPS in a system automatically.

Now test its output with a multimeter. Here are the Voltage levels

YELLOW WIRE=+12VDC
RED WIRE=+5VDC
ORANGE=+3.3VDC

12 Vdc and 5Vdc is used often in the circuits. I have never used other pins of the SMPS.

Building your own DC supply
However, we may design our own power supply.



Above circuit shows how to produce +/-12V, 5V from 230V AC line.
It basically consists of a bridge rectifier with a capacitor filter and a voltage regulator.



First connect primary side of transformer with free end of power chord.
To identify which side is primary follow either any one of these steps

• Always red color wire end will be primary and black color wire end will be secondary.
• if you have multi meter check the resistance, primary side will have higher resistance and secondary will have lower resistance (Sometimes you may not notice the difference as you might be using transformer for smaller size)
• or else if possible supply lower AC voltage on either side and check the output on other side. By that you can identify which side is stepping down.

Than connect secondary sides to the board having the circuit of bridge rectifier and voltage regulator.
Note
The rectified dc output will never be greater than 2*VP/pi.
Vp - Peak Voltage Transformer's stepped down peak voltage rating should be chosen accordingly, e.g, for 7812 to generate the input v should not be less than 14.5V. Transformer of 15-0-15 as shown in figure is used accordingly to meet the requirements
If you invert polarity of the capacitors then sometimes it will burst because

all the capacitor here are electrolytic capacitors. 

If you invert the polarity of the diodes then this circuit won't work.
If you invert the polarity of the voltage then it may burst

Here you can see that -12V is with respect to ground, remember that we measure all quantities with respect to ground. Suppose if you want -12V , don't say that you connect 0V to +ve and 12V to -ve lead because we say -12V with respect to ground and ground(0V) is a common terminal.

Voltage Regulators

Voltage regulators produce fixed DC output voltage from a variable DC (i.e. a DC with small amount of AC on it). Normally we get fixed output by connecting the voltage regulator at the output of the filtered DC(see in above diagram). It can also used in circuits to get a low DC voltage from a high DC voltage (for example we use 7805 to get 5V from 12V). There are two types of voltage regulators
1. fixed voltage regulators(78xx,79xx)
2. variable/configurable voltage regulators(LM317)


1.  Fixed Voltage regulators
This include 78xx voltage regulators. The most commonly used ones are
7805 and 7812. 7805 gives fixed 5V DC voltage if input voltage is in (7.5V,20V). if input voltage is <7.5 V or some 3V, the regulation won't be
proper.
-ve voltage regulators are of 79xx family
2. Variable Voltage Regulators
Most commonly variable voltage regulator is LM317 although other
variable voltage regulators are available. The advantage of variable voltage
regulator is that you can get a variable voltage supply by just varying the
resistance only. I have never used any variable voltage regulator
Note:
Remember about the heat sink of the voltage regulators before touching
the voltage regulator IC  because it will be in the heated state normally .