voltage reduction help needed

Discussion in 'The Projects Forum' started by markosillypig, Jul 29, 2009.

  1. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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    hi all
    i have made these circuits from a vellaman kit scamatic and all is fine but i now want to add the lights to my aeroplane but the problem is the voltage of the unit as 9v but on my plane it is only 4.8 so could any one help me on what components need to be changed to make the unit 4.8v
    scamatic included
     
  2. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
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    thanks but the scamatic is 9v and i want to drop it to 4.8 but it needs modifying to work on the 4.8v
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Not enough information about the 9V kit. I couldn't find it at Velleman USA.

    The 22 ohms for the current-limiting resistor seems far too low for the BC548 transistor and ordinary 25mA LEDs.

    If the LEDs are 3.6V white or blue ones then 2 in series need at least 7.2V and won't light with only 4.8V.

    A single white or blue LED will quickly burn out if the value of the current-limiting resistor is not increased.

    The flash rate will be almost half unless the timing parts are reduced in value.
     
  4. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
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  5. Audioguru

    New Member

    Dec 20, 2007
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    The LEDs are white. Then they might be 3.6V each.

    But the 22 ohm resistor has 1.6V across it if the transistor has a saturation voltage loss of 0.2V so the current is 73mA which will quickly burn out the LEDs.

    The LEDs need a total voltage that is far more than 4.8V.
    Then circuit can be modified to drive two separate LEDs, each with its own current-limiting resistor like this:
     
  6. markosillypig

    Thread Starter Active Member

    Jul 21, 2008
    184
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    muchas gracias senor
    i will give it ago tomorrow :)
     
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