Problem with PNP switching circuit

Discussion in 'General Electronics Chat' started by shivaaa, Jan 19, 2011.

  1. shivaaa

    Thread Starter Member

    Jan 5, 2011
    55
    0
    I am trying to make a simple switching with BD140 PNP. The LED is not glowing. plz tell me wats the problem. attached is the circuit image
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Without a current limiting resistor to limit the current to the LED, the LED and possibly the BD140 PNP transistor are both killed by overcurrent.

    Learns how to calculate the resistor for a LED with different supply voltage so this won't happen again.

    You can find out voltage across the LED(it depends on LED color) from the Internet, then apply simple Ohm's law. Subtracts this voltage from the supply voltage then divides the result by the current(usually 10~20mA) you want to use will give you the required resistor value.
     
  3. shivaaa

    Thread Starter Member

    Jan 5, 2011
    55
    0
    I added 330ohms and 100ohms series to LED as well . but still not working
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    An LED that is burned out does not glow when the proper current-limiting resistor is added in series with it.
    Get a new LED and transistor and don't forget to limit the current.
     
  5. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    Do you have the LED hooked up in it's proper lead configuration?
     
  6. soda

    Active Member

    Dec 7, 2008
    174
    13
    This is how your circuit should look like
     
  7. shivaaa

    Thread Starter Member

    Jan 5, 2011
    55
    0
    I bought new LED , New BD140 and tried the same, still not working..
     
  8. soda

    Active Member

    Dec 7, 2008
    174
    13
    Did you connect it the way i showed you? Make sure the LED anode is connected to the collector and the cathode to the 0v rail. On one side of the led is a flat spot which mark the cathode, the other side is the anode.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Here are what the parts look like:
     
  10. shivaaa

    Thread Starter Member

    Jan 5, 2011
    55
    0
    Thanks, its working now, i have added a 2K in base, and replaced the BD140.

    Before this, I added 1K in base and closed the switch as per the circuit diagram, it was started glowing and stopped. again I tried but dint work. so now replaced the BD140 and working thanks for all your help.
     
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