POWER SUPPLY DESIGN

                                                              POWER SUPPLY


As sounds, power supply gives power to all electronic circuits and if we consider it in a broader way, power supply is essential for all the activities happening around us, even we the human being need power to perform our day to day life.  So, good, clean and regular power is must for all activities including electronic equipments.

In this post, I will emphasize on how to design a good simple power supply.  Here under in fig-1 block diagram is given mentioning the essential elements of a power supply. 

A.C. mains 220/230 V (in some countries 110 V) is first stepped down to a required level, rectified, filtered and then regulated (if required).  Most of the circuits require a combination of unregulated and regulated supply out puts.  Below in Fig-2 a simple zener regulated power supply is given.  This power supply gives 12 volts at  50 M. amp.  It is suitable for reference voltages or an op-amp.

If higher current is a requirement, power supply shown in Fig-3 below is more suitable.  In this supply transistor 2N3055 is used as a series pass element.  This supply can deliver upto 500 M. Amp. which is suitable for most of the circuits.  Proper size heat sink is necessary for the transistor to safeguard from undesirable heating. 80 MM X 40 MM X 3 MM thick aluminium heat sink is sufficient.  This supply does not have over load or short circuit protection, therefore a 500 M. Amp. fuse may be added between power supply and the load.  To obtain adject 12 volts diode D5 has to be inserted between anode of zener ZD1 and ground as shown in the figure as the base emitter junction of transistor Q1 drop 0.6 volts.   

While designing a power supply, the first component is transformer. 
Transformer must be capable to handle the current which it has to deliver continuously.  An ideal consideration is the transformer should have double the capacity of required load, if it has to supply continuous current and 1.5 times the capacity of required current if it has intermittent load, just as an audio amplifier. I shall discuss how to design a transformer separately.  Here I give two ideas to test a transformer.
  
         a)  Put a dummy load, preferably rheostat across the secondry winding of transformer under test.  Connect primary winding to A.C. mains and adjust the current to 25 % of its capacity.  Now set the mains voltage to 275 volts through variac ( for 220 volts transformer) and leave it for at least one hour.  The transformer should not heat up.  It should be just warm 30 -32 degree celsius.

      b)  Do not connect any load to secondry winding,  connect primary winding to A.C. source and set the input voltage to 300 Volts.  Leave it for one hour. The temperature of transformer should remain at room temperature.

Above in Fig-4, darlington pair of transistors Q1 and Q2 used to enhance the current capacity of power supply.  A larger heat sink is required to cool down the series pass transistor Q1.  An additional transistor Q3 with heat sink may be added to further enhance the current capacity of power supply, when single transistor is not able to pass sufficient current required by the load, darlington pair is a good solution to resolve the issue.  Diodes D5 & D6 in series with zener diode ZD1 be added to obtain adject 12 volts, since Q1, Q2 together drop 1.2 volts through base emitter junction.

Stepped down A C is to be rectified by a rectifier.  Two points are to be kept in mind while selecting a rectifier diode, current capacity & the break down voltage.  Diode 1N4007 has a current capacity of 1 amp. and break down voltage 1000 volts, hence 1N4007 is suitable for most of the power supplies.  For up to 3 amp. supplies 1N5408 (current 3 amp., break down 1000 V) and up to 6 amp. diode 6A4 (current 6 amp., break down 400 V) is suitable.

Filter selection is also very important for a good power supply.  Use only best available quality capacitors ( cost doesn't matter).  A quite simple consideration for selecting a capacitor is 1000 Mfd. for each one ampere of load i.e. up to 1 amp. 1000 Mfd., 2 amp. 2200 Mfd, 3 amp. 3300 Mfd and so on.  For variable load such as audio amplifiers an additional 1000 Mfd. may be added to the capacitor value to maintain voltage at higher volume thus audio quality too.

Voltage regulation is most critical part of any power supply.  there are many ways to keep the voltage at constant level under line and load variations.

Besides this, over load and short circuit protection also has equal importance.   
 

