What is TRANSISTOR?

TRANSISTOR

A three lead semiconductor device that acts as:

• an electrically controlled switch, or
• a current amplifier.

• Transistor is analogous to a faucet.

• Turning faucet’s control knob alters the flow rate of water coming out from the faucet.
• A small voltage/current applied at transistor’s control lead controls a larger current flow through its other two leads.

 

Transistor Types: BJT, JFET, and MOSFET

• Bipolar Junction Transistor (BJT)

• NPN and PNP

• Junction Field Effect Transistor (JFET)

• N-channel and P-channel

• Metal Oxide Semiconductor FET (MOSFET)

Depletion type (n- and p-channel) and enhancement type (n- and p-channel)

                          BJT                JFET            MOSFET

BJT Types

• NPN and PNP.

• NPN: a small input current and a positive voltage applied @ its base (with VB>VE) allows a large current to flow from collector to emitter.
• PNP: a small output current and a negative voltage @ its base (with VB<VE) allows a much larger current to flow from emitter to collector.

 

NPN BJT: How it works —I

• When no voltage is applied at transistor’s base, electrons in the emitter are prevented from passing to the collector side because of the pn junction.

• If a negative voltage is applied to the base, things get even worse as the pn junction between the base and emitter becomes reversebiased resulting in the formation of a depletion region that prevents current flow.

 

NPN BJT: How it works —II

• If a positive voltage (>0.6V) is applied to the base of an npn transistor, the pn junction between the base and emitter becomes forward-biased. During forward bias, escaping electrons are drawn to the positive base.

• Some electrons exit through the base, but because the p-type base is so thin, the onslaught of electrons that leave the emitter get close enough to the collector side that they begin jumping into the collector. Increasing the base voltage increases the emitter-to collector electron flow.

• Recall, positive current flow is in the direction opposite to the electron flow current flows from collector to emitter.

BJT Water Analogy

 

 NPN (VB > VE)

 

PNP (VB < VE)

NPN Transistor in a Complete Circuit —I

•Normally OFF.

•No current passes from collector to emitter when base is not activated.

 

NPN: VB = VE→OFF

NPN Transistor in a Complete Circuit —II

• When VB > VE we have an operating circuit.

• Current passes from collector to emitter when base is activated.

 

NPN: VB > VE→ ON

Transistor Experiment —LED On/Off

• Turning the switch on/off turns the LED on/off.

 

JFET

• Junction field effect transistors like BJTs are three lead semiconductor devices.

• JFETs are used as:

• electrically controlled switches,
• current amplifiers, and
• voltage-controlled resistors.

• Unlike BJTs, JFETs do not require a bias current and are controlled by using only a voltage.

• JFETs are normally on when VG - VS = 0.

• When VG - VS ≠ 0, then JFETs become resistive to current flow through the drain-source pair → “JFETs are depletion devices.”

JFET Types

• Two types of JFETs:

• n-channel and p-channel.

• In n-channel JFET, a –ve voltage applied @ its gate (with VG < VS) reduces current flow from drain to source. It operates with VD > VS.

• In p-channel JFET, a +ve voltage applied @ its gate (with VG > VS) reduces current flow from source to drain. It operates with VS > VD.

• JFETs have very high input impedance and draw little or no input current – → if there is any circuit/component connected to the gate of a JFET, no current is drawn away from or sunk into this circuit.

 

MOSFET

• Metal oxide semiconductor FET.

• Similar to JFET.

• A metal oxide insulator is placed @ the gate to obtain a high input impedance @ the

gate

• gate input impedance approx. 1014Ω.

• Use of insulator as described above yields a low gate-to-channel capacitance.

• If too much static electricity builds up on the gate, then the MOSFET may be damaged.

MOSFET Types

• Enhancement type:

• Normally off, thus no current flows through drain-source channel when VG = VS.
• When a voltage applied @ the gate causes VG ≠ VS the drain-source channel reduces resistance to current flow.

• Depletion type:

• Normally on, thus maximum current flows through drain-source channel when VG = VS.
• When a voltage applied @ the gate causes VG ≠ VS the drain-source channel increases resistance to current flow.

