DC Power Supply

Discussion in 'The Projects Forum' started by Andy Larson, Apr 11, 2015.

  1. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Hello community,

    I'm working on a 1.5A 0-30V full wave bridge rectifier regulated DC Power Supply. I have run into a recurring problem. No direct and simple answers for what I need exactly. There are tutorials, but they don't answer many questions. I have read a lot(10 or so) of forum posts and tutorials on designing a power supply. I have tried using a designer program, that was last updated 4 years ago

    What I know so far: Requires a Transformer, 4 diodes or a rectifier ic, a smoothing capacitor, and some kind of voltage and amp regulation. Filters (RC, LC, RLC, EMI, etc.) can be added with the smoothing capacitor. Voltage can be adjusted via a LM3xx Mosfet circuit, higher current varies require a heatsink on the Mosfet. A diode should be use to prevent a short. There are ways to regulated/adjust current.

    My questions:
    1. What specs for a transformer do I need to figure out?
    2. What kind of Capacitor do I need and how many?
    3. How many uF, nF or mF do i need for a minimal ripple (maybe <5%)
    4. What difference do those filters make (I know that EMI stops interference from radio waves)?
    5. Is 1.5A a good current for powering and testing circuits?
    6. Is it a good idea to add a Zener Diode voltage regulator with the LM3xx Mosfet?
    7. What should the amp rating on the fuse be?
    8. Is surge protection needed?
    9. (Optional but curious about)
      1. What kind of On/Off LED should i uses? I was thinking a piranha super flux.
      2. Are there any cool or unique fuse holders? I was hoping for a spring loaded, turn locking sliding fuse holder.
      3. Short indication via a LED? I have seen it with a PNP gate.
      4. I'm considering adding a USB socket for charging devices, what should the voltage and amps be for fast charging?
      5. What's the pro and cons of using the different kinds off output sockets? Banana plug, Weidmuller, Speaker Spring, etc.

    My wall power specs: 120V 60Hz (US Mains Power)

    Please if you can, provide formulas. So others people can figure these things out. I'm a semi-experienced high school junior (I know the basics of electronic, ohms Law, how circuits works and how to solder) so please try to make it simple for me and others to understand.

    A million thanks,
    Andy
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
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    Look there are plenty of plans out there and some very good and some very bad .. Are you building this for a school project or for home use ??? Also do you have the tools and knowledge to understand test the circuit??
     
  3. Andy Larson

    Thread Starter Member

    Mar 26, 2015
    51
    0
    As a home project for fun. Yes I do understand how to test a circuit for continuity, amps, volts, and ohms.
     
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
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    Do you have the tools ??
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,179
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    I'm not sure you can get everything you want in a simple design. More about that later.
    To get 30 VDC out your transformer must have a bit more than 30VAC(peak).
    To convert VAC(rms) to VAC(peak) you multiply by the square root of 2, approximately 1.414
    If your transformer secondary is 24VAC(rms) you get 33.94 VAC(peak)
    Then you go through the bridge rectifier and you loose a bit from the peak, say 1.6 volts leaving 32.3 VAC(peak) at 2 times the powerline frequency or 120 Hz.
    Now for a capacitor, of say 470 uFd. and you are left with about 32 VDC with some ripple.
    Are you with me so far?
     
    Alec_t likes this.
  6. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Yep. The essentials: pliers, wire cutters, wire strippers, and a multimeter
     
  7. Andy Larson

    Thread Starter Member

    Mar 26, 2015
    51
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    Yes
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    Right and the next problem is power management. Lets say that we want to set our output at 15V and we want the load to draw 1.5 amperes.
    So 15VDC * 1.5 Amperes = 22.5 Watts.
    The input to our regulator is going to be 32VDC and we need to draw that same 1.5 amperes. The regulator will have to dissipate
    (32V - 15V) * 1.5 amperes = 25.5 Watts
    I don't know if you know anything about power management but that is a great deal of power to manage as heat. It will literally cook the integrated circuits.
    So a linear regulator like an LM317 will just not be up to the job.
    If we want a 5V output it gets worse.
    5VDC * 1.5Amperes = 7.5 Watts, but
    (32V - 5V) * 1.5 Amperes 40.5 Watts.
    You can see where this is going
    What do you think are possible solutions to this power management problem?
     
    Alec_t likes this.
  9. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    add a heat sink or figure a way to make the regulator to be additive not subtractive or as a last resort us a LM337. I do have 4 heat sinks, one is from a half wave rectifier high temp diode
     
    Last edited: Apr 11, 2015
  10. LDC3

    Active Member

    Apr 27, 2013
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    You can get a mains connector, switch and fuse holder as one unit.
     
  11. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    I forgot about those. Thank you
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    I don't know what that means. Can you explain it to me?
     
  13. Andy Larson

    Thread Starter Member

    Mar 26, 2015
    51
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    Instead of the linear regulator subtracting voltage from line, it would add voltage to the line. Still you would have maybe one or 2 V line that is also being fed by a 30 V line that will have a potentiometer on it. But I know nothing about circuit design or mechanics and behavior of electrons
     
  14. Papabravo

    Expert

    Feb 24, 2006
    10,179
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    Well linear regulators, like the LM317, only work one way by converting some power into heat and the rest into a lower voltage. The process is simple, but not very efficient. A switching regulator, on the other hand can be efficient, but it is a more challenging project. Even they do not add voltage, what they do is power conversion. Let me give you an example. Suppose we have a 5VDC power supply and we want to use that to make a 12VDC source. Let us also say that we need the 12VDC to provide 1 Ampere.
    Code (Text):
    1.  
    2. So 12 Volts times 1 Ampere = 12 Watts of output power
    3. Let assume that the conversion process is 85% efficient
    4. 12 Watts / 0.85 ≈ 14.1 Watts of input power
    5. 14.1 Watts of input power / 5 VDC ≈ 2.82 amperes
    6.  
    In any power conversion process if voltage goes one way (up or down), then current goes the other way. So the challenge in your project is to pick one or more ways to mange the power conversion process. This is one reason why adjustable laboratory power supplies are expensive. They are not simple and they are not cheap to design or assemble. You might want to refine your requirements since there are numerous things to learn in order to get to your goal.
     
  15. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Well I have seen multiple linear regulators used. I'm willing spend a little bit (<300$). So I'm open to all ideas and solutions
     
  16. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Would a Op-Amp work to? I have just done a lot of reading on this. Use a low volt transformer to save heat. Or use a series of transformers. down to say 2V. Use a Op-Amp to up to 30v or more if I wanted to
     
  17. Papabravo

    Expert

    Feb 24, 2006
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    No, opamps don't source or sink much power. They work in the tens of milliamperes range. Except to monitor the output voltage and provide closed loop control, they are not much use for the process you have in mind.
     
  18. Andy Larson

    Thread Starter Member

    Mar 26, 2015
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    Ok thxs for the clarification. What would work? I'm open to all ideas. Would this regulator work? LT3085 or LT3080?
     
    Last edited: Apr 16, 2015
  19. Papabravo

    Expert

    Feb 24, 2006
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    You still need to pay attention to the power dissipation limits of the package. If you use a 30 VDC input and want a 5 VDC output, then you must be able to dissipate (30 VDC - 5 VDC)*LOAD_CURRENT
    The low dropout feature just means that you can get your regulated output with the input voltage much closer to the output voltage.
     
  20. Andy Larson

    Thread Starter Member

    Mar 26, 2015
    51
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    So I'm either going to need a big arse heat sink or..?
     
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