BASIC POWER SUPPLY

                                         POWER SUPPLY


Each and every electronic circuit requires a power supply. Power supply is a heart of any electronic circuit whether  simple or complex, analogue or digital.

It is well established fact that the performance and life expectancy of an electronic circuit largely depends on the performance of its power supply. 

Alternating current it self cannot operate an electronic circuit.  It has to be converted into direct current to give life to an electronic circuit.

Three basic power supplies are given below.

1.  Half Wave Rectifier Circuit.

This is the most common and simple power supply.  Diode D1 allows positive half cycle to charge the capacitor C1. During negative half cycle diode D1 does not allow capacitor to charge and during this period capacitor C1 discharges slightly through load, however, it is recharged during the next positive half cycle.  Due to this out put D.C. voltage does not remain constant which results  in rippled D.C. voltage.  These ripples increase as the load current rises.  Therefore, such power supplies require larger filter capacitor.  Due to this factor half wave rectifier power supply is not suitable for higher and critical loads. It supports only small load (100 to 200 m.A.). e.g. relay driver circuits. 


2.  Full Wave Rectifier Circuit.

 

Full wave rectifier power supply is same as half wave circuit.  The difference is centre tapping transformer and two diodes are used instead of one.  One of the diodes allow current during positive half cycle and the other diode conducts during the negative half cycle.  This enable filter capacitor to charge during the both half cycles.  The resultant ripples in D.C. line are one half of the half wave rectifier supply.  This improves the quality of out put D.C.


2.  Bridge Rectifier Circuit.

Bridge rectifier power supply produce same results as the full wave rectifier does.  This type of supply uses four rectifier diodes connected in a bridge.  The main advantage of bridge rectifier is that it does not require centre tapping transformer thereby reducing size and cost.  During the positive half cycle diode D2, D3 conduct in series and diodes D1, D4 remain reverse biased.  While during negative half cycle diode D1, D4 conduct and D2, D3 are reverse biased.

P.S. -  Compare the out put wave form with grey shaded area in Fig1 and Fig-3, 4.  

BASIC ELECTRONICS - 2

                                                                         DIODE


As clear by name diode has two electrodes or terminals.  One is called anode and other is cathode.  It allows electricity to flow in one direction only.  It does not allow the current to flow in reverse direction.  Diode has very important role in electronic circuits. 

ANODECATHODE

 

The working of diode can be seen in the diagram given below.

Diode conducts only if forward biased i.e. positive voltage applied to anode and negative to cathode.  In Fig-1 diode is forward biased and allows current to flow therefore lamp glows.  In second part of Fig-1 the negative voltage is applied to cathode of diode, in this configuration diode is also forward biased therefore, lamp glows.

In Fig-2, positive voltage is applied to cathode of diode, which causes reverse biasing of diode and lamp does not glow.  similarly in second part of Fig-2, diode is reverse biased because negative voltage is applied to its anode therefore, lamp does not glow.

When diode conducts it utilises little power to conduct (push current through it).
Therefore it has a small voltage drop across it and this voltage drop is called forward voltage drop and it is 0.65 volts for normal silicon diode.
The most common use of diode is in power supply as a rectifier i.e. it converts alternating current to direct current.  All electronic circuits work on D.C. therefore, diode plays a vital role to provide power to the electronic circuits.  The figure given below shows how diode convert alternating current to direct current.

 In the above fig-3 alternating current is applied at the A.C. input.  The input A.C. waveform is shown at the left hand side of the circuit and the converted waveform is shown at the right hand side of the circuit which is available at the D.C. output.  It is clear from the diagram that the diode allows current only for the positive half cycle of the A.C. waveform and does not allow current to flow during negative half cycle.  Therefore, negative half cycle is eliminated at the output. it is rippled D.C. and can be smoothed by adding suitable electrolytic capacitor at the output terminal.  

The circuit given below shows how positive half cycle can be eliminated from the input waveform to obtain negative voltage by just reversing the direction of the diode. 

