Very small 220V AC to 5V DC 100mA power supply

Discussion in 'Power Electronics' started by T3STY, Oct 4, 2016.

  1. T3STY

    Thread Starter New Member

    Oct 4, 2016
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    Hello Everyone,

    I have a 24V power supply (PSU from now on) with a load attached to it, inside my PC that I want to either turn on automatically when I switch on my PC, or to manually turn it on even if the PC is switched off. I will be using the PC 5V output to detect when the PC is switched on.
    I will use a 5V relay between the 220V mains and the 24V PSU, but I need a very small 5V PSU that would be able to keep the relay on. The relay will be requiring around 100mA at most (generally the requirement is around 70mA).
    Basically, I am trying to achieve the following (pseudo-)schematic:
    autoPower.png

    The 5V PSU does not require too much attention to filtering, regulation, precision, ect. As long as it keeps the relay turned on without flickering, it's good enough for me (it should keep the voltage between 4.5V and 5.5V though). And it will also be used for this only, single purpose; not even an extra LED will be attached to it.
    And obviously, I am posting because I need help in designing one that use as few, small and inexpensive components as possible.
    Do you have any suggestions?
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    375
    I think you can do this without the extra PSU. S1 switches the load either direct to mains or via the relay to the mains. The relay is connected to one of the PC supply outputs (5V or 12V) and will switch on the load when the PC is on.
    upload_2016-10-4_22-36-13.png
     
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  3. T3STY

    Thread Starter New Member

    Oct 4, 2016
    16
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    Thank you very much for replying!

    That was my idea at first as well, but I am not really sure about the safety of it. I mean for PC's PSU, not for me.
    Connecting and disconnecting contacts in a switch often causes sparks internally - small and for very short times, but sparks. Is there a possibility for any spark to "jump" into PC's PSU and cause trouble?
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,886
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    No. Even if there was, PC power supplies are exceedingly well protected from almost anything you can imagine - short-circuit, overcurrent, over-voltage, under-voltage, overheat...
     
  5. T3STY

    Thread Starter New Member

    Oct 4, 2016
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    Now that I am looking at your schematics, it seems I have also read it wrong at first. I see the relay is placed between the mains and PC's PSU, which totally avoids this issue.
    So my last question to this is about that relay attached directly to the PC PSU. I'm definitely thinking about using a solid state relay that does not have the inductor actuator like traditional relays. But in case I use a traditional one, is there any additional protection needed on PC's 5V line? Like the usual catch diode?
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,886
    375
    The diode is needed when a switch (or transistor) disconnects the coil from the power. That does not happen in this case. When the PC is switched off, the PSU output will fall and the back emf will just maintain the supply voltage for very, very slightly longer. There will be no large voltage generated.
     
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  7. T3STY

    Thread Starter New Member

    Oct 4, 2016
    16
    0
    Thank you very much Albert!
    I think I should have waited for my Aha! moment before posting (reading *this* article at the moment), but I guess I didn't want to wait too long.
     
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