Using 2 transistors as relay

Discussion in 'General Electronics Chat' started by FroceMaster, Oct 18, 2015.

  1. FroceMaster

    Thread Starter Member

    Jan 28, 2012
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    Hi
    Will try this instead of Relay.
    Can it be done, and what value for resistor ?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Your circuit will work in concept. The BC337 can carry up to 500 mA (keep it below 300 mA to be safe (R = 39 or more). The problem is that your TIP42 can only carry 6 amps for a continuous current (10amps pulse with short duty cycle). Also a problem with TIP42 as a switch means the gain is too low. The TIP42 has only 15x gain when over 3 amps. That means you need base current of one amp or more. Most people would suggest even higher for a fast switch.

    For a 10 amp motor, you would be best off with a P-channel mosfet. MOSFETS operate differently and are commonly used as a relay replacement. And they generate much less heat.

    Use something like a irf9540. Or something more modern. You can drop it right into the drawing I posted below to replace the PNP. It will not work in your drawing.

    This is more typical.

    image.jpg
     
    Last edited: Oct 18, 2015
  3. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi GT,
    Is that top 100 Ohm resistor a typo.?
    E
     
  4. GS3

    Senior Member

    Sep 21, 2007
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    TIP42 will not stand 10 A collector current. And 100 ohm base resistor might be too high for a minimum gain of 15 or less at high collector current.
     
  5. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    i will not fried anything, so i need to know if this setup will Work for output up to 16A 12-15V
    max drawn current at input must be 10 mA
     
  6. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    BD912 is also a solution
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Can't you use a P-MOSFET to switch the load? More power-efficient.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Here is a preferred design


    image.jpg
     
  9. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    everything is possible, have asked here before, but no help.
    Need help to calculate the value of resistors and help chosing the right transistor and mosfet.
    only thing, to-92 or similar to transistor and TO-220 to mosfet, and do i need cooling ?

    the input is a constant 12-15v not a pulse.
     
  10. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    IRL3716 and a 2N3904 ?
    Cheap ones. :)
     
  11. GS3

    Senior Member

    Sep 21, 2007
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    I think you will find that people are more willing to help those who are trying to learn than those who just want the work done for them. This is very basic stuff about transistors, gain, etc.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The BC337 is a TO-92 package in my post #8, the MOSFET is a TO-220 and the best option I see is
    http://www.st.com/web/en/resource/technical/document/datasheet/CD00002632.pdf
    As the P-channel. It can handle 80 amps and should not need cooling (heat sink) at 15 amps unless you are pulsing it (PwM).

    Good luck
     
  13. Roderick Young

    Member

    Feb 22, 2015
    408
    168
    If you can afford to switch the negative terminal of your load, may I suggest
    4110.gif
    • single TO-220 device
    • won't need a heat sink at 10A, if not switched quickly
    • can be driven directly with 12-15 volts (but don't exceed 20)
    • static current draw on input essentially zero (much less than 1 uA)
    • will withstand transients of 100V on the drain (if this is an automotive application)
     
  14. FroceMaster

    Thread Starter Member

    Jan 28, 2012
    400
    4
    Control of negative is not possible.
    Have done it with the above components
     
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