Selenium diode (rectifier)           Germanium diode            Silicon diode


Selenium diodes or metal diodes were in use mainly to convert (rectify) A.C. current in to D.C. current and they had the disadvantage of ageing process i.e. increasing resistance with use and they were suitable for low frequency only.

Germanium diodes have low forward voltage drop (0.3V) compared to silicon diode forward voltage drop (0.7V).  Older germanium diodes had larger leakage current and low reverse breakdown voltage (50V).  Germanium diodes are more heat sensitive and can be damaged while soldering.  So care should be taken when soldering.  Germanium diodes are hard to find in component shop.

The lower voltage drop for germanium diodes becomes important in signal detection for AM/FM radios.  As the signal strength in radio intermediate frequency (IF) is very low. 

Generally, germanium diodes are low current diodes, if one needs higher current diode germanium transistor can be used as a diode by shorting collector base lead together.

Silicon diodes are currently in use.  The advantage of silicon diodes are very low or negligible leakage current and high reverse break down voltage.


                                                  
                                               ZENER DIODE

                                  ANODE                            CATHODE


Zener diode is like a general purpose diode which behave like any other diode if biased in the forward direction, but, if reverse biased and voltage applied across it, corsses the zener voltage (predetermined value of zener) it breaks down and start conducting to maintain the voltage (zener voltage) across it.  For the period as power supply remains above the zener voltage, the voltage dropped  across the zener diode will remain at constant level.  If the applied voltage increases further zener diode heats up and damaged if, connected directly to the power supply.  Therefore, zener diode must be reversed biased through a current limiting series resistor to sustain it over a wide range of voltage.

This quality of zener diode is utilised to regulated the D.C. voltage against supply and load variations.

Simple voltage regulator circuit.  

Zener diodes are used to stabilise or regulate D.C. voltage as shown in above circuit. Unregulated D.C. voltage between points A & B is applied to zener through a current limiting resistor R1.  The regulated D.C. voltage is available across the zener and points C & D.  Capacitor C1 plays a role of noise filter as the zener diodes, sometimes generate electrical noise which goes into D.C. supply.

The circuit shown in Fig-5 can be used to power small circuit which require low current, say 15 to 20 milliamperes.

                                                          Fig-5A

Every silicon diode has the quality to maintain voltage across it over a vide range of supply and load regulation.  This characteristic of diode can also be utilised to obtain larger voltage regulation. 

In the above Fig-5A, 10 silicon diodes are connected in series and forward biased through a series resistor.  Since, silicon diode has forward voltage drop (or forward breakdown voltage) of 0.65 V.  The net breakdown voltage of 10 diodes will be 6.5 V,  hence the output voltage will be regulated to 6.5 V.  

An Improved Voltage Regulator Circuit.


The above circuit is an improved version of a voltage regulator.  In this circuit transistor Q1 is a series pass element.  The collector of transistor is connected directly to power supply and base is connected to the cathode of zener diode, a regulated voltage source.  The regulated output is available at the emitter of transistor Q1, rest of the circuit is same as in Fig-5. 

The difference between two circuits is, the load draws current through a series pass resistor R1 in Fig-5, while in Fig-6 the load draws current from the collector emitter path of transistor. The circuit in Fig-5 is suitable for 15 to 20 milliampere current, while the circuit in Fig-6 can deliver much greater current depending upon the capacity of transistor Q1.

Zener diodes are sensitive to temperature like any other semiconductor device.  Excessive temperature will destroy zener diode, and because it conducts and drop voltage, it produce heat (W=V x I).  Therefore, care must be taken while designing the regulator circuits, so that the power dissipation in a zener diode does not exceed the safer limits.  When zener diode fails due to excessive heat , they are shorted instead of being open.  It is easier to detect such a faulty zener diode.  



