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Very low power, power supply

Discussion in 'Electronic Components' started by Chris W, Mar 17, 2005.

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  1. Chris W

    Chris W Guest

    I am looking for a power supply to power a small LED night light. I
    want it to run of the 120V AC and I need it to be very small. I don't
    want the completed night light to protrude more than 1/2" from the
    outlet when it is plugged in. I could probably live with 3/4" if I had
    to though. Since this is to power LED's it would be nice if it had a
    current limiting feature. It only needs to run 6 20mA white LEDs. So
    far the smallest I have found is about a 1" cube. It takes 120V AC to
    5V DC.

    Chris W

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  2. Don Pearce

    Don Pearce Guest

    You don't need a power supply. Put all the LEDs in series with each
    other, and with a 5.6k resistor. Get a resistor with a power rating of
    about 5 watts.


    Pearce Consulting
  3. Paul Burke

    Paul Burke Guest

    Or a 250Vdc/160Vac 0.47uF cap. (assuming 60Hz) (about 50p from Farnell),
    then it doesn't get warm.

    But either way I wouldn't be happy unless the LEDs were covered. Blow
    one up and there's two little prongs sticking out with mains on them.

    Paul Burke
  4. Hi Don,

    What a bad advice.
    Lots of wasted power, lots of wasted LEDs.
    LEDs dont like exessive reverse voltage, typically only about 5 volts.

    So better use a 470nF capacitor capable of 150V AC, a small bridge
    rectifier (needs only to survive the summed LEDs forward voltage) and
    a little 100 ohms resistor.

    (switch to fixed pitch font to view)

    --- 470 nF 150V
    120V AC |
    60Hz .-.
    | | 100R 0,25W
    | |
    | .-----. the LEDs
    -----| ~/ +|- ->|->|->|--|
    o-----------|~/ -|-------------|

    You can vary the LED current with the capacitor.

  5. You need some resistance in series to prevent the inrush current zapping
    the LEDs and/or the cap. For 120 V mains, 470 ohms 0.5 W may be enough.
  6. Ken Smith

    Ken Smith Guest

    Also: Make 2 banks of series out of the LEDs. Hook the two banks in
    reverse parallel.

    ! !
    --- V
    ^ ---
    ! !
    --- V
    ^ ---
    ! !
    --- V
    ^ ---
    ! !

    This way, the current has a path for both directions.
  7. mc

    mc Guest

    Does your night light have to be an LED rather than a neon lamp? An NE-2
    neon lamp can run directly from the 120-volt line through a 220k resistor.
  8. René

    René Guest

    or use 2 leds anti-parallel, or build de rectifying bridge out of
    leds, feeding a resistor or....

    (or I would buy a ready made one for a few $ in the local DIY shop:)
  9. Hi Ken,

    Well, this will work.
    But imagine what happens if one the LED fails open...

  10. Here is the usual way of doing this, with a capacitor to drop the
    voltage. You will have to connect all 6 LEDs in series. Make sure the
    capacitor is X2 rated for connection across the AC line, and all parts
    are insulated to prevent a shock hazard. The typical .47 uF X2 rated
    cap is about a half inch thick, but you could put two smaller, thinner
    capacitors in parallel.

    BTW, if you don't use a filter capacitor, the flicker becomes
  11. And because you forgot to include a 1N4004 diode, the 6 LEDs will be
    subjected to 160V peak in the reverse direction, causing them to
    breakdown since they're only rated for 5V reverse x6 = 30V.

    I don't think I'll be asking for your consulting services in the near
  12. Fred Bloggs

    Fred Bloggs Guest

    You have to run the LEDs off DC or you will never be able to get 20mA
    average current through them at 60Hz sinusoidal without significant
  13. I read in that Watson A.Name - "Watt Sun, the
    The reverse current is limited by the series resistor or capacitor. For
    the resistor, it's 90 V/5600 ohms = 16 mA. Whether that's acceptable or
    not can be found on the data sheet.
  14. Jim Gregory

    Jim Gregory Guest

    Red/amber low-current LEDs can glow at 1mA, but I'm not sure about the
    "white" ones.
    What's wrong with using a 120V neon indicator and integral dropper? It will
    last for up to ten years.
    Or go for a self-indicating, internally soft-lit translucent plug (used as
    nursery/corridor courtesy light)?
  15. The overcurrent isn't the problem, it's the flicker. Most white LEDs
    are rated for 30mA max, 100mA peak. But the flicker is really
    noticeable especially 60 Hz half wave.
  16. The usual white LED is rated at 5V max in the reverse direction, this
    arrangement far exceeds that voltage. As for power, the max dissipation
    is about 3.3V * 30mA or 100mW. 5V at 20mA would be 100mW, but the
    breakdown voltage is usually much higher than that, some say the LEDs
    will break down in the tens of volts range. In that case, the
    dissipation would far exceed the 100mW max. That's plainly not
  17. Rich Grise

    Rich Grise Guest

    A 5K, 2W resistor, and an antiparallel 1N4004. Put the LEDs in series.
    (and the 1N4004 in parallel with the series string, but opposite
    polarity.) 5.1K might be easier to find - it's not terribly critical.
    But it _will_ dissipate two watts, so you might want to use a 3W R.

  18. Rich Grise

    Rich Grise Guest

    Or, just get a low-profile night light:

  19. Ken Smith

    Ken Smith Guest

    If one LED fails, the circuit is broken. After the "BOOM" the circuit is
    still broken.

    Actually there is no "BOOM" the back biased LEDs will fail shorted and the
    series RC will still limit the current.
  20. Ken Smith

    Ken Smith Guest

    If you use two strings anti-paralleled, the flicker is very hard to
    detect. The frequency is 120Hz and most of the time there is light
    produced. The eye detects spikes in light better than dips.
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