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.