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Low Power Embedded DC supply from 120Vac Mains

Discussion in 'Power Electronics' started by Fish4Fun, Jan 1, 2020.

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  1. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013
    For years, a recurring theme in my DIY projects has been a "cheap" DC supply from AC Mains .... and to that end Wall-Warts have generally been the answer ... But sometimes a Wall-Wart simply isn't a good answer ....

    Over the years I have spent many, MANY hours conjuring up cost effective DIY solutions ... and like most DIY solutions they all fundamentally failed in at least one (and generally more) of the important four categories:

    1) Safety
    2) Cost
    3) Reliability
    4) Efficiency

    Recently I have been working on a project that really isn't suited for a Wall-Wart and needs less than 75mA @ 5V .... the temptation to use a voltage divider + capacitor arrangement was strong! But before I wandered too far down the path of dangerous, unreliable and inefficient heaters, I checked ebay ... and guess what?

    Yep, for ~$1.40 each my problem would appear to be solved, and it appears (once they arrive and I can determine the spacing) I will be able to design my DIY project's PCB so that this power supply can simply be mounted (like a daughter-board) via 4 header pins!

    I am sure everyone reading this has known about these little modules for years and that I am the last to find out about them, but in the event there is anyone else out there who hasn't discovered them, I thought I would share.

    Happy New Year!

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Personally I'd prefer a closed type supply like this one.
    Possibly internally the same as the cheaper open type power supply, but more safe due to the insulating case.
    davenn likes this.
  3. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013
    Hey Harald!

    Thanks! Those are way nicer than the ones I ordered. I actually saw some modules similar to that, and agree 100% fully enclosed offers safety advantages. The particular project I ordered these for is cost sensitive (for absolutely NO good reason), and I plan to build at least half a dozen .... even though this project has been on the **round-to-it list** for a decade or so, with the Chinese New Year approaching, I felt pressured to order enough to complete the project .... I didn't want to order prototype versions and then be caught with a two + month lead time to build the second one, LoL.

    In all likelihood I won't even start building the first unit until long after the CNY has come and gone, but that is just one of the problems with A.D.D. .... that is, getting in a hurry so that when the time comes, proper procrastination can be employed ;-)

    The really sad part about this is that I figured out a way to "cheat" a low voltage source in this particular project ... after I ordered the power supplies, of course :)


  4. Bluejets


    Oct 5, 2014
    The link in post #1 shows an isolated supply which is fine, all-be-it of open construction.
    If enclosed in a plastic case as an example, no problem.

    In the #2 post there is no spec that I saw which showed it is isolated design and if used in any product with free access to the low voltage directly is mostly considered as "dangerous".
    Reason being any fault condition can put mains voltage directly in the low voltage circuitry.
    Many instances of cheap Chinese plug packs with this "non-isolated" design used as phone chargers for example, being fatal and therefore banned from use or sale in many countries.

    I have my suspicions but would be interested to see how you managed to "cheat " a low voltage from the mains.
    Last edited: Jan 2, 2020
  5. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

    Aug 27, 2013
    I want the 5V supply to power an ATTINY2313 to provide PWM dimming capabilities to an AL9910 used to drive a 3 series X 4 parallel array of LEDs, rated @ ~12Vfmax 900mA Imax. I realized somewhat late bound that I could use the AL9910's externally available internally regulated Vdd (max 1mA) for the pull-up of a PC817 photo-coupler. The optical isolation in turn allows me to power the ATTINY2313 from an existing voltage drop across one of the series connected LEDs.... Like this:

    AL9910 Floating Supply.jpg

    Another late-bound thought was to simply wind a secondary on one of the existing toroid inductors (my actual circuit uses 6 x 1mH toroid inductors in series to achieve the 6mH) and then rectify/regulate the secondary ... How many turns ??? I have no idea, start @ 20 or so and test it out ... worst case if the voltage drops out the PWM will stop and the LEDs will operate @ 100% of design Imax.

    While in theory 100% current would be 4 * 900mA = 3.6A; experience with these "10W LED chips" has taught me that they operate much more reliably @ less than 250mA. The difference in brightness between 750mA and 250mA is barely perceptible while the difference in heat generation is tremendous ... The fixtures I built with no Dimming have a design current of 400mA (100mA per LED) and a typical Forward Voltage Drop ~28V (10.8W) and are considerably brighter than Identical fixtures with traditional bulb sockets and a pair of "good quality" 13W LED bulbs in them. As an added bonus, reducing the power consumption to < 1W/Chip extends their life considerably.

    Anyway, It will be a month or so before I get time to do any testing. By that time the $1.40 each 5V supplies I ordered will be here and I suspect I will just use them. My out-of-pocket cost on the components in either of the proposed solutions would prolly exceed the $1.40 5V "Big Red Easy Button" anyway, LoL.


  6. Bluejets


    Oct 5, 2014
    Yes, as I suspected, non-isolated supply.
    Refer to #4 and note the dangers.
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