Dual Power Supply

Discussion in 'General Electronics Chat' started by Diego Bonilla, Sep 19, 2014.

  1. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
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    0
    Hi i have a 12V switching power supply, but i need 12V, 5V and 3.3V.
    Based in this premise i design this circuit...i need opinions

    thanks for the help

    upload_2014-9-19_9-44-56.png

    somebody can help me?
     
    Last edited by a moderator: Sep 19, 2014
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    How much current do you need on each output?
    Keep in mind that the current taken from the 3.3 Volts regulator also goes through the 5 Volts regulator.

    Bertus

    PS No bumping within 24 hours please
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,047
    D3 may not really be needed unless reverse polarity hookup is a possibility. It does help drop the voltage a bit.
    As Bertus has noted, you may want to consider running the 3.3V regulator directly off the supply so that its current is not added to the 5V regulator. The decision depends in part on the current demand from each.
     
  4. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
    35
    0
    Hi Waynech

    Run the 3.3 regulator dont cause so much heat?
     
  5. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    These are 800 ma regulators. LDO = 1V
    OP seems to have followed instructions properly.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,123
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    Can you restate the question? I don't understand.
     
  7. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
    35
    0
    Excuse my english isnt very good.

    trying to say: put the 3.3V Regulator Direct at 12V; did not generate much heat dissipation in this IC(3.3vRegulator)?
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,123
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    Yes, whatever the current demand is for the 3.3V regulator, driving it from 12V will increase its heat dissipation compared to driving it from 5V. However moving it to the 12V source will cause the same amount of extra heat to be removed from the amount that must be dissipated by the 5V regulator. So the question is, which chip do you want to take the heat load? If the 5V regulator is already warm because of the current demanded at 5V, adding the 3.3V heat might overdo it.
     
    Diego Bonilla likes this.
  9. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
    35
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    What chip you recommend me?
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,123
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    You haven't said what your specifications are.
     
  11. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
    35
    0
    So

    This is the recommended Circuit?

    upload_2014-9-19_13-52-44.png
     
  12. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
    35
    0
    approximately :

    5v - 500mA(max)
    3.3V-250mA(max)

    the 12V are 1.5A
     
  13. wayneh

    Expert

    Sep 9, 2010
    12,123
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    Option 1
    Heat dissipation in 5V regulator: (12-5) x 0.5 = 3.5W
    Heat dissipation in 3.3V regulator: (12-3.3) x 0.25 = 2.175W

    Option 2
    Heat dissipation in 5V regulator: (12-5) x 0.75 = 5.25W
    Heat dissipation in 3.3V regulator: (5-3.3) x 0.25 = 1.45W

    Personally, I would choose Option 1, your last schematic. Option 2 comes too close to the max rating of the IC, 750mA versus 800mA.
     
  14. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
    35
    0
    You refer options to the 2 schematics?
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Yes, but I gave them numbers in opposite order. Sorry. Option #1 = schematic #2
     
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  16. Diego Bonilla

    Thread Starter Member

    Sep 18, 2014
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    ok...so the best option are:

    [​IMG]
     
  17. #12

    Expert

    Nov 30, 2010
    16,284
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    That method keeps both chips close to the same temperature, compared to the other method.
     
  18. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi Diego,
    Looking on the web using your part no: AML1117 there a many different ready built AML regulator PCB's, some have heat sinks, others appear to have minimal AML1117 cooling methods.
    Which version of these ready built regulators are you buying.?

    E
     
    Diego Bonilla likes this.
  19. ScottWang

    Moderator

    Aug 23, 2012
    4,854
    767
    For a light load I will in series all the regulators, as this.
    In your case, if you want to connecting all the inputs from 12V, for the heat dissipation that I will using power resistor to bear it.

    Vmin_U1 = Vo+Vdrop = 5V + 4V = 9V
    Calculating the current limiting resistor.
    V_R-limit1 = 12V - 9V = 3V
    R-limit1 = 3V/500mA = 6 Ω

    Vmin_U2 = Vo+Vdrop = 3.3 + 3V = 6.6V
    V_R-limit2 = 12V - 6.6V = 5.4V
    R-limit2 = 5.4V/250mA = 21.6 Ω, you could choose 20 Ω to get a little more current.
     
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