Project: Simple Power Supply

Discussion in 'The Completed Projects Collection' started by T.Jackson, Dec 8, 2011.

  1. T.Jackson

    Thread Starter New Member

    Nov 22, 2011
    328
    14
    5VA Linear Variable Regulated Supply


    • 1.25 - 15VDC (good for a few hundred mA on the middle of the scale)
    • Uses safe low-voltage mains plug pack
    • Low-cost and simple to build
    • Regulator IC is near bullet-proof with internal overload, thermal, and short circuit protection
    [​IMG]

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    Last edited by a moderator: Dec 10, 2011
  2. Wendy

    Moderator

    Mar 24, 2008
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    Mr. Jackson is a talented fabricator and a decent technician. Unfortunately he could not help making personal comments about other users, which resulted in his banning.

    I wish him well, and hope he can find a home someday were his creative talents can be exercised in full.

    This is his 3rd entry in the Completed Projects forum. I like all his submissions, they are clear, concise, and meet every requirement stated in guidelines with a minimum of fuss. If you want to know how it is done you need only look at his examples.
     
  3. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Hum! A project without instructions? It should have more specs at least.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Try looking on the top of his post.

    I've been regularly rejecting those that are only You Tube videos. You may complain, but it meets all criteria stated.

    It is also an excellent project for beginners.
     
  5. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    I've read all the post, but those "specs" are not enough. It doesn't tell, for instance, the maximum current. Although accessible for beginners, it might be hard to follow for those, since there is no PCB layout or, in this case, a layout for the perfboard.
     
  6. atferrari

    AAC Fanatic!

    Jan 6, 2004
    2,644
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    Having joined on November this year, he managed to make 330 posts. Wow!

    Never ready one from him.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Do you remember how ticked off you were about Audio Guru? Now you want to continue this tradition? You are talking about someones submission who can not come back.

    I have no problem with the entry as is. I even rate it superior in many ways, there is plenty of information to duplicate it. It is not going away, and given the circumstances your criticism is unwarranted. Whom do you expect to respond anyhow?

    If I were doing this project (and I have a power supply that is remarkably similar) I would have added a fuse. Mine also has a meter, but that is a preference.

    I am one of the few who do draw their protoboard layouts, I have never seen anyone else do this. A schematic is considered adequate, it is the one thing I consider necessary. The other pictures do show how it is laid out. There is more than enough information for a rookie to successfully duplicate this project.

    To be completely blunt, I have approved this entry. It needs nothing else.

    *******************************************************

    Other than the correction of the schematic by Bertus. I missed what was wrong there, thanks.
     
    Last edited: Dec 10, 2011
  8. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Bill is right.

    But about the project, I doubt that given the size of the components you could actually get a clean 5 amp for very long ( if ever ). Should be fine for lower currents. This said without calculations or looking at spec sheets, so I may be wrong.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    My version is lucky if it exceeds ½A, but in general you don't need much else.
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Sorry if I was misunderstood. My criticism doesn't compare to Audioguru's. I was being constructive since I gave suggestions of improvement. Yes, I remember how ticked off I was, and I agree with you on that. However, you must agree with me on this: Audioguru always pointed secondary to non-existing "defects" and never gave a single suggestion of improvement. I didn't wanted to talk about this once again, but Bill, you were the one you mentioned Audioguru. This is only a disclaimer on my behalf. Hope you understand! Offence not taken, for sure! :)

    And I'm sorry if I was unaware that the OP wouldn't come back.

    The LM317T is prepared for a maximum of 1.5A, according to the datasheet. If it uses a 1A trafo, I would expect it to deliver no more than 0.55A, according to my calculations (Iac = 1.8 x Idc for a bridge rectifier). Iac is the RMS current considered for the trafo (not for the diodes). For the diodes, I usually consider the average current, with a safety factor of 2 (that is, 0.55A, since the average current for each diode is half the average current across the bridge, so the safety factor is effectively "cancelled"). The project is well calculated, as long as the current doesn't exceed 0.55A. Then again, correct me if I'm wrong.
     
    Last edited: Dec 14, 2011
  11. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,343
    Hello,

    A larger heatsink (as the one shown in the picture) would improve the capacity of this simple powersupply.
    The LM317 will shutdown when it gets to hot.

    Bertus
     
  12. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Yes, an heatsink will increase the current that the LM317 can deliver, if limited by the thermal shutdown protection. But besides thermal protection, the LM317 has a fixed current limiting little above 1.5A, perhaps 2A (another protection). So the current will be limited even if the IC doesn't get hot enough to enter thermal protection. Another limit (and a serious one) is the transformer. This is a serious one, because a transformer that delivers 1A AC RMS, ends up delivering about 0.55A DC after rectification and filtering. That is because filtering increases effective voltage with a cost, but also due to the form factor of the rectified wave. Both factors in conjuntion will give a factor of 1.8 proximately.

    So, my suggestion is to consider a trafo with more amps or VAs if you wish more current. I also recommend using slow-blow fuses both on the primary side and on the secondary side of the transformer. It will give valuable extra protection. Never use the current limiting capabilities of the LM317 (either using thermal shutdown or current limiting) to limit the current. It is not safe for the trafo.
     
  13. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    I thought the project was titled 5 amp supply but it was 5 VA, not sure what that even means, there is a Volt Amp ( sort of a watt ) but not usually used to rate a dc power supply.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    While not completely accurate think in terms of multiplication. I suspect the OP was referring to the transformer.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    The heatsink is way too small for more than 150mA at 1.25V. Maybe the fitting at the top of the case is for liquid nitrogen cooling when the voltage is set low.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    AG, you are better at math than that. At 150ma it is just a little under 3W for the case dissipation. The heat sink will help with that, and the case will probably need a few holes to keep it from becoming an oven. Liquid nitrogen is a bit over the top though.
     
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  17. cumesoftware

    Senior Member

    Apr 27, 2007
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    3W dissipation, considering that deltaT is 30ºC (Tamb = 40ºC and Tcase = 70ºC), will give a thermal resistance of 10ºC/W (just the expected rating for the heatsink shown). In a situation like that, the heatsink will be capable of doing its work without getting too hot. Sure the IC will give more than 150mA (perhaps 300mA) with that same heatsink, but it will get very hot. It doesn't need liquid nitrogen, though. That would be overkill and expensive (sarcasm).
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    The 16VAC transformer has a peak voltage of 22.6V and the full-wave bridge rectifier and filter capacitor produce 20.6V at the input of the LM317. The LM317 will have 19.4V across it when its output is 1.25V so with a current of 150mA it will dissipate 2.91W.
    Its chip will be at its max allowed temperature if it dissipates 1.9W without a heatsink outside the box. It will be too hot inside the box with the little heatsink shown.
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    Gee, I think I said that, but it is a far cry from needing LN2, don't cha think. Holes would probably suffice in the case. Any air movement would fix it.
     
  20. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    Nope! Thermal resistance junction to case is about 2ºC/W max. Case to heatsink is about 0.5ºC/W. The chip won't reach 125ºC. The chip will stay at 80ºC, and the case close to 70ºC (considering that the heatsink is stationary at 70ºC and the air temperature is 40ºC). That heatsink of 10ºC/W shown is good for 150mA.

    <snip>
     
    Last edited by a moderator: Dec 16, 2011
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