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Problem with LM317T

Felipe Coelho
Prodigy 30 points
Other Parts Discussed in Thread: LM317

Hello!

I made a DC source using LM317T, the circuit is on this website: http://www.newtoncbraga.com.br/index.php/mini-projetos/171-fontes-e-carregadores/7770-fonte-de-tensao-variavel-com-o-lm350t-lm317-min353. 

  • 0
    TI__Mastermind 24975 points

    Hello Felipe,

    Welcome to the forums!

    What kind of problems are you having? Is it not regulating properly?

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Prodigy 30 points

    Hi Michael,

    Thanks for asnwering!

    No, it is not regulanting correctly. I made some tests in it. When I put a voltimeter in the output of the source, and I'm changing the potentiometer, the output oscilates of 0V to 26V. When I put some charge in that, like a resistor, the output voltage falls. For example, I put just a resistor, of 10k, forming a serie circuit with the source. Therefore I expect to have the same 0V to 26V, because all the voltage will be at the resistor, but it didn't happened. The output changes of 0V to 10V in this case. And with another components, the source changes too. I don't know why. Maybe is something wrong with the circuit? Or with the LM317T?

  • 0 in reply to Felipe Coelho
    TI__Mastermind 24975 points

    Felipe,

    Could you please provide a schematic of your circuit?

    What is your input and output voltage? What is the output current?

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Prodigy 30 points

    Michael,

    The schematic is on this picture:

    The input is a center tape transformator, with 24+24 on secondary. The output, like a sad before, with a resistor of 10k, is only 1mA, because the output maximum is 10V in this case. The LM317 has a heat dissipator, and it has gotten a little hot.

    P.S: In this link, I put the Proteus archieve with the schematic complete, including the board circuit. www.4shared.com/.../projeto_fonte.html

    Thanks,

    Felipe

  • 0 in reply to Felipe Coelho
    TI__Mastermind 24975 points

    Felipe,

    What voltage do you measure at the input node of the LM317T?

    Also, you mentioned that if you put a load on the output, it has a smaller range of regulation. One thing to note about these linear regulators is that they typically need a minimum current draw in order to regulate properly. For this part it is possible that you need a current as high as 10mA in order to ensure regulation.

    On the other hand if you have too much current on the output then the device could heat up beyond the maximum junction temperature and damage the device.

    Best,
    Michael