NE555 PWM 2N3904 emitter resistor values

Discussion in 'The Projects Forum' started by tracecom, Dec 17, 2012.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    In the attached NE555 PWM circuit, how do I determine the value of the emitter resistors?

    Thanks.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    what is the current your drawing through the load, the maximum per transistor is 200mA,
    these resistors will limit the current in the load in the transistors, try using 4.7 ohms at 1/4W
     
    tracecom likes this.
  3. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    I am planning to PWM some table top photo lights that I built using LED's. There are a total of 8 strings of LED's, and each string is conducting 18mA, for a total of 144mA, so I suppose that I could get by with one 2N3904, but two would allow for some headroom. (I showed four transistors just as an example for discussion.) In the case of two, what should the value of the emitter resistors be, and please explain how you arrive at the answer.

    Thanks.
     
    Last edited: Dec 17, 2012
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