### What is the difference between alternating current and direct current.(College Level)

A battery cell gives "d.c." or "direct current" which means that a steady voltage is available to drive your radio or whatever. Less well understood is "a.c." which stands for "alternating current". However, it's important to understand the difference because it could cost you money! You wouldn't dream of connecting your 12 volt radio directly to a mains power plug because you know that it gives at least 230 volts. But do you know that a 12 volt a.c. transformer can do almost as much damage? The reason is that electronic equipment needs not only LOW voltage but low D.C. voltage. Let's take a quick look at the method of making electricity.
sine wave voltage

In a power station, electricity can be made most easily and efficiently by using a motor to spin magnetic wire coils. The resultant voltage is always "alternating" by virtue of the motor's rotation. Fig. indicates how the voltage goes first positive then negative - rather like turning a battery cell continually backwards and forwards in its clip.

There are several ways but the simplest is to use a transformer to reduce the voltage to, say 12 volts a.c.  This lower voltage can be fed through a "rectifier" which combines the negative and positive alternating cycles so that only positive cycles emerge. This "rectified" voltage is suitable for running things like filament bulbs and electric trains but it is still no good for electronic circuits. So a "12 volt dc transformer" is no good for electronic devices. What you need is "regulated d.c." which truly simulates the steady voltage that you get from a battery.

The first step is to connect a large value capacitor to the output of the rectifier. A capacitor acts as a voltage reservoir and has the effect of smoothing the "ripples". This is still not the same as a battery produces but it's often good enough for charging batteries in mobile phones, personal stereo equipment and similar full wave rectified voltage with reservoir capacitor

The final step is to pass this "rippling d.c." through a regulator unit. This effectively chops off the ripple to leave almost pure "regulated d.c." at a steady voltage.

So, to provide a suitable voltage for electronic circuits you need a power supply which gives a "regulated d.c." output. If your power supply doesn't use those words then it may not be suitable for use with electronic circuits. In this case, saving cost might lead to expensive smoke!

### COMMON COLLECTOR CONFIGURATION OF A TRANSISTOR

COMMON COLLECTOR CONNECTION

In  this  configuration  the  input  is  applied  between the  base  and  the  collector and  the  output  is  taken  from  the  collector  and  the  emitter.  Here  the  collector  is common to both the input and the output circuits as shown in Fig.

Common Collector Transistor Circuit

In  common  collector  configuration  the  input  current  is  the  base current  IB  and  the output current is the emitter current IE. The ratio of change in emitter current to the  change in the base current is called current amplification factor.

It is represented by

COMMON COLLECTOR CIRCUIT

A test  circuit  for determining the  static characteristic  of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits.   To   measure   the   base   and   the   emitter   currents,   milli   ammeters   are connected in series with the base and the emitter circuits. Voltmeters are connected   across the input an…