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Power Supply Woes

Discussion in 'Power Electronics' started by MrClamperSir, Feb 4, 2016.

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  1. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Hello!! I am new to this forum and new to electronics. I am a tattooer of 16 years and I have been repairing (replacing parts) on antiquated power supply's for a couple years now. I don't care for the switched mode supply's because of the rippled output so I stick with the linear supply's.

    Unfortunately for me I am one of the last of a dying breed when it comes to my preference of supply. That being said, I have built my own supply and am mostly happy with it. However, it runs a little too hot (most of my older supply's get real warm but his one is HOT). I'm hoping someone can help me verify the schematic, and see if there is a way to decrease the Vin to Vout while maintaining the current needed to drive my tools.

    I'm thinking about lowering the volts on the secondary side of the transformer to 14V. But what about the amps? Is 1A too much or too little?

    My machines run on a variable of 3V - 8V drawing 0.8A - 1.3A

    Below I have attached the schematic drawing. I will try my best to answer any questions but like I said, I really new to this and self taught so bear with me please.

    phot1.jpg
     
    Last edited by a moderator: Feb 4, 2016
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    Hi there
    welcome to EP :)


    If you are only needing 3V DC, then 14V AC is a serious overkill as that is giving you almost 20V DC into the regulator chip
    it will have to drop 16 V at say 1A = 16W ... yeah the regulator WILL get hot !!
    get a transformer with a ~ 8 VAC out and a 2A capability and the reg chip wont have to work so hard
    It is ALWAYS wise to have the PSU capable of supplying more current than what is needed, that way it's never working at its maximum

    just looking at your circuit ... that transistor connection definitely doesn't look right for doing current pass
    note that the base and emitter are short circuited

    you can do away with the transistor and just use a LM 338 reg which is capable of up to 5A, instead of the LM317


    Dave
     
    Last edited: Feb 4, 2016
    duke37 likes this.
  3. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Thank you for your reply.

    I need a variable output the ranges from 3v-8v. Not sure if that changes anything you suggested.

    I'll look into the LM 388 right now.

    Was this not an option in the early 90's?
     
  4. davenn

    davenn Moderator

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    The LM338 is an adj reg ... just a higher current version of the LM317

    not sure when the 338 became available ?
     
  5. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Right now I have the secondary side of the transformer at 25VAC. The regulator doesn't get too hot (that I'm aware of) but the transistor definitely does.

    So if I move the secondary side down to 8VAC at 2A and use the LM 388 (I'm assuming mounted on a heat sink) this will be a better output/PSU to power my tools?

    ** These changes can be made while still using the same circuit at the same values??
     
  6. davenn

    davenn Moderator

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    yes 8 - 10 VAC secondary ( 10V would be best if available will allow for a bit of voltage drop under load)

    actually .. just redid calculations ... definitely go for a 10V, 2A secondary rated transformer

    heatsink definitely


    yes and there is no need for your R2 - 47 Ohms


    Dave
     
  7. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Looking into the LM338 it looks like it's often used with the LM7812. It also looks like adj reg or a transistor
    [​IMG] [​IMG]
    What's the difference? Is one better than the other?
     
  8. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Thanks Dave!! I've been alone in this endeavor for the past year and I've spent the last month banging my head on the wall!! I'm feeling energized o get back to it!!
     
  9. davenn

    davenn Moderator

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    use the TO3 metal cased one

    that's good to hear :)


    D
     
  10. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    I'll try and wrap my head around building this circuit!!
     
    davenn likes this.
  11. duke37

    duke37

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    Jan 9, 2011
    If you use a boost transistor, it needs to be a pnp transistor such as a 2N2955.
    The circuit you show has the pass transistor permanently turned off so I do not see how it should get hot.
     
  12. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Is there a way to test a component to see if it's pnp or npn? I have a transistor in a working circuit but the values (markings) are not present.

    Also, how did you conclude that the pass transistor is permanently turned off? because I listed it as a 2N3055 when it should be a 2N2955?
     
  13. BobK

    BobK

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    Yes, an NPN transistor would be turned off as shown.

    A PNP transistor would act as a forward biased diode, shorting out the regulator.

    So you probably have a PNP transistor acting like a diode and no voltage regulation. The transistor would get hot.

    Bob
     
  14. BobK

    BobK

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    Jan 5, 2010
  15. davenn

    davenn Moderator

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    because as I told you in an earlier post, it is wired incorrectly ... the base and emitter are shorted out
     
  16. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    I'm not sure how the schematic compares to the wiring of the actual circuit considering the schematic was drawn for me by a guy who had never seen the supply in person and I wired the circuit using my existing supply as an example. So I'm not sure if the transistor is PNP or NPN but I do know its getting really hot and I'm trying to figure out why.
     
  17. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Yes, I see that now. Thank you. I went back the circuit and changed the schematic to represent what I was seeing in the circuit. Is this how it should look, or am I'm I missing something or.....? Phot3.JPG
     
  18. duke37

    duke37

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    Jan 9, 2011
    Look up the data sheet for the 317, it normally gives details of how to boost the output.
    R2 is not necessary in your circuit. Q1 just works as an emitter follower.
    If you use a pnp transistor, then R2 should come before the 317 and the collector should be connected to the 317 output.
    You can compare a known pnp and known npn transistor with the unknown with a resistance measurement, this can tell you which is the base and whether it is pnp or npn.

    If a lot of voltage is dropped with a high current, then a lot of heat will be produced and a massive heat sink will be needed.
     
  19. MrClamperSir

    MrClamperSir

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    Feb 3, 2016
    Thanks, I will look up the data sheet right now.

    In this circuit R2 is after the LM317 (which your saying isn't necessary) and the collector is on the input side of the regulator. Does that mean that it is a NPN?
     
  20. davenn

    davenn Moderator

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    Sep 5, 2009
    not quite, do that and you go back to a transistor doing nothing with it's base and emitter shorted ;)


    a resistor is needed but not in that config., and the base should be on the input side of the 317
    I will find the correct circuit for you

    OK here we go

    dd.GIF

    This is for a 317 in a fixed output mode ... hence no trimpot
    it's the placement of the transistor and it's resistors I want you to note :)


    Dave
     
    Last edited: Feb 4, 2016
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