Powersupply with LM78L05, uC, IRL2703 and LM338K

Discussion in 'The Projects Forum' started by nerdegutta, Jan 11, 2015.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi.

    I'm working on a powersupply, and I would appreciate some second thoughts.

    Background:
    I got a Raspberry Pi B+, and a D-Link router. The Pi works on 5v at appx. 1200mA. The D-Link works on 7.5v at 1500mA. They are connected with standard network cable. The router is set up as a DHCP.

    I'd like to make this batteryoperated, and I've drawn the attached schematic.

    Schematic:
    The circuit is planned working from a 12v car battery, that's connected to X1. The LM78L05 regulates the volt from 12v to 5 volts, and power the uC. This is a PICAXE 08m2. The uC, checks the voltage from the 12v battery, using ADC. If the voltage is more than 11v, the uC sets pin C.2 high, which turns the IRL2703 on, lighting the green LED and opens for power to go to the two LM338. These in turn regulates the voltage to the Raspberry Pi with 5v, and the other regulates the voltage to 7.5v to the router. If battery voltage is less than 11v the red LED goes on and stays on.

    The circuit has a ICSP pin header to program the uC. This is the PICAXE program:
    Code (Text):
    1.  
    2. ' Read volage on C.4
    3. ' If voltage is greater than 11v then set C.2 high
    4. ' If voltage is less than 11v then set C.1 high
    5. ' PICAXE 08M2
    6.  
    7. #no_data
    8.  
    9. symbol OutputPinFail = c.1
    10. symbol OutputPinOK = c.2
    11. symbol ControlPin = c.4
    12. symbol AdcValue = b0
    13.  
    14. init:
    15. low OutputPinFail
    16. low OutputPinOK
    17.  
    18. main:
    19. do
    20.     low OutputPinFail
    21.     readadc ControlPin, AdcValue
    22.     if AdcValue < 210 then
    23.         goto BattLow
    24.     else
    25.         goto BattOK
    26.     endif
    27.    
    28. loop
    29.  
    30. BattLow:
    31.     low OutputPinOK
    32.     high OutputPinFail
    33. '    debug AdcValue
    34.     goto main
    35.    
    36. BattOK:
    37.     high OutputPinOK
    38. '    debug AdcValue
    39.     goto main
    40.  
    This will make sure the RPi and router have enough juice. (At least that's what I hope.)

    I'm not too familiar with voltage dividers, and Mosfets, so any thoughts and comments are welcome.

    LM338.png
     
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    If you want this to be battery operated then the 7805 (linear regulators in general) is going to be a poor choice. You would do better with a switching regulator. Lots of choices out there. TI has a couple that are very easy to wire up. Or the 34063 has been around for a very long time. Not the most efficient chip but tons of support on it. If you don't feel like wiring it yourself you can pick up one of those automobile USB chargers. That is what is usually inside.


    But you will have to "roll your own" or hack the USB charger to get your 7.5V.
     
  3. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    The idea was that the LM78L05 would power the uC, the uC would control the IRL2703, which in turn controls the LM338K. There's two LM338, one is configured to regulate 12v to 5v, and the other to regulate 12v to 7.5v.

    I'll check into that chip you mention.

    Thanks.
     
  4. takao21203

    Distinguished Member

    Apr 28, 2012
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    there are LM2596 PCBs which can output upto 2A

    its not so interesting to build these circuits yurself all the time, infact, its pretty boring and takes a lot of time.

    anyway, yu can get the blank ICs too + required part
     
  5. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    It might be simple and boring to you and that's ok with me. :) I added the uC, to widen my horizon. I'll check the LM2596 PCB. Thanks.
     
  6. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    It really doesn't matter how you configure them, those linear regulators will cause you to dump a lot of your energy in heat. Especially stepping down from 12V.

    That 34063 while old is really versatile. You can configure it to step up, step down or invert.
     
  7. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    I will order a few of them.

    Thanks!
     
  8. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Take a look at the datasheet first. You would need coils, caps and a sensing resistor. I guess you can always use a trimpot if you can't find the proper value.

    I assume you have a "dollar" store in your town? A place here called 5 and below. Everything there is at or under $5 US. They sell those USB chargers for the car. Everything you need will be in one of those except maybe the proper value sensing resistor.
     
  9. takao21203

    Distinguished Member

    Apr 28, 2012
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    But it cant handle 1.5 A

     
  10. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Rats I missed that in the requirements.

    Well the OP might also want to check out The RBs website

    http://www.romanblack.com/

    I will bet there is a switching regulator that will fit the bill.

    Also TI has a number of easy to use switching regulators with some pretty cool design calculators.
     
  11. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    I'll check the TI calculator. Thanks.
     
  12. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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  13. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Thanks, I've been playing around on TI's design tools and component guide.

    Great site!
     
  14. takao21203

    Distinguished Member

    Apr 28, 2012
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    Sure, did you use the IC some time?
     
  15. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    No idea what you mean, again.

    I have a 34063 right in front of me that I built. I have not yet tested the current limits of the circuit.
     
  16. takao21203

    Distinguished Member

    Apr 28, 2012
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    you'll find you dont get 1.5 Amps out, sooner than that, the IC gets very hot

    Even in optimal situation the efficiency isnt much greater than 80%, and theres is some limit what the small IC case can distribute. You dont want a sizzling hot IC perhaps.

    From my experience 0.5 Amps is the absolute limit you can do with the IC
     
  17. takao21203

    Distinguished Member

    Apr 28, 2012
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    1.5 Amps output is demanding even for the larger LM2576, which has a thick metal tab (TO220)
     
  18. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    I've played around on TI's web page and imported a BOM to my Mouser account.

    When I entered my criteria, one of the suggestions that came up was LM25085.

    Vin 11
    Vinmax 14
    Vout 7.5
    Iout 5

    It came with a nice schematic, and a BOM. :)
     
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