transistor as switch

Discussion in 'General Electronics Chat' started by sangpo, Sep 9, 2013.

  1. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Hi all. When search in the internet Transistors are used as amplifier and switch. I have even watched video clip demonstrating transistor using as swich to glow the LED.

    But what is the idea behind using transistor as switch instead of ordinary/normal switch.
    Pleases kindly explain about it.

    Thank you.

    Sangpo
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    You often need to activate the switch electronically, like for example to light a led when battery charger has finished, and that is kïnda hard to achieve with a mechanical switch.
     
  3. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Thanks kubeek. You have even cited example.When battery charge is finish. it is obvious that the LED will not glow any longer even if we use transiistor . And when battery charge is finished the LED will not glow even if have used mechanical switch. my confusion is not fully cleared. Please kindly explain with more example. Thank you.

    sangpo
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    Using bjt to light up the LED.
    Using bjt to drive the Buz.
    Using bjt to drive the speaker.
    Using bjt to drive the relay.
    Using bjt to drive the motor.
    ...
    Also Using MOSFET to light up the LED.
    ....
     
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  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It depends on what that LED is supposed to indicate. It could be that a red led is on when the charger is charging, it blinks when no batteries are inserted, and a green turns on when charging is finished. Or anything else the designer thinks is needed.
    The led can be on whenever you like to indicate something, even based on something really complicated such as ethernet cable passing some test.
     
  6. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Thank you kubeek very much indeed for explaining with example.Does Transistor use as switch for LED only? If it is used as a switch for other purposes also , kindly give the example. Thank you
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Transistors are used to switch basically anything, could be large motors, power inverters, solenoids, or many other applications.
     
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  8. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Dear Sir. You have given lot more example what transistor are used for. But still I am not convinced why normal or mechanical switch is not used to run the motor or speaker. Could you please kindly mention advantage of using BJT against normal switch ?
    Thanks

    Sangpo
     
  9. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Thanks a lot. I have in my mind that , in case of just to switch on to run motor, just mechanical switch would do. Why transistor is used instead?Could give example of using transistor switch for running motor? I am asking all these questions which might be simple or funny for you, becasue , I am just stared leaning basic electronic

    Thank you
     
  10. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    You are making a robot. You need to control it by a wireless remote. A mechanical switch will not work in this situation.

    You are making a smoke detector. To turn on the buzzer when smoke is detected, a mechanical switch is not suitable.

    You have designed a digital logic circuit. You want to turn on a motor, light a remote LED, and sound a siren. A mechanical switch is not suitable.

    Several different examples, for you.
     
    Last edited: Sep 9, 2013
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Another example, you want to switch a high-power load, for example a few kW heater or motor. A mechanical switch will be very large, cost a lot, and the contacts will eventually wear out. On the other hand a transistor rated for the same application will be smaller and cheaper and if the circuit is properly designed it will never wear out or break.
     
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  12. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Thank you very much. Now I got idea of using transistor as switch.
     
  13. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Thank you for explanation.
     
  14. sangpo

    Thread Starter Member

    Aug 17, 2013
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    Can same trsnsistor which is used as switch used as amplifier. For instance. can BC547 transistor be used both for switch and power amplifier?
     
  15. BobTPH

    Active Member

    Jun 5, 2013
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    Or say you want to switch something on and off 1,000,000 times per second. How would a mechanical switch do at that? A transistor switch can.

    Bob
     
  16. ScottWang

    Moderator

    Aug 23, 2012
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    When you want to light up a LED, if you using your hands to switching the switch, the speed should be less than 10 times/sec.

    When you want to using pwm to control the LED of brightness, the frequency of pwm will be more than 10Hz and much more, how can you do with your hands?

    The difference between using a bjt as a switch and your hand, those are the automatic and frequency, you can think about what are the automatic products in our daily lives.
     
  17. LDC3

    Active Member

    Apr 27, 2013
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    Microprocessors use thousands (and thousands) of transistors as switches to execute a program. They also use transistors as amplifiers (in the analog to digital and digital to analog converters). Memory chips use millions (and millions) of transistors as switches.
     
  18. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    1) Also, don't forget that transistors can source more current than microchips. Therefore, specialty IC's such as the 555 timer and other things that "do a special job" can run loads, using a transistor, that they could not drive themselves.

    Example: a timer that turns on a relay, which requires 800 mA to latch. A 555 can only source 200 mA, so you use a BJT to switch the relay on instead, protecting the chip while still maintaining the ability to accurately control the timing.

    2) If you have a very high load or high voltage circuit, you can use power transistors to switch even those. A mechanical switch may arc in this application, causing a fire hazard....the control voltage would be very low, and safe. This allows engineers to place all controls in a control room (like in a power plant), yet have the 'dangerous' machinery safely in another location (remote switching). This even works for NASA, with Mission Control - the device being switched by transistors may be on Mars!!
     
  19. wayneh

    Expert

    Sep 9, 2010
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    Yes, there is no difference. The manufacturer might make small optimizations to improve it for one application or the other, but either type of transistor can be used for other applications.
     
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