                            LIGHT EMITTING DIODE (LED)

                                    ANODE                       CATHODE

Light Emitting Diode (LED) is a semiconductor which is used mainly for indicator and display purpose.  since, it is a diode therefore, it allows current in one direction only.  In the beginning LEDs were made in low intensity and colours such as red, green and yellow, but modern versions are available in many colours and high brightness.  Initially, LEDs were used in expensive  equipments such as test & measuring instruments and later in calculators, TVs &  audio equipments due to high cost.  As the technology grew and cost came down LEDs were used in domestic appliances. 

Most of LEDs come in 5MM and 3mm round package.  Now the LEDs are available in various shape and sizes.  The first high bright blue LED was made in Japan in 1994.  Typical current drawn by 3/5 MM LED is 15 to 20 mA and power dissipation of such LED is 40 to 50 Milliwatts.  However, bright white power LED dissipates up to 1000 milliwatts of power. 

The life time of a LED depends on operating temperature and current drawn.  If operated within safer current and temperature limits LED can work for 10 or more years.  High operating current and temperature can damage LED easily.

Observe the circuits given below.

Above two circuits are given, first, try (A) circuit and observe what happens.  As the LED is connected to 12 V battery it will glow for a moment and damaged immediately.  This is because current flow in LED rises abruptly to high level and heats up.  In the second circuit (B) a current limiting resistor is inserted into the circuit.  Try 2K2 resistor at first and observe the light intensity, it will glow dim.  Now check the current and try lower value resistors step by step and observe that the current drawn by LED comes within 15 to 20 milliampere.  You will see that at this current level the LED is glowing at its normal intensity.  The resistor R1 value should be 560 / 680 ohms. 

Now, take a look at the following circuits.   

In Fig-8 (A) two LEDs are connected in the circuit.  Keep the value of R1 to 680 ohms and observe the light intensity of LEDs and measure the current drawn, it may be around 10 to 12 milliamperes.  To achieve full intensity of light emitted by LED you have to reduce the value of R1 to near 400 ohm, practical value is 390 ohms. Similarly, in Fig-8 (B) you have to further reduce the value of resistor R1 (330 ohms) to achieve the current up to 20 milliampere.  

Testing of Diode :

Diode can be tested by Digital Multi Meter (DMM). Set the DMM to diode test range, connect the red (+ve) probe to anode and black (-ve) probe to cathode of diode under test as shown in Fig-9 here under.  If diode is healthy the meter will show the breakdown voltage of diode, otherwise 0.00 if diode is bad.

Diode as a Temperature Sensor :

Connect a diode as shown in Fig-9 and see the reading, now place soldering iron near to diode carefully and observe the reading.  As the diode temperature rises the reading (breakdown voltage) falls down. This deviation can be utilised in temperature control circuit, however, one must have a good temperature meter to calibrate the circuit.  IN4148 or a zener diode will be more suitable for this purpose.

SHORT DURATION PULSE GENERATOR

SHORT DURATION PULSE GENERATOR

IC 555 has been used in many many applications. It is up to you how you manipulate it.  I have designed this circuit for use in Inveverters.  When inverter trip due to over load.  The user comes under complete darkness and he /she needs a light source to locate inverter to reset it.  This is a very awkward situation,  to avoid this I have designed this circuit.  If inverter tripped due to overload it will reset the power after every 4-5 seconds indicating inverter is overloaded, thus user can reduce the load and inverter works fine.

It is simple multi vibrator circuit producing positive going short duration pulses with a gap of 4-5 seconds.  LED L6 gives visual indication of pulses.  Positive going pulses are available at pin3 of IC3 and negative going pulses are available at collector of transistor Q9, one can use these pulses as per circuit requirement.

