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    Power supply circuit help

    Hi

    I have been trying to repair a simple power supply from a student microscope base (3 of them in fact). It has a 230 to 12V transformer which gives a 12V AC input to the attached schematic (which I think is correct but I made it myself, so there may be errors - please correct me where needed).

    My knowledge of electronics is very limited and I'm struggling to understand what role the transistor plays in the attached circuit (and if it is shown with the correct orientation).

    I've attached photos of the board (the image of the rear side is flipped to match the top).

    The good news is that I think I have identified the problem on 2/3 of the boards (new 7805's needed).

    The last board is more difficult. I've replaced the 7805 and transistor (BC337) on this board but am now getting the LED permantly on and non-dimming, some odd voltage readings and a short across R1 (even without the resistor present) and I wonder whether I may have the transistor inserted the wrong way (even though it appears the same orientation as the original).

    Any help much appreciated.

    Cheers,

    John
    Attached Files

    #2
    Re: Power supply circuit help

    One end of R1 is ground, the other end goes to the transistor base, this would suggest the transistor is shorted E to B. The print for the transistor may have been for a different transistor that had a different configuration. If this is how the original BC337 was installed, and it worked before, this must be correct.
    The transistor base voltage is controlled by the variable control, the transistor then controls the current through the load led and its brightness.
    Attached Files
    Last edited by R_J; 05-29-2022, 12:03 PM.

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      #3
      Re: Power supply circuit help

      Thanks very much for your help R_J.

      Yes, I can see that R1 is attached to ground as per your schematic. My mistake.

      I will replace the transistor and also double check the orientation vs. the original.

      Does anyone know what role the transistor plays in this circuit?

      Comment


        #4
        Re: Power supply circuit help

        Originally posted by Pugsly0191 View Post
        Thanks very much for your help R_J.

        Yes, I can see that R1 is attached to ground as per your schematic. My mistake.

        I will replace the transistor and also double check the orientation vs. the original.

        Does anyone know what role the transistor plays in this circuit?
        The transistor base voltage is controlled by the variable control, the transistor then controls the current through the load led and its brightness.
        Check the transistor with your meter to see if IT is shorted.
        Last edited by R_J; 05-29-2022, 12:08 PM.

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          #5
          Re: Power supply circuit help

          Originally posted by R_J View Post
          Check the transistor with your meter to see if IT is shorted.
          Yep, it is indeed. E to B. I will change it.

          Originally posted by R_J View Post
          The transistor base voltage is controlled by the variable control, the transistor then controls the current through the load led and its brightness.
          I thought it must be something like that but I couldn't figure out how the variable resistor was affecting the base voltage. But now I see my schematic was wrong and that caused the confusion.

          Thank you so much for your help!

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            #6
            Re: Power supply circuit help

            Changed the transistor and it works like a dream.

            What a great feeling!

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              #7
              Re: Power supply circuit help

              That's good to here, glad to help.

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