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Simple Variable DC Power Supply

Discussion in 'Power Electronics' started by jjdans, Jul 13, 2013.

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  1. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Hey guys, I'm a new poster.

    I've joined because I'm looking at expanding my knowledge of all things electronics.

    My first project I've started on is building a Variable DC Power Supply for a friend who is a tattooist.

    I'm hoping I can recieve some pointers from anyone willing to help.

    - The input for my power supply is going to be just a standard 12VDC Power Supply
    - The output voltage needs to range from somewhere between 4.5VDC-12VDC (with a little leeway)
    - The current drawn will be max 2A (unlikely to reach that high)
    - ON/OFF Isolator
    - LED Indicator
    - Standard female jack for the input (common size)
    - 1/4" (guitar jack) for the output

    Earlier today I went to purchase some components from an electronics store and seek some help with what I actually need. I was told to buy a 10K Pot for the Voltage reduction and then I bought an LED, Switch, Enclosure & the input/output jacks.

    I wired up this little project only to find it doesn't work. After it hit me I'm pretty sure the Pot will only reduce the current flow (correct?) being resistance is the opposition to current flow.

    What do I need in its place? A variable regulator? something else that I'm not familiar with?

    Thumbs up to anyone willing to shed some light!
     
  2. GonzoEngineer

    GonzoEngineer

    321
    2
    Dec 2, 2011
    You can start by researching "LM317, Adjustable Voltage Regulator."
     
  3. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Thank you very much seems like it could be what I need!
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    652
    May 8, 2012
    Gonzo gave you good advice but who exactly was it that told you a 10K pot was your ticket? Was it a salesman at the electronics supply? Who was the supplier?

    Chris
     
  5. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Yeah it was a Salesman. I just said what I'm after and I remembered reading that I could need a regulator and he told me "no a regulator is a fixed voltage reducer so you'll need a pot". I'm an apprentice electrician so I know a couple of things but not a whole lot about dc as I mainly just do AC lighting & power so I thought about the resistance effecting current flow and then it hit me!
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Was it Radio Shack? They always give stellar advice. :rolleyes:

    BTW, welcome to EP!

    Chris
     
  7. jjdans

    jjdans

    15
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    Jul 13, 2013
    No it was Jaycar which is Australia's equivalent of Radio Shack I suppose!

    Thank you very much glad to be here!
    Can't wait to dive into more projects
     
  8. GonzoEngineer

    GonzoEngineer

    321
    2
    Dec 2, 2011
    Also bear in mind that at 2A, you are going to need a heatsink on an LM317.

    To explain simply, if you put 12V in, and adjust it for 5V out, the device is going to have to dissipate as heat the other 7V. At 2A, this would be 14 watts.

    Also be aware that with 12V in, you will not be able to get 12V out. This is because the device needs a certain amount of "headroom" to regulate. If I remember correctly you will need at least around 14V in to get 12V out.

    If you have any questions, feel free to ask.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    The data sheet for the LM317 indicates a 5K pot for typical applications. If you've soldered the leads of your 10K pot, in which case they won't accept a return, you can shunt it with a 5.1K. I hope your pot is linear. An audio taper will work but it will be a cramped scale at one end.

    Chris
     
  10. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Thank you!
    Is there a certain type of heatsink I need? As in a certain rating for this application?
    The power supply will most likely be set at 6.5VDC and sometimes as high as 8VDC I originally said 12VDC as I assumed you could have the same output as the input but 12VDC is not essential.
     
  11. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    The pot itself was quite cheap so if I need to buy another one that's not the end of the world. And I believe it is linear yes.

    Thanks Chris
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    It's always a good idea to be as precise as you can here. 6,5V is a long way from 12V.

    Chris
     
  13. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    I was trying to be a specific as I could with the specifications I was given.
    I know for a fact for part of the tattooing process it's set at 6.5V but the they do increase depending on what techniques are being used. My apologies for not being clear but it could still fluctuate from 6.5-8 or maybe even 10 or 11. But not having 12V will be fine I think.

    Thanks Chris
     
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Just remember what Gonzo told you. The minimum input V must be n(V) higher than your maximum output V. The exact spec should be listed in the data sheet.

    Chris

     
  15. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Hey guys. I bought the LM317 and a couple of resistors & caps that I saw listed with another circuit online. I'm a little unsure of how everything is connect.

    This is my parts lists to start with (hopefully I'm not missing anything):
    - 1x LM317T
    - 2x 270Ohm Resistor
    - 1x 10K POT
    - 1x 5K POT
    - 2x 1/4" Jack
    - 1x DC Power Supply Jack
    - 1x Isolator - 1x 12v LED
    - 1x 0.1 Micro Farad Cap
    - 1x 1 Micro Farad Cap
    - Steel Enclosure (TI was advised to use this as my heatsink)
    - 2x Small PCB's
    (not all of these are essential but its just a list of what I have to use if needed)

    Here's the circuit I drew up to see how it was all going to go together
    http://www.circuitlab.com/circuit/yuc538/variable-dc-power-supply/
     
    Last edited: Jul 14, 2013
  16. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The circuit you show will not work !

    The LM317 will only work with direct current input.
    So,
    Rectify the input AC
    Smooth the DC
    Pass through the LM317
    Make a DC to AC converter (audio amplifier?)

    What are you trying to do?
    Heat sinks are often connected to one of the chips terminals so should be insulated unless the chip is insulated from the heat sink.
     
  17. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Sorry that circuit could be way off but the input is 12VDC.
    I'm just trying to make the 12VDC input variable so I can alter between 5-10VDC output

    I'm a novice and I apologize for my stupid questions and incorrect explanations and diagrams. Hopefully years down the track I can be as smart and knowledgable as you guys
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    Here is a fixed circuit

    [​IMG]

    It's still not perfect, but it shows you the sorts of things you need to do.

    It is also recommended that you have input and output capacitors.

    Find a good datasheet for this device and you'll see what you have to do (they have sample circuits)

    Look at the top left corner of page 2 of this.
     

    Attached Files:

  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Usually I'll draw and post the circuit for you but I told you before that all the information you need will be found in the LM317 Data Sheet. Just Google "LM317 Data Sheet". The data sheet will include application notes with included schematics. The heat sink data will be included too. In fact there should be nothing that we could add, that is LM317 specific, that isn't already included.

    Chris

    Edit: Sir Steve is faster than me. His Eminence posted a schematic for you in the time it took me to type this post. :p
     
    Last edited: Jul 14, 2013
  20. jjdans

    jjdans

    15
    0
    Jul 13, 2013
    Thanks Chris & Steve. I have looked at some data sheets and a lot of it doesn't make a whole lot of sense to me. I guess the main issue is knowing how to physically put all the components together. Thanks again
     
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