Calculate RL where's thersaurus an LDR in the circuit

Discussion in 'The Projects Forum' started by Lewismm99, Jan 21, 2016.

  1. Lewismm99

    Thread Starter New Member

    Jan 21, 2016
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    Can someone please help with this, I haven't got a clue where to start
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    380
    hi 99,
    You have not been given sufficient information about the components in that circuit to enable you to calculate the exact values.
    The LDR as you may know is a Light Dependant Resistor, the LED operating current is not specified.

    Is this Homework.?
    E
     
  3. Lewismm99

    Thread Starter New Member

    Jan 21, 2016
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    Nope it's an assignment for my college work!
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,501
    380
    hi,
    OK, Lets assume that the LED is a general purpose type, say 25mA maximum current.
    Its usual to under run this maximum current in order to extend the life of the LED, so assume 20mA, also assume a 2V drop across the LED.
    Also assume that the voltage drop across the transitor [ collector to emitter ] is 0V.

    Can you calculate the value of Rload that allows a current of 20mA to flow.?
    E
     
  5. Lewismm99

    Thread Starter New Member

    Jan 21, 2016
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    I should be a to, I'm in college now I'll check with my tutor sometime soon and I'll let you know, thanks for the help!
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Try a 470 ohms resistor for starters..
     
  7. Lewismm99

    Thread Starter New Member

    Jan 21, 2016
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    Do we know it's a 470? Or am just trying 470?
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    470 ohms will give approx 21mA led current, without knowing what the spec of the transistor and led, your guessing.
     
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