• This topic has 2 replies, 2 voices, and was last updated 5 years ago by LectronFan.
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    • #2484
      LectronFan
      Moderator

        Hi all,
        When looking at the circuit 11, I noticed something weird.
        The LDR block is connected in series with a very low resistance (120 Ohms).
        This is much too low in value, a current of 20mA can flow through the LDR and 10mA into the base of the transistor, which are relative high values.
        In the older circuit with PNP transistors, this resistor is 1,5K, which is a better choice, but still a big current is flowing into the transistor.
        So, let’s study and calculate better values for the circuit.
        Instead of the 120 Ohm or 1,5K resistor, we mount a 10K resistor.
        The max. current flowing through the LDR is now 0.88 mA instead of 22 mA.
        We add a base resistor of 5,6K to reduce the base current to 1,2 mA.
        Now the circuit has a better performance and will prolong the life of the components.
        Here are the circuits :
        Circuit 11 with NPN transistor

        Circuit 11 with PNP transistor

        Greetings

        • This topic was modified 5 years ago by Michael.
        • This topic was modified 5 years ago by Michael.
        • This topic was modified 5 years ago by Michael.
        • This topic was modified 5 years ago by Michael.
      • #2488
        Michael
        Keymaster

          Hi Frank,

          Thanks again for this helpful and important revision and optimization of the experiment #11! 

          The links for the graphic did not show up working however.  I see the referring url in text view.  Not sure why the embedding is not working.

          At least the tags are not showing up now!

          I am working on the embedding of graphics function as a further enhancement.  Thank you for bringing this to my attention!

          Best regards,
          Michael

          • This reply was modified 5 years ago by Michael.
          • This reply was modified 5 years ago by Michael.
        • #2494
          LectronFan
          Moderator

            Improved circuit 11 with NPN transistor :
            Circuit 11 with NPN transistor
            Improved circuit 11 with PNP transistor :
            Circuit 11 with PNP transistor
            Note that we also use a resistor of 47 Ohms to reduce the current flowing through the lamp.

            Here are some measurements I made :
            Ic = 46,6mA (Collector current = current flowing through the lamp)
            Ib = 0,8mA (Base current = current flowing the 5,6K resistor)
            Ildr = 1,76mA (current flowing through the LDR when it is bright light)
            The voltage across the 47Ohm resistor is about 2,25V, so the lamp gets about 6,75V at max.

            All these currents are now safe for our transistor and components.

            Feel free to experiment :
            Interchange the LDR with the 10K resistor : now the lamp only lights when the LDR is completely darkened.

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