1. tengiga

    Thread Starter New Member

    Sep 12, 2013
    2
    0
    Dear All,
    I am trying to make soft start circuit for halogen light using this circuit

    http://i187.photobucket.com/albums/x.../softstart.jpg
    reference: http://www.bikeforums.net/archive/in.../t-357675.html

    This is DC circuit with battery connected capacitor to p-mosfet which then connects to resistor and diode.

    I am trying to replace mosfet with BC 547 NPN transistor.
    Datasheet
    http://datasheet.octopart.com/BC547-...sheet-7074.pdf

    Can anyone advise me whether I can do this using the same circuit as shown in pic above or do i need to add more component for usinG NPN transistor safely.

    I am assuming it is 12 v battery?

    An advice greatly appreciated.
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,288
    1,255
    The BC547 is much to small for a 20, 30 watt bulb. The FET circuit is not so good as drawn either. Can you find a large NFET?
     
  3. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    ronv is on the right track. Still, it would be helpful if you told us what voltage the light is designed for instead of having us guess.
     
  4. tengiga

    Thread Starter New Member

    Sep 12, 2013
    2
    0
    Thank you gents for your reply and brief explanation.
    I am trying to soft start 50 W DC halogen bulb @ 12 V. Hence P =VI, i.e. I = 4.2 Amp
    http://i187.photobucket.com/albums/x.../softstart.jpg
    reference: http://www.bikeforums.net/archive/in.../t-357675.html
    Further I have only LM317T or BC 548 available with me.
    Can I not use LM317T instead?
    I am trying to soft start 50 W DC halogen bulb @ 12 V. Hence P =VI, i.e. I = 4.2 Amp
    1) Selecting right transistor:
    BC 547 - Based on datasheet
    http://datasheet.octopart.com/BC547-Fairchild-datasheet-7074.pdf
    My understanding that why BC 547 is not acceptable is because Ic (collector current)
    is limited to 100 mA whilst required current can be as high as 4.2 amp. Please advise
    if different.
    LM317T
    Can I use LM317 T instead as this the only one available to me?
    http://www.fairchildsemi.com/ds/LM/LM317.pdf

    2. Resistor
    R = (Power Supply-Voltage drop through bulb)/ Current.
    In this case I believe that voltage drop will differ through halogen bulb.
    Is there any other way to calculate resistor and capacitor required.
    Thanks for your time.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,288
    1,255
    It is difficult to soft start a halogen bulb because it's resistance when cold is very low. Probably about .2 ohms so the current is very high until it warms up. Tell us why you want to soft start it maybe that will give some ideas.
     
  6. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    The LM317T will only pass 1.5 amps. You can not get there with a 317 and a 100ma transistor. You can not do it with a resistor an a capacitor. You need an active component to be instructed by the resistor and capacitor. You will need a different transistor and some design work.
     
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