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 .

getting started with electronics stuff-2

MULTIMETER:
Multimeter is used to measure different parameters like voltage, current, resistance, etc... by setting the dial at a proper position.
A very important feature of multimeter is continuity test-used for debugging ckts.You can check if the circuit connections are proper as per the circuit diagram.
It is a good habit to check continuity of every new connection while making the circiut on a board else it becomes difficult to find any continuity error at the end.
A cheap n best multimeter(yellow ones) used generally by the students is shown below.

NOTE:connect the multimeter in parallel to measure voltage and in series to measure current.

DIODES AND LEDS:
Diodes are two terminal devices which conduct electricity in one direction.

LEDs contitute an important element of digital ckts-used to display output as a monitor shows the output of a c program you execute in your computer.
LED has two leads- cathode and anode.They are identified by the length of the lead.Cathode lead is of lesser length.However this is not a reliable identification, for confirmed identification see the figure below:

Connect LED to the output of any IC with a resistor in series.LEDs have a small resistance,so a resistor in series decreases the loading effect of any output.If you connect the output of 7805 directly to an LED then the voltage output of 7805 reduces to 3.85V from 5V.

Relays
a relay can make a 5V DC signal from a circuit output to switch a 230V AC mains circuit. The input section has a coil which generates magnetic field when a small voltage from an electronic circuit is applied to it. This voltage is called the operating voltage. Commonly used relays are available in different configuration of operating voltages like 5V, 9V, 12V, 24V etc. The output section consists of contactors which connect or disconnect mechanically. In a basic relay there are three contactors: normally open (NO), normally closed (NC) and common (COM). At no input state, the COM is connected to NC. When the operating voltage is applied the relay coil gets energized and the COM changes contact to NO. Different relay configurations are available like SPST, SPDT, DPDT etc, which have different number of changeover contacts. Figure given below shows how a relay basically operates.

 

getting started with electronics stuff-1

You must be knowing a lot about circuit elements like resistors, diodes, capacitors,etc.
So in this post the intention is not to let you know about these elements.
I have depicted only few ideas but very important that you may need while making ckts.
BREADBOARD
Breadboard is used to make circuits. But mostly after testing your circuits on breadboard you will be making PCB.Breadboards costs from Rs.80-120(depends on place where you are in India).

All you require is to do a good wiring.Following figures show good wiring practices

 Below you can see what connections you should NOT have to do.

However breadboards are not reliable. It is the PCB on which u have to make the circuit finally.
Some breadboards will be difficult to insert in IC's like 7805,LM317 etc due to the small sized holes.
some parts of the breadboard don't work after sóme days of use as the internal shorts in the breadboard get damaged. Always press the ICs gently into breadboard to avoid such damages rather than pressing them hard.
  
RESISTOR
resistors are used in digital circuits generally for voltage divider circuits,or in controlling the voltage across leds.
Resistances range from 10 ohm to 56Mohm(or more) and power ratings from 1/8W to 20W.These 1/8W resistors are used in low power devices. The one which available in shops are of 1/4W which we mainly use.

you can use a multi meter to measure resistance.
Generally you have to select a resistance from set of resistors and in that case it becomes difficult to measure the resistance of each to find the desired one. You should be accustomed to use colour coding of resistors.
Here is the 4 band colour code:

POTENTIOMETER ( ' POT ' )
These are resistors whose resistance can be varied. Potentiometers are available from 100 ohm to 470Kohm(or more).

Potentiometer is used as a voltage divider.If we connect Lead A to Vcc and Lead B to ground then you can get voltages from 0 to Vcc by taking voltage at LeadW and LeadB.
Given figures show different types of potentiometers available in market.

CAPACITORS:
A capacitor is used to store charge. Like resistors there is fixed as well as variable capacitor also.
There are capacitors with no polarity and polarity.
Ceramic and Mica capacitors available are of no-polarity, but electrolytic capacitors are of polarity.
Capacitance vary from 22pF to about 15000uF.
Values <.1uF are mainly mica and ceramic capacitors and C>=1uF are electrolytic capacitors.

We can identify negative lead of electrolytic capacitor by checking the length of the lead, one with less length is -ve.On the body of electrolytic capacitor -ve symbol is shown. Be careful about electrolytic capacitor because inverting polarity can make 'explosion' (not firing) of capacitor(sometimes it can hurt your body).

Every capacitor has two factors - value of its capacitance and other the maximum voltage rating.