Size of resistor and transistor

Discussion in 'General Electronics Chat' started by FroceMaster, Oct 4, 2014.

  1. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    Hi
    Need to get this working.
    have an output 12v 20ma max.
    need to lid up 3 lights 3*21w
    how to connect the transistor, and witch ?

    Rgds.
    Sten
     
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  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well simply use a MOSFET Irfz44n

    1a.PNG
     
  3. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    not an option.. the "ground" on bulps is connected as shown on schematic.
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    So use irf4905 instead.

    1a.PNG
    Also you 12V power supply for the bulps must be able to deliver at least 7A.
    And also small heat sink for the MOSFET will also be needed.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I hope the power supply is rated for 20A; not 20mA
     
  6. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    the input can only Draw 20mA. the powersupply is at least 300A.
    why at least 7 A ? 3*21 = 63w. 63/12 is not more than 5,25.
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    With R3 = 1K the input current is smaller than 12V/1K = 12mA.
    Also you can increase R3 to 10K and R4 to 100K
    But you can also remove Q2, R3, R4 , R2 but then low state (close to 0V) at the input will light the bulbs and high state (12V) will turn-off the bulbs

    Very good.

    Just as a safety margin.
     
  8. ronv

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    Nov 12, 2008
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  9. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    desided to make it differently.
    will this Work, and not Draw more than 20 mA to the transistor ?
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    What is the DC resistance of the coil?
     
  11. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    360 Ohm.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Your new circuit should work. You could even increase the resistor value to, say, 3.3k to reduce the current draw.
     
  13. panic mode

    Senior Member

    Oct 10, 2011
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    why do you have three diodes in series with the bulbs?
     
  14. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    Use of diode . Yes good ? , really dont remember why. Its for use to turn on taillights on new cars. When deliver from factory there is no taillight in the day. Many custumor will have them on. I think it is so that left/right/licenceplate light still is on each fuse
     
  15. Papabravo

    Expert

    Feb 24, 2006
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    So the transistor will be sinking about 33 mA in the on state for 12 mA of base current. You don't need that much base current to turn the transistor on as has already been noted. You should have a resistor from the base to GND in case the driving source becomes disconnected.
     
  16. FroceMaster

    Thread Starter Active Member

    Jan 28, 2012
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    ok if i chance the 1k to 3.3k and where to put the extra resistor. before the base resistor or between base and resistor. and what value ?
     
  17. Papabravo

    Expert

    Feb 24, 2006
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    Between the base resistor and the base of the transistor. Check the earlier schematic that was offered in this thread for the value.
     
  18. takao21203

    AAC Fanatic!

    Apr 28, 2012
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    automotive?
     
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