ULTRA-FAST ACTING ELECTRONIC CIRCUIT BREAKER

ULTRA-FAST ACTING ELECTRONIC CIRCUIT BREAKER

ABSTRACT

The steadily increasing population has more demand and consumption of electric energy in the market as raised and that of equipment’s used like electrical and electronics are also costlier. So to protect the electrical system from overload or short circuit here is one possibility, which is by ultrafast acting electronic circuit breaker. A circuit breaker is automatic operated switch designed to shut down the power supply when overloaded. The tripping depends on the current passing through the CT’s which is connected in series with load. It uses the PIC- microcontroller into which program is dumped for the operation. The unit is extremely fast and over comes the drawback of thermal type circuit breaker like MCB based on a thermal bimetal lever-trip mechanism which is very slow. Here an electronic circuit breaker is designed which is based on the current sensing across a series element typically a CT (current Transformer). The current sensed which is compared against the preset value proportional to the voltage by comparator which is inbuilt in PIC Micro controller to generate an output that drives a relay through a MOSFET to trip the load very fastly.

Keywords: Microcontroller, CT, Relay, MOSFET.

INTRODUCTION

In this project electrical system can be protected from the over load condition. Industrial instruments or home appliances failures have many causes and one of the main causes is over load. The primary of the distribution transformer or any other transformer is designed to operate at certain specific current, if that current flowing through that instrument is more than the rated current, then immediately the System may burn because of over load, through this project we are going to protect the system from over load condition. In this project work for generating high current or over load current more loads are applied to the circuit; so that the current will be increased. Whenever the over current is drawn by load the circuit will be tripped. To trip the circuit we are using one relay which will be controlled through PIC microcontroller. When over load occurred the relay will trip the total circuit. And it will be monitored on the LCD. LCD displays are used to display the status of circuit breaker. For protection from over current condition first we have to measure the total load current. Here we are using CT for measuring the load current and the output of CT is given to ADC for converting analog output of CT into digital data. Hence ADC output is given for monitoring purpose. When current increases behind certain limit then we are going to trip the load by using relay. In this project we are using 230v bulbs as a load. We are going to increase the load by increasing the number of bulbs ON. When we ON more bulbs it causes over load condition and microcontroller will detect that and it will trip the total load by using relay through MOSFET which acts as switching circuit.

EXPLANATION

The main power supply is given directly to load through CT and step down transformer. 230 volt is Step down to 12v and supplied to regulated supply unit which consist of bridge rectifier to convert ac to dc and passed through 7805 regulator to get 5v supply for working of microcontroller, capacitor filters are used to remove the ripples to get pure constant dc voltage. The current passing to load is sensed by the current transformer and output of CT will be in analog form is given to the ADC pin of PIC microcontroller for converting the analog output to digital data. The current sensed is compared with the inbuilt comparator of microcontroller which as pre-set reference value. If the current sensed is less than the pre-set value than MOSFET will be in OFF state and relay will not trip the supply to load. As we increase the load current drawn is more so if the current is increase than the pre-set value than MOSFET will turn ON and energizes the relay. Thus LED used as an indicator is properly biased, and it glows. The relay coil gets energized, causing the armature to shift its position to the normal open point from the normal closed point.

The AC supply to the load is thus cut off from the load and the load is tripped. Once the circuit is tripped it must be reset for further use using reset button as shown in Fig.1. In either case, the microcontroller is programmed so as to show the status of the output on the LCD interfaced to it. In case of normal operation microcontroller will pin will receive 5v dc from regulator and accordingly displays the status on the LCD. In case of any abnormalities, the microcontroller pin doesn’t receive the 5V input signal and the related status is accordingly displayed on the LCD.

 

Fig.1. Block Diagram of Proposed System.

TABLE I: Trip Current Setting

 

Fig.2. PIC Microcontroller.

During this implementation an 8 bit peripheral interfaced controller microcontroller is used with following specifications as shown in Fig.2.

·       RAM 256 bytes

·       ROM 4 kilo bytes

·       8 bit Processor

·       4 I/O Ports

·       Timers 2

·       Power saving mode

·       RXD TXD interfacing

HARDWARE IMPLEMENTATION

The fig.3 shows the hardware implementation of ultra fast acting electronic circuit breaker using microcontroller.

 

Fig.3. Hardware implementation.

CONCLUSION

Now A days the protection and control of equipment plays a very important role. To avoid electrical failure we use fast responding circuit breakers because of its considerable accuracy in fault detection and cut off- time, and also its smooth operation compared to conventional type. Future Scope: The operating time of electromagnetic relay can be improved by using sophisticated electronic components.

REFFERENCE

[1]Raj kamal –Microcontrollers Architecture, rogramming, Interfacing and System Design.

[2]Mazidi and Mazidi –Embedded Systems.

[3]PIC-Microcontroller Manual – Microchip.

[4]A M S Almadji & J G J Sloot review of Current limitting IEEE.

[5]T. Genji , Nakamar: high speed circuit Breaker for electric power system.

[6]200WAS RI BIN ABU ,HARUN:Over Current protection using PIC Microcontroller IEEE.