DUAL MODE LED FLASHER

DUAL MODE LED FLASHER  

This LED flasher is built around timer 555.  The circuit is very simple and self depictive.  It is a free running oscillator, oscillating at 1 Hz rate approximately.  Transistor Q7 is used to control LED mode.  When high logic is given to the base of Q7, pin 2 of IC1 is pulled down below 2/3^rd of VCC resulting in high output at pin 3 and LED D3 glows constantly.  The low logic at base of Q7 keeps LED flashing.  It can be connected to many circuits.  For example Automatic Battery Charger showing two states of  charger i.e. battery charging and battery charged.  It can be connected to 12 V battery charger as it is.  However, for 24 V charger any of combination shown in fig 2 or 3 may be added. Since IC 555 works fine if connected to +5 to +15 volts (Min-Max).

POWER ON DELAY

ON TIME DELAY CONTROL CIRCUIT

This simple time delay circuit is very useful for refrigerator, deep fridge, air conditioner or any power sensitive equipment.  It gives 3-4 minutes time delay when A.C. mains power supply resume.  If power fail during the running of compressor, it is necessary to give time gap to resume power to the compressor specially to the reciprocating compressors, since they require more power to compress the refrigerant in comparison to rotary compressors.

At power resumption both transistors remain in cut off position and base biasing of 3.6 V is available at the base of transistor Q5 through potential divider R10, R11.  Since capacitor C4 starts charging through resistor R12.  Transistor Q5 will not conduct until the voltage at its emitter remains below its base voltage.  When the voltage at emitter of transistor Q5 goes above the voltage available at its base (3.6V) it starts conducting and transistor Q6 gets base biasing since its base is connected directly to the collector of Q5.  The emitter of Transistor Q6 is connected to the gate of SCR through resistor R5 which causes SCR to fire and the relay is energised.  Capacitor C3 connected at the gate of SCR prevents it to misfire.  As SCR comes in conducting mode capacitor C4 is immediately discharged through diode D4 so that the circuit is ready for the next time delay cycle.

Power supply can be obtained from the existing device in which this circuit is to be placed or through a 12 Volt 300 milliampere transformer, a bridge rectifier and a smoothing capacitor of 1000 Mfd/25 Volt.

RELAY PROTECTION

RELAY PROTECTION CIRCUITS

Sometime it is observed, that in some equipments relay(s) energised or remain in on state continuously, for example Time Delay circuit and Reverse Battery protection circuit.  The relay coil heats up and burn sometime.  These two circuits prevent relay from damage, specially if the voltage across the relay coil, crosses the specified limit for longer period.

Both circuits are very simple to understand.  First circuit (fig.1) is suitable for switching relay according to logic given at the base of transistor Q1.  As soon as high logic given to base of transistor Q1 the relay draws maximum current through capacitor C1.  As the capacitor is charged resistance R1 maintains the hold current for relay.  Relay required 25% less current to hold latch state .

Fig2 is a reverse battery protection circuit.  When connected to power the transistor Q1 gets base biasing voltage through capacitor C1 & resistor R2 and the relay is energised through collector-emitter path.  As the capacitor C1 is charged the transistor Q1 is cut off and relay gets hold current through resistor R1.

FLASHER CONTROL

EXTERNALLY CONTROLLED FLASHER

This extremely simple circuit is built around two npn transistors BC547.  R C network R3 and C2 decides the flash rate of LED.  When high logic is given to the open end of resistor R1 (10K) transistor Q1 immediately starts conducting and LED glows.  Capacitor C2 gets charging through resistor R3.  As soon as the voltage at the base of transistor Q2 via resistor R2 reaches above the voltage at the emitter of Q2+0.6 V transistor Q2 starts conducting.  The voltage at base of transistor Q1 immediately pulled down to ground level resulting in switch off the LED and capacitor C2 gets discharged through R3, R4 and R5.  Thus the cycle repeats till the logic at the open end of R1 is removed.  R4 is current limiting resistor for LED.

TOUCH SWITCH

ELECTRONIC ON-OFF  TOUCH SWITCH

 

You can give sophisticated and good look to many electronic equipments such as Inverters, Battery chargers, UPS and D.C. power supplies to name a few. 

