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Advice about MC34063 based buck converter diagram

Discussion in 'Power Electronics' started by HellasTechn, Apr 10, 2019.

  1. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Hello to all forum members.

    I am working an a small project of mine that involves an arduino board and a few relays.
    My question has to do with the power supply section of my project.-
    The arduino board needs 5V to run and the relays 12V.
    I have a 12V 2A switching power supply. My intention is to use it to power the whole project. The relays can be hooked directly to it but power to the arduino board has to be stepped down to 5V and the current consumption is about 250ma more or less.
    I tried the 7805 solution with input and output capacitors but it gets really hot. after 5minutes of running it reaches tempratures about 60-70C while ambient is 22C.
    I am not even sure sure if that is normal...

    So i Googled for a simple buck converter diagram to solve the heating problem and came across this site:
    https://circuitdigest.com/electronic-circuits/12v-to-5v-buck-converter-circuit-diagram

    I am asking about cmments and thoughts since i know nothing about switching PSU's

    Thank you !!!
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    With 12 V input and 250 mA the power dissipation of your 7805 is 7 V*0.25 A= 1.75 W. Depending on the case type of your regulator this amounts to:
    TO220: Rthja = 65 K/W or a junction temperature of 114 ° + 22 ° = 136 °C. The max. allowed junction temperature would be 125 °C. Your regulator is clearly overloaded without cooling.
    You may add a heatsink to reduce the thermal stress, but that will cost space and money plus it is not energy efficient.

    Not a very good prerequisite for rolling your own switch mode converter. Even with today's moderns controller ICs this type of regulator can be tricky and requires careful layout for stable operation. The upside is you can learn a lot along the way...

    How about using a 7805 drop in switch mode replacement such as e.g. this one? Easy to handle (almost as the venerable 78xx chips).
     
    hevans1944 likes this.
  3. dave9

    dave9

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    Mar 5, 2017
    I would just buy a $1 buck converter board off ebay, or aliexpress, or wherever you prefer. It will cost a small fraction of what the individual parts would cost to build your own, and save hours of time for your first try. Here is one example, there are a dozen of them that would be suitable for 12V input, 5V output, at a fair margin over 250mA:

    https://www.ebay.com/itm/DC-5-24v-t...ep-down-Voltage-Regulator-Module/172825665907
     
    HellasTechn and Bluejets like this.
  4. Bluejets

    Bluejets

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    Oct 5, 2014

    Use them often...work really well.
    I usually hook them up to my power supply and adjust before fitting to the Arduino, avoids any smoke problem.:):)
     
  5. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I think i've got the point.
    I use a TO-220 7805 with a small heatsink on it. the project will be housed into a small ABS plastic box. I am concerned that after hours or days of operation it will turn the box into a miniature oven :p

    Yes . That is why i am asking for your opinions :)

    I am not really interested in the ebay modules mainly because i am going to build my own pcb for this project.
    The 7805 dropin is not a bad idea but i still want to make the converter myself.
     
  6. Bluejets

    Bluejets

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    Oct 5, 2014
    Best of luck with that.
    Have you seen the size of these things..??
     
    HellasTechn likes this.
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    In that case I suggest you go for the website of a manufacturer of your choice with good reputation and have a look at the available chips and the application notes.
    For your comparatively low power requirements lok for a chip with integrated power MOSFETs, that will save you some of the trouble of correctly driving the power MOSFETs.
     
  8. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Thank you Sir !
    The specific circuit i am asking for can definitely fit into a 2x2 inch board.

    Understood.

    I assume noone has experience with the MC34063 ?
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Sorry, no.
     
  10. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I am having trouble finding an appropriate coil for this application.
    The only information i have is that it needs to be 100uH and be able to handle say 500mA.
    On the other hand i could slightly modify the values of the components and instead of 100uH i could use a 220uH inductor that can handle 500mA.
    What kind of inductor should i use ? iron powder core? ferrite core ? smt or torroid ?
    I am sure there are people with experience in coils here :)
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Iron powder or ferrite should work equally well here. Typically ferrite cores are used.
    SMT or toroid are not really alternatives.
    SMT or THT would be alternatives. It depends on your preferences. THT is usually easier to handle by hobbyists, but if you're going to create a pcb, there's no reason not to use smt. SMT typically has the advantage of less stray inductance.
    Toroid would refer to the type of construction (in contrast to e.g. a cylindrical coil). I really suggest you use a toroiodal construction, ideally a shielded type (example) to minimize radiated noise. Observe closely the layout notes in the application section of the datasheet as layout is critical for a switch mode converter for good efficiency, high stability and low noise.
     
    HellasTechn likes this.
  12. dave9

    dave9

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    I just remembered that years ago I tore apart a Belkin F8V718 vehicle lighter outlet to phone charger adapter that used an MC34063, and had made pics of the PCB for it:

    [​IMG]

    [​IMG]
     
    HellasTechn likes this.
  13. HellasTechn

    HellasTechn

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    That looks useful !
    So it uses a torroid iron powder core inductor.
    But i suppose a shielded inductor would be better since the psu will be on the same board with the microcontroller.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It probably won't make much difference unless you're dealing with low level or analog signals.
     
  15. HellasTechn

    HellasTechn

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    I have no analog signals on the PCB. Should be ok with both.
     
  16. WHONOES

    WHONOES

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    Try the attached circuit. It uses a 555 timer + some other bits. It runs at about 112KHz which means that you can use a smaller inductor. Come back If you want to try it and need more info.
    Also see the graph which shows outputs. The Blue trace is the O/P volts, the Red trace is the switching O/P of the 555 and the green trace is the control O/P from the TL031.
     

    Attached Files:

    HellasTechn likes this.
  17. Bluejets

    Bluejets

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  18. JWHassler

    JWHassler

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    Dec 22, 2014
    Have you seen this?
    It's been kicking round for a while and I've used it often.
     
    HellasTechn and (*steve*) like this.
  19. HellasTechn

    HellasTechn

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    Ive been up to my ears with work and i am still. Ill play with it on the breadboard though !

    Yes it looks perfect. It's just that i want to build it on my own PCB together with the microcontroller.

    Actually i have not. It calculates all the values automatically. Super !

    I now have enough info to keep experimenting. I am going to get back to you with results and question !!!
     
  20. dave9

    dave9

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    Mar 5, 2017
    It could be on the same PCB as a daughter card, just put pins on your board to form a riser, solder to the I/O on the regulator board. That would work better with the one Bluejets linked than the one I linked because it would have pins on all four corners to support it.
     
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