lm338 power drop

Discussion in 'The Projects Forum' started by Danilo pedroso martins, Feb 18, 2016.

  1. Danilo pedroso martins

    Thread Starter New Member

    Oct 29, 2015
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    0
    I've buld a project using lm338 that will use some Amps, (I'll supply power for a Beagleboneblack, 7" Lcd by 4dsystems, and another pcb). for that, that 5Amps that lm338 can provide should be more than enough..
    so I bult this following the datasheet(my schematic is attached below) to give me 5volts i need.
    besides that in my project i get 12v In to output this 5V, because i need this same 12v to feed the other pcb, in parallel.
    everything looks nice as my pcb is on all alone with no things attached as bbb or anything. As output i have arround 5.12V, seems nice.
    But, when i attach my pcb to the lcd like a shield, the bbb start to boot up(i can see the leds), screen doesn't power up(only leds below).. after +-5 secs it suddenly power off and leds blink a little bit like the power is not enough.. i can see in the multimeter that my power is by this time sort of 3.78V and still dropping(oscope pic attached)
    I've read this forum and yes, I am using heatsink and a great dissipation unit :D
    things seems to not getting hot,
    resistors used in the lm338 are not smd
    maybe something missing?
    what can be wrong?

    thanks, in advance.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    Can you measure the output current as well as the voltage.?
     
  3. Danilo pedroso martins

    Thread Starter New Member

    Oct 29, 2015
    7
    0
    should i measure with the things plugged in? or just my project pcb?
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    Yes, measure the current at the output of the regulator, and connect each stage up one by one and note the output voltage and current.
     
  5. Danilo pedroso martins

    Thread Starter New Member

    Oct 29, 2015
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    I get around 480mA peak current until it drops, with everything attached,
    only the supply powered on i get 15mA(I have a 3mm led).
     
    Last edited: Feb 18, 2016
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    With everything on, whats the input voltage and current of the supply, unless your regulator is wired wrong, could be your 12v psu.
     
  7. Danilo pedroso martins

    Thread Starter New Member

    Oct 29, 2015
    7
    0
    hello, sorry for the delay, after everything is turned on, the voltage drops to 3.7x Volts and the max current i can get is 480mA.. maybe I should try another schematic, the "Precision Power Regulator with Low Temperature Coefficient(attached)", since i've used the example below.
    do you have any ideas for a 5V better power supply using the lm338 with a great current and no drops?
    my 12v power supply is no a problem since i tried in a pc power supply, and in siemens plc 24v power supply.

    thanks for your help
    really appreciate it
     
  8. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    There's nothing wrong with the first circuit diagram, it must be your wiring errors..,
     
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