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Problem with 5v power supply

Discussion in 'Power Electronics' started by bigone5500, Oct 16, 2014.

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  1. bigone5500

    bigone5500

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    Apr 9, 2014
    I have built a simple power supply using a 7805CT regulator, a 47uF cap, and a 104 ceramic cap. I also have a red led with a 100 ohm resistor for power indication.

    I have it connected as follows:

    5v PS.jpg

    My problem is when I apply 12v to the regulator it gets hot fast. If I disconnect the LED then it's ok. I have screw terminals for the input and for the output.

    *edit* schematic updated.
     
    Last edited: Oct 16, 2014
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Try redrawing your circuit:
    • The 0V rail (negative side of input and output voltages) should run horizontally only, all the way along the bottom of the diagram;
    • The regulator should have its input on the left and its output on the right;
    • Positive voltages should be higher up in the diagram than the 0V rail;
    • Input voltage should be shown;
    • Part number for the regulator should be shown.
    • All text should be horizontal and read from left to right.
    Once you've done that, the problem should become clear to you. If it doesn't, repost it here.
     
  3. bigone5500

    bigone5500

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    Apr 9, 2014
    Even though the schematic is not a standard one, it should be clear as to how I have it connected. I can redo it but I do not know what I am doing wrong. I will post an updated one.

    *edit* ... LOL! HAHA! I see what did wrong on the schematic. However, I do have it connected right on my PCB.
     
  4. davenn

    davenn Moderator

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    hi Jared

    still 2 main things wrong with your schematic
    1) you didn't follow Kris's point 2 .... as a result your PSU is back to front ( rotate it 180 deg with input on the left side of the diagram )

    2) your LED is around the wrong way

    3) not a major problem but ... C2 should be labelled 0.1uF not mF

    4) I would up C1 from 47uF to at least 220uF or more


    Dave
     
  5. bigone5500

    bigone5500

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    Apr 9, 2014
    Ok. I fixed it. But as I said before, I have the regulator connected correctly on my PCB. If I take the led out of the circuit, then I get 5.03 volts at the output. The regulator does not get hot. With the led it gets warm fast.
     
  6. bigone5500

    bigone5500

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    Apr 9, 2014
    Here is the board.

    tmp_19316-20141015_2023401233591454.jpg
    tmp_19316-20141015_202415281189861.jpg
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Are you sure the input voltage is 12V?

    Are you sure the LED's series resistor is 100Ω?

    Assuming the LED's forward voltage is about 2V the LED current will be about 30 mA. This will cause the regultor to dissipate about 0.2W (if the input voltage really is 12V) which will make it detectably warm but not noticeably warm with that heatsink on it.
     
  8. bigone5500

    bigone5500

    712
    120
    Apr 9, 2014
    Input is a sealed lead acid 12v 20AH battery. Resistor is brown black black.

    tmp_25662-20141015_2043311233591454.jpg tmp_25662-20141015_204425-1281189861.jpg
     
  9. davenn

    davenn Moderator

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    is the LED actually lighting up ?

    what happens to the output voltage of the reg when the LED and resistor are connected
    does it drop significantly or stay ~ 5V ± 0.1V ?
     
  10. bigone5500

    bigone5500

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    Apr 9, 2014
    LED lights up when connected to the battery. Output voltage is 4.97v.
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Brown black black is 10Ω
     
    hevans1944 and davenn like this.
  12. bigone5500

    bigone5500

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    Apr 9, 2014
    And with that I will go and hide...
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    Doh ... I didn't even pick up on that
     
  14. bigone5500

    bigone5500

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    Apr 9, 2014
    HAHA! Neither did I!!!

    Heck, I was just testin' ya!
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    You may have damaged the LED too. 300 mA is probably too much.
     
  16. bigone5500

    bigone5500

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    Apr 9, 2014
    And I thought it was a blinking LED...
     
  17. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Ho ho ho! Like the little Christmas tree lights? Must be a pretty good LED if it somehow goes into thermal shutdown at 300 mA and then lights up again after it cools off. Oh, wait, you were kidding us about the "blinking," riiiight?

    This must be the time of year to replace LEDs. Working late into the evening, I burned out two of them on the same day this week, one because I read the color-code wrong on a 1/4 watt current-limiting resistor, another when I accidentally shorted two wires together on the breadboard, probably connecting the LED directly across the power supply rails. The 12 V 1.5 A voltage-regulated power supply didn't even hiccup, but the resistor was toast, as were the two LEDs.

    Good thing they were just two out of a hundred or so I purchased for about five bucks (late in the last century) from Mendelson Electronics, a surplus store here in Dayton. Lessons re-learned: (1) wear the magic magnifying 2X four-diopter OptiVISOR and (2) use spaghetti insulation over those pesky long leads of the resistors and capacitors until they are transferred from the breadboard to a circuit board. <sigh> This was so much easier twenty years ago. Plus, I was allowed to ride my motorcycle then.
     
  18. bigone5500

    bigone5500

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    Apr 9, 2014
    Ok. I got around to 'repairing' the resistor that someone colored wrong. So I put a 100 ohm in and the regulator doesn't get hot anymore. The temp is very negligible. So...I am wondering if it really needs a heatsink.
     
  19. hevans1944

    hevans1944 Hop - AC8NS

    4,547
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    Jun 21, 2012
    That depends on what kind of a load you plan on using. Or did you just want to light up the LED (which I presume it does now)?
     
  20. bigone5500

    bigone5500

    712
    120
    Apr 9, 2014
    At most it will power an Arduino mega 2560.

    And yes the LED is shining nicely.
     
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