i cant make transistor work as relay

Discussion in 'The Projects Forum' started by psoke0, Nov 17, 2017.

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  1. psoke0

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    Mar 31, 2017
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    hi all. im trying to trigger transistor only when voltage of the cap reaches 130v. but its just doesnt work like relay. even if i short base to ground its still showing voltage at transistor's collector. i dont want this. i want it work like relay. when voltage is not 130v yet it is should not show any voltage like real relay. reason is the real project im making using differant voltage source not like this simulation. its rising the voltage slowly charges the cap and when cap reaches 130 v its should trigger the transistor but before 130 v its shouldnt show anything so that cap can be charged otherwise the cap voltage is not reaching 130 v its allways in use . im charging the cap with joule thief circuit. i hope i could make it clear whats the problem is can u guys help me please?



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  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Use a zener diode in series with the Base to increase the voltage strike, problem is when the transistor turns on it will pull the capacitor voltage down with R4, and will therefore turn itself off and on continuously.
     
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  3. psoke0

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    Mar 31, 2017
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    you mean reverse bias or as a normal diode
     
  4. AlbertHall

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    Jun 4, 2014
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    So the transistor should off while the voltage is less than 130V. When the voltage reaches 130V you want the transistor to switch fully on. At that point the voltage will begin to fall. Do you want the transistor to stay switched on so it discharges the capacitor and then switch off again, then the capacitor begins to charge again?
     
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  5. psoke0

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    Mar 31, 2017
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    both sounds good :) after switch on its should at least wait a little until cap is discharges otherwise its will just keep oscillating.
    Edit: and yes i want this :D how can we make it wait until its discharges and after that swtich off until cap is 130v again
     
  6. AlbertHall

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    To do that you need to add a comparator with hysteresis to sense the voltage and drive the transistor base. That could be an IC like LM397 or LM339. Alternatively you could construct a schmidt trigger circuit with two transistors to do the job. The problem with either of these is that they would need a power supply of their own.

    If you don't need the 130V to be very accurate you could use two neons in series (about 120V to 140V) and trigger a thyristor. This would not need a separate supply.
     
  7. psoke0

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    Mar 31, 2017
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    i see thank you very much guys :) i will work on it and see what i can do
     
  8. Colin55

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    Aug 27, 2015
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    What transistor are you using?
     
  9. psoke0

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    Mar 31, 2017
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    EB 13001 transistor
     
  10. Colin55

    Active Member

    Aug 27, 2015
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    150v 300R 500mA Tr is 300mA Gain at 250mA = 10 Nothing is going to work.
    You will notice none of the replies had any idea how to work out the problem.
     
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  11. psoke0

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    Mar 31, 2017
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    i dont understand why is it not going to work. not in any shape or form this will work or is tehre a solution ?
     
  12. AlbertHall

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    How about this circuit?
    upload_2017-11-18_14-17-41.png
    upload_2017-11-18_14-18-12.png
     
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  13. panic mode

    Senior Member

    Oct 10, 2011
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    first you need to understand how transistor works... for example relationship between output voltage and input current.
     
  14. psoke0

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    Mar 31, 2017
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  15. psoke0

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    Mar 31, 2017
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    i know because the base current is not enough transistor cant turn on fully
     
  16. panic mode

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    Oct 10, 2011
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    QUOTE:
    "even if i short base to ground its still showing voltage at transistor's collector."


    of course it does, output voltage (collector voltage) is inversely proportional to input... if you short the base to ground, you get maximum voltage on collector.
     
  17. psoke0

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    Mar 31, 2017
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    that is interesting . and very true i understand now. so when im not shorting to ground and if transistor is not fully turned on the max voltage cap is giving gets divided between transistor and resistor. in order to make the resistor to consume all the voltage (almost all) i have to turn the transistor fully on. but here is the question. when we shorting base to ground voltage at collector will be high but its using no current right ?
     
  18. AlbertHall

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  19. psoke0

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    Mar 31, 2017
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    i see :D okay thank you but that is alot of components and components that i dont have :D so i will try something else but i should say what im going to do with this project. i will buy a watch Casio F91W and that watch beeps when alarm time comes. so i made a joule thief circuit that will activate when alarm beeps and joule thief will charge the cap and when cap is around 120 to 130 v its will shock my hand and wake me up. that 300 ohm resistor is actually my hand in worst case scenario when my hand is wet or something. :D
     
  20. panic mode

    Senior Member

    Oct 10, 2011
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    correct...

    btw, you really need to work with low voltage before you gain enough experience.
     
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