The working of this circuit is very simple as shown in schematic diagram.  When power supply is connected initially the relay L3 remains in de-energise (off) mode and voltage across the relay coil will be same i.e. +12V.  Therefore, the base of transistor Q3 will be high through resistor R7 to keep it in cut off state.  When push to on switch S2 is pressed momentarily Q4 start conducting and relay is latched and the base of Q3 will be pulled down and LED L2 will glow indicating power is on. The voltage available at the collector of Q3 will keep the transistor Q4 in on state through resistor R8.  Now, when push to on switch S1 is pressed momentarily the base of Q3 is pulled up and high base biasing will cut off transistor Q3 and the LED L2 will go off.  This will cause pull down base of Q4 through resistor R8 which will result in de-energising the relay L3.

The power to the equipment (connected through circuit) will be available through N/O & common points of the relay.  If you want to use it with an Inverter the relay contacts will be placed in series with the D.C. line to Inverter card. To use it with UPS, you have to use double contact relay, one contact will be used to cut A.C. 230 V line to the load and second contact will be in series with D.C. line to inverter card.  If D.C. 12 V is not available in the equipment in which you intend to use it.  You can built a 12V supply  by adding a 12 V 300 milliamp transformer, a bridge rectifier and a 470 Mfd. electrolytic capacitor. 

 

 

 

CAPACITOR

Capacitor

Capacitor (also known as condensor) as clear by its name it has capacity to store electrical energy.  It is made of two metal foils separated by a insulater or dielectric, when connected between +ve and -ve end of a power supply it stores +ve charge to one foil (or plate) and -ve charge to another foil.  Its value measured in farads and called capacitance.  Narrower the gap between foils or plates higher will be the capacitance of the capacitor.  Every capacitor has a certain working voltage beyond this voltage it breaks down.  This called electric field strength of a capacitor.  Capacitors are used in electronic circuits generally for blocking D.C. current and allowing A.C. current to pass.  Capacitors are made in many types such as polyster capacitor, ceramic capacitor, tantalum capacitor, electolytic capacitor etc.  Care should be taken while connecting electrolytic capaitors to the circuit as these have polarity and must be connected according to it.  These are all fixed value capacitors, another type is variable capacitor which is used mostly in Radio frequency circuits (RF circuits) one example of variable capacitor is in radio receivers which is used to tune in to receive radio station signals.

Capacitors can be connected either in series or in parallel like resistors.  The calculation of resultant value of capacitors connected in series or parallel will be different from the calculation of resistors as shown in figure below. 

Above in fig-1 capacitors C1, C2 and C3 are connected in series. the resultant value between points A and B will be derived from the equation mentioned therein.

In fig-2 capacitors C1, C2 and C3 are connected in parallel, the resultant value between points A and B will be derived from the equation given above therein.     

BASIC ELECTRONICS - 1

 

 

BASIC ELECTRRONICS



 

 Electric current

Electric current has two types D.C. or direct current and A.C. or alternating current. D.C. or direct current flows in one direction only, and does not change its path. A.C. or alternating current flows in both direction and changes its direction after a certain period of time. The number of time it changes its direction in one second is called frequency. In India the power line come to our homes has a frequency of 50 Hz. In some of countries it has frequency of 60 Hz.

Voltage

 

Voltage is a force or source of energy to push –ve charge substances from +ve terminal to –ve terminal of battery or power supply. For example, chemical reaction inside a battery provide energy required for current flow from +ve terminal to –ve terminal. It can be understood by an example, a fan while rotating produce a force to flow air. Higher the rotation of fan the more pressure will be produced to air flow. In an electric circuit this pressure is called voltage. The higher the voltage (pressure) the more current will flow into the circuit.

