Wednesday, May 2, 2012

Electronics Project: Basics for doing it yourself - 2


In this post I will be going through a regulated power supply. It is necessary to create your own power supply circuit for the projects you will be doing. Now lets discuss about the components that are used to make a regulated power supply unit.

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors. It consists of two windings primary and secondary. When alternating current is applied to primary winding, a varying magnetic flux is created in transformer's core and thus a varying magnetic field through secondary winding. This varying magnetic field induces varying electromotive force (EMF) in secondary winding. If a load is connected to secondary, current will flow in the secondary winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. This effect is called inductive coupling and well known transformer equation has been established:

Here, voltage in secondary coil (Vs) is in proportion to the primary voltage (Vp) and is given by the ratio of number of turns in the secondary (Ns) to the number of turns in the primary (Np). Thus, we can use transformer to step up or step down the AC voltage. While purchasing a transformer its cost depends on how much voltage and current it can couple on secondary coil. For our application, we will use a 500mA transformer that is rated 12V. This transformer when connected to 220V AC line will produce 12V at secondary coil. So, if the source AC voltage fluctuates the output voltage at secondary coil also fluctuates accordingly. 
It is very essential to distinguish primary and secondary coils of transformer when you want to use it. The primary coil in general has only two wires and the secondary coil has three wires coming out of the coil as shown in the figure. To check a transformer whether it is working or not we simply connect its primary wires to AC line. Then using a voltmeter, we have to measure voltage at secondary coil. Among the three wires from secondary coil, the wire that is coming from centre is common wire and the two wires coming from edge gives rated output voltage of the transformer. We connect common wire to common probe of multimeter and touch one of the edge wire. Place the multimeter in AC Voltmeter mode and measure the voltage. Then touch other wire and measure the voltage. Generally, the voltages are equal however, in some cases it might be different. Mostly, we can find the output voltage marked on the transformer such as 12-0-12, 12-0-9, 9-0-9 etc. which means at secondary coil the output is 12V at each edge wires and 0V at centre wire. 

Voltage Regulator IC:

As we know that, the AC source voltage fluctuates a lot and transformer also produces fluctuated AC output. For the proper operation of circuit, a constant DC voltage is always required. This constant DC voltage is obtained with the help of voltage regulator IC. Mostly we use 78xx series voltage regulator IC such as 7805, 7809, 7812 etc. The 79xx series voltage regulator IC are used for obtaining negative voltage level for application that requires negative voltage levels such as an audio amplifier. The 78xx series IC has three terminals named IN, GND and OUT. At IN terminal we apply input DC voltage which must be greater than required output voltage. The GND terminal is connected to ground of the circuit. The OUT terminal gives output voltage as specified by that IC. For 7805, 7809, 7812 IC we obtain +5V, +9V, +12V as output respectively. The voltage regulator IC also has a hole to connect heat sink with it. The heat sink is used to absorb heat dissipated by the IC which ensures its good performance and longer life. The heat sink is always connected to grounding of the circuit. 

Circuit Diagram:
This is the circuit diagram for 3-way regulated power supply. Where you can obtain three different voltage levels from same circuit. Here I have used 5V, 9V and 12V for output. The full-wave bridge rectifier is made up of four diodes where, we connect it as shown in diagram. This gives us two pair of terminals one pair for connecting AC source voltage and the other pair for obtaining corresponding DC voltage. The terminals can be distinguished as AC terminals are formed by junction of P and N side of each diode and DC terminals are formed by junction of P-P or N-N. 

When we connect the primary wires of transformer to AC line, the transformer steps-down the source voltage of 220V to 12V AC. Then this AC voltage is applied to AC terminals of full-wave bridge rectifier. When 12V AC is applied to it corresponding 12V DC is obtained at DC terminals. The polarity of this DC voltage can be determined since at P-P junction we always obtain negative half cycle of AC and at N-N junction we obtain positive half cycle of the AC. Thus N-N junction is positive and P-P junction is negative terminals of DC output. Since, AC voltage is a function of sine we obtain ripples at output DC voltage. To eliminate these ripples we have to use a capacitor as a filter. The capacitor charges slowly and discharges quickly hence, it do not allow ripple formation and smooth DC voltage is obtained after filtering by the capacitor. The positive terminal of the capacitor is connected to positive terminal of the rectifier and negative terminal is connected to negative terminal of the rectifier. Here we have used 1000uF/25V capacitor for our purpose. This 12V DC voltage is now applied to input terminal of different voltage regulator IC. The 3-way selector switch selects, each of the input terminals of the IC at different position. At one time only one of the three voltage regulator IC is given the input DC voltage. When the switch is changed to different position different output is obtained at output terminals of the circuit. The LED is used to indicate which voltage level is appearing at the output terminals.

Variable Voltage Regulated Power Supply:
The circuit we discussed earlier can be used for obtaining discrete voltage levels such as +5V, 9V and 12V. But, sometimes we are required 7.5V in this case the above circuit is not useful. Here, I would like to show a simulation of variable voltage power supply in which output varies from 1.2V to 12V. The main component in this circuit is LM317 variable voltage regulator IC. 
In the figure you can see the pin names of the IC and a typical circuit for connecting LM317. There is also a formula to calculate output voltage Vout of the IC. The variable resistor R2 is varied and corresponding voltage is obtained at output. In the circuit simulation shown in the video you can see when the resistance of variable resistor is varied it causes output voltage to vary from 1.2V to 12V. 

I hope you would like the circuit and give it a try. Please comment and post your suggestions.
Thank you.