Pls help--classic LED FLASHER ckt

Discussion in 'General Electronics Chat' started by mrinal81, Jun 10, 2010.

  1. mrinal81

    Thread Starter New Member

    Jun 10, 2010
    3
    0
    This is about the classic LED flasher ckt using 2 transistors (refer to the ckt in attached file). why the positive side of the capacitor is connected to the collector and the negative side is connected to the 100k registor ? plaese help , I am new to electronics.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,733
    759
    Collector is positive compared the 100K R
    and polarized caps should not be connected in reverse
     
  3. mrinal81

    Thread Starter New Member

    Jun 10, 2010
    3
    0
    The 100k R is connected to +9v. how the collector can be more positive?
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,733
    759
    compared to the collector load (resistance) it is negative
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,671
    899
    The current gain for the 2N3904 is around 50 to 100 at 10 mA. Assume 10 mA though the LED (it really doesn't matter). The voltage drop across the 470 ohm will be 4.7 volts. Base current at gain of 50 will be 0.2 mA. Voltage drop across the 100K will "20V." Of course, that is not possible, but you should see that if the current gain is less than the ratio of 100K to 470 (approx. 212), then the voltage drop across the 100K will be higher making the polarity correct as shown.

    John
     
Loading...