Resistance
 
Resistance is a basic component in electronics. It reduces flow of electric current (A.C. or D.C.) in a circuit, if connected in series resulting in drop of voltage across it. How much current or voltage will drop depends on the value of resistor. Each resistor has its certain value printed on it in colour code. However, the value printed on wire wound resistor is in numeric term. The resistor comprises three types carbon film, metal film and wire wound. The vlaue of resistors is measured in ohm and displayed as Ω. The unit ohm named after German scientist Georg Simon Ohm. In A.C. circuits, electrical impedence is also measured in ohms (example - loud speakers) By nature or by character the value of a resistor remains constant under a certain range of voltages, temperatures and other paremeters too. This is the one type of resistor which is used frequently in electronic ciruits. The other type of resistor is thermal resistor or thermistor. The important point is its value changes significantly with temperature. Thermistors have two types, NTC or Negative Termperature Coefficient and PTC or Positive Temperature Coefficient. The value of NTC decreases with the rise of temperature whereas the value of PTC increases with the rise of temperature.  NTCs are mainly used in inrush current limiter circuits, temperature sensor circuits etc.  PTC are used in Colour Television Degaussing circuit.

 
How to read the colour code of resistor.

 

The above figure shows the value of each colour and how to read the value of resistor. Hold the resistor in your hand so as the group of three colour is on your left hand side and the tolerance colour code is on your right hand side. The first two colours have most significant value i.e. the actual value of each colour and the third colour represents number of zeros to be placed after the value of first two colours i.e. most significant value. The fourth colour ring is for tolerance, gold represent 5% and silver represent 10% tolerance.

 

 

For example, in the above figure, at right hand side, first two colours are brown and black, the most significant value is 10 and third colour is orange which has the value of 3, therefore 3 zeros will be placed after 10. It means the value of resistor will be 10,000 ohms or 10 Kilo Ohms or 10K (in practical term). At left hand side the value of resistor is 100 ohm since the third colour is brown and brown has its value of one, so one zero will be placed after 10 (value of first two colours).



Another example is given in above figure, at left hand side, the value of resistor is 10 ohm, since the third colour is black and it has its value of zero, so no zero will be placed after 10 (value of first two colours). At right hand side the value of resistor is 5.6 ohm, in this case the third colour is golden and if the third colour is golden the value of first two colours (the most significant value) will be divided by 10 to work out the actual value of resistor.

How to use resistor in a circuit

A resistor can be connected either in series or parallel in a circuit. If a resistor connected in series with load (lamp) will reduce the current flowing in the circuit (Fig-2) and if connected in parallel with load (lamp) will increase the current flowing in the circuit (Fig-3).  See figure below.

How to calculate resultant value of resistors

Below in Fig-1 resistors R1, R2 and R3 are connected in series, the resultant value between points A and B will be derived from the equation mentioned below.

In Fig-2 resistors R1, R2 and R3 are connected in parallel, the resultant value between points A and B will be derived from the equation mentioned below.

CURRENT CONCEEPT

What is electricity

In simple terms, electricity is a flow of energy.  Electricity flows through electrical wires and components like lamp, heater, motor etc.  It flows like water flows through steel or plastic pipes.

An electric current is a flow of positive charge substances from +ve to -ve (conventional theory).  While later theory says electricity is a flow of –ve charge electron flowing from –ve to +ve.  The electron theory was discovered much later than electricity discovered.  At that time it was not possible to find out the direction of electric current flow.  Therefore, at that time it was accepted that electricity was a flow of positive charge substances from +ve to –ve terminal of a battery or power supply.  When electron theory came into effect scientists discovered that flow of electricity is a flow of electrons (-ve charge particles) from –ve terminal to +ve terminal of a battery or power supply.  By the time the flow of electric current theory (from +ve to –ve) was established and accepted and is still in the practice.  So, don’t be confused to understand the electric current flow, since positive charge particles flowing form +ve to –ve are equivalent to –ve charge electrons flowing from –ve to +ve terminal.

 

But what !      

 

Metals are not only conductor of electricity.  For example, what happens if you place your fingers across live electric outlet socket or a naked live wire, one would suffer a dangerous or lethal electric shock.  During this painful experience there was certain electric current flow in the body.  Are there electron flow in human body ?  Some other examples are fluorescent tubes, lightening in the sky in stormy weather, electrolytic capacitors and batteries too.