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Interfacing two powers

Discussion in 'General Electronics Discussion' started by steve_venkat, May 22, 2017.

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  1. steve_venkat

    steve_venkat

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    May 22, 2017
    In this circuit this is an LED connected with controller and push button also....
    The LED will blink in different time interval...Ex.10sec to 200 secs.
    And the LED should on when the push button is pressed means also...
    My doubt is while running the LED from controller during this time if the switch is pressed means what will happen in TIP125..?
    When push button is pressed more than 2seconds means the output from controller will turned off(I didn't mention in schematic)....
    I this while is their any damage will occur in TIP125 or controller..?
     

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  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    What is the emitter-collector voltage of the transistor when the push-button is pressed? Compare this value with Vce (max) of the datasheet and you will immediately see what happens.
     
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    Why are you using a TIP125 power darlington rated at 5000mA when the 5V LED it is driving is using only 7mA? An ordinary little PNP transistor (not a power transistor and not a darlington) like a 2N3906 will work fine.

    Why is the current in R2 five times more than the current in the LED?
     
  4. steve_venkat

    steve_venkat

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    May 22, 2017
    I'm going to use more number of LEDs.It will consume nearly 4A..So only I used TIP125.
     
  5. steve_venkat

    steve_venkat

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    May 22, 2017
    How to check emitter-collector voltage..?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Put a voltmeter between emitter and collector?
     
    hevans1944 likes this.
  7. Audioguru

    Audioguru

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    Sep 24, 2016
    If your circuit has a 4A load then your schematic posted here should not show a 7mA load.
     
  8. steve_venkat

    steve_venkat

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    May 22, 2017
    When I put multimeter across emitter and collector it showing 1V.When I pressed buttonit going to 0V...
     
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    The datasheet for the TIP125 shows a typical collector-emitter saturation voltage drop of 1V when its collector current is about 1.3A and its base current is about 5.2mA. Your 330 ohm base resistor causes a much higher base current then maybe your collector current is 4A. It is normal for the collector-emitter voltage to be zero when the switch shorts it then the TIP125 darlington has no collector-emitter current which will not harm it.
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This was to be expected.
    What does the datasheet tell you about the max. emitter-collector voltage? Compare this max. value to your measured value. The result should answer your question in post #1.
     
  11. steve_venkat

    steve_venkat

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    May 22, 2017

    I observed this from datasheet...
    Is their any time specification for pressing the switch...?
     

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  12. Audioguru

    Audioguru

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    Sep 24, 2016
    Why don't you understand that when you press the switch then NO CURRENT goes through the darlington transistor, it goes through the switch instead. You can press the switch for hundreds of years and the darlington transistor just sits there with no current and no voltage except for the small amount of base current and voltage.
    The datasheet shows that the MAXIMUM Vce saturation voltage is about 3V when the collector current is 4A and the base current is about 16mA. Your base current is much higher at about 30mA so the Vce will be less and maybe your darlington is better than the worst ones, so your saturation voltage loss is lower at only 1V. Your darlington will heat with 1V x 4A= 4W so it should have a heatsink if it is turned on longer than a few seconds.
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    There is not as your circuit is not part of the datasheet.
    Vcesat is not the relevant parameter. Vce=0 V is way below Vcemax and tehrefore a safe operating condition.
    Or as @Audioguru wrote no current will flow throughthe transistor in this case.
     
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