Showing posts with label transistor. Show all posts
Showing posts with label transistor. Show all posts
Saturday, March 30, 2013
The Transistor Tester Circuit
This is a very simple design circuit that can be used to check the hfe of transistors. Both PNP and NPN transistors can be checked using this circuit. Hfe as high as 1000 can be measured by using this circuit. The circuit is based on two constant current sources build around transistors Q1 and Q2. This is the figure of the circuit.
Operation of the circuit is the Q1 is a PNP transistor and the constant current flows in the emitter lead. The value of constant current can be given by the equation; (V D1 -0.6)/ (R2+R4).The POT R4 can be adjusted to get a constant current of 10uA.
The Q2 is an NPN transistor and the constant current flows into the collector lead. The value of this constant current can be given by the equation; (VD2-0.6)/(R3+R5).The POT R5 can be adjusted to get a constant current of 10uA.This constant current provided by the Q1 circuit if the transistor under test is an NPN transistor and by Q2 circuit if the transistor under test is a PNP transistor is fed to the base of transistor under test. This current multiplied by the hfe flows in the collector of the transistor and it will be indicated by the meter. The meter can be directly calibrated to read the hfe of the transistor. The Zener diodes must be rated at least 400mW. J1 and J2 are transistor sockets.
Operation of the circuit is the Q1 is a PNP transistor and the constant current flows in the emitter lead. The value of constant current can be given by the equation; (V D1 -0.6)/ (R2+R4).The POT R4 can be adjusted to get a constant current of 10uA.
The Q2 is an NPN transistor and the constant current flows into the collector lead. The value of this constant current can be given by the equation; (VD2-0.6)/(R3+R5).The POT R5 can be adjusted to get a constant current of 10uA.This constant current provided by the Q1 circuit if the transistor under test is an NPN transistor and by Q2 circuit if the transistor under test is a PNP transistor is fed to the base of transistor under test. This current multiplied by the hfe flows in the collector of the transistor and it will be indicated by the meter. The meter can be directly calibrated to read the hfe of the transistor. The Zener diodes must be rated at least 400mW. J1 and J2 are transistor sockets.
Thursday, March 28, 2013
Fire Alarm Circuit Based on Transistor
This is a design for circuit o f the fire alarm. This circuit is work based on 3 transistors. When there is a fire breakout in the room the temperature increases. This ultra compact and low cost fire alarm senses fire breakout based on this fact. This is the figure of the circuit.
The operation of the circuit will based on this explanation. The transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3 (AC 128).So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie, light, bell, horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched. All capacitors are electrolytic and must be rated at least 10V.
The load can be connected through the C, NC, NO points of the relay according to your need. The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply. Keep the tip of soldering iron near to the Q1.Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON. This is not a latching alarm, when the temperature in the vicinity of the sensor decreases below the set point the alarm stops. The circuit can be powered using a 9V battery or a 9V battery eliminator.
The operation of the circuit will based on this explanation. The transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3 (AC 128).So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie, light, bell, horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched. All capacitors are electrolytic and must be rated at least 10V.
The load can be connected through the C, NC, NO points of the relay according to your need. The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply. Keep the tip of soldering iron near to the Q1.Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON. This is not a latching alarm, when the temperature in the vicinity of the sensor decreases below the set point the alarm stops. The circuit can be powered using a 9V battery or a 9V battery eliminator.
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