transistor for transistor switching?

Discussion in 'The Projects Forum' started by Sharpen047, Jan 14, 2013.

  1. Sharpen047

    Thread Starter Member

    Apr 19, 2012
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    Hey guys i am trying to power 4 led lamps with a single 555 dimmer.
    The circuit already has a single transistor and id like to use some of my fje3303 transistors. so my question is can i power my transistor by the output of the 555->1k ohm resistor->transistor->1k ohm resistor-> transistor? output freq of the timer is 126 hz.
    power is a 12v car battery being charged by solar panels.
    Thanks!
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Somebody is going to ask for a schematic, so it might as well be me.
     
  3. Sharpen047

    Thread Starter Member

    Apr 19, 2012
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  4. tracecom

    AAC Fanatic!

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    And what NPN transistor are you currently using on pin 3 of the 555? And what is the voltage drop and forward current required by each of the 4 LEDs.
     
  5. Sharpen047

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    Apr 19, 2012
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  6. tracecom

    AAC Fanatic!

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    If you want to allow for the maximum current of 700 mA through the string of 4 LEDs, you will need a fairly large current limiting resitor: 6.8 Ω at 6 watts. Do you have one?

    Secondly, you seem to indicate that you have already built the PWM circuit with a transistor already attached to pin 3. Is that correct or are you just planning to build the circuit?
     
  7. Audioguru

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    Dec 20, 2007
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    The trtansistor is rated for very high voltage but low current. Its current gain is low.
    If the LEDs are in series then their current can be 700mA if they do not burn out.

    The datasheet for the transistor shows that it saturates fairly well when its base current is 1/4 to 1/5th of the collector current. 1/4 of 700mA is 175mA. The 555 will get very hot if you limit its current to 175mA.
    Replace the transistor with a Mosfet.
     
  8. Sharpen047

    Thread Starter Member

    Apr 19, 2012
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    i am asking if i can power another fje3303 with that one meaning another step of transistors, so that transistor powers another transistor. This is more for learning than powering with a mosfet. I have plenty of mosfets i just want to see if i can power a transistor with another transistor.
    Rather than throwing a bunch of parts together and hoping i dont blow parts.
    i also do have 15w resistors ready but ill be dimming them too.
     
    Last edited: Jan 14, 2013
  9. Audioguru

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    Dec 20, 2007
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    A transistor that powers another transistor is called a Darlington Pair. Look at it in Google. It has a voltage loss. A Mosfet has almost no voltage loss.
     
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  10. Sharpen047

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    Apr 19, 2012
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    thanks a lot! does it still need the 1k ohm resistor between first and sencond transistor?
    or was the 1k ohm resistor only to protect the 555?
     
  11. Audioguru

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    Dec 20, 2007
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    The 1k base resistor for the transistor limits the base current. If the 1k resistor is replaced with a wire then the base of the transistor and the output of the 555 will both burn out with a very high current.

    With a 12V supply the output of the 555 goes high to about +10.5V and the base of a single transistor goes to about +0.7V then the base current (and the output current of the 555) is 9.8V/1k= 9.8mA. Then the transistor can drive an LED with 39mA or 98mA.

    The base of a darlington will be turned on at about +1.3V so with a 1k base resistor the darlington will be able to drive an LED with a current as high as 9.8A.
     
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