Connect with us

A wall-wart alternative

Discussion in 'Home Power and Microgeneration' started by [email protected], Jan 5, 2006.

  1. Guest

    I finally got tired of replacing the 2 C-cells in a clock with a moving
    pendulum, so I measured the current (3 volts at 340 microamps) and built
    a charge pump like this, viewed in a fixed font:

    0.047 uF @400 V
    || |
    || | | |
    | | 3V
    120 VAC --- --- to clock
    ^ _
    0.047 uF @400 V | |
    || | |

    I used two caps in case one shorts and for some ground isolation. The diodes
    are 5.1 V zeners ($1.29 for 2 at Radio Shack) to limit the clock voltage if
    someone unplugs the batteries. This circuit supplies 390 microamps. I hope
    to avoid replacing the batteries until their shelf life runs out in 2012 :)

    This could be useful for lots of low-power stuff, eg smoke detectors.

  2. Guest

    How much will they overheat if overchaged at 3Vx50uA = 150 microwatts? :)

    Happy new year,

  3. It is, but it's a current-limited shock hazard. Never pass UL, but it
    works. Well, until the wrong cap shorts, then someone dies. 8*|
  4. Jeff Wisnia

    Jeff Wisnia Guest

    And at that low a current, why not ferget the caps and just use a couple
    of 150K resistors? The additional power loss will hardly spin your
    meter off the wall and you'll avoid the possible cap failure problem.

    Jeff (Ducking and slinking off...)

    Jeffry Wisnia

    (W1BSV + Brass Rat '57 EE)

    "Truth exists; only falsehood has to be invented."
  5. Guest

    It might pass UL, in its "double-insulated grandfather clock" :)

  6. Don K

    Don K Guest

    It seems counter-productive to build booby-traps into smoke detectors
    and the like.

    Unlike a wall-wart, it will require special precautions not to accidentally
    touch anything when you go to change the battery or maybe even set the time
    on a clock. You will never know whether a capacitor has shorted and whether
    you might be killed the next time you provide a path to earth ground.

    There's nothing to limit the current when capacitors fail, so that failure
    will cause a fire. There's no transient voltage protection, so the odds
    of capacitor over-voltage failure is pretty high.

  7. Vaughn

    Vaughn Guest

    A few points of data:

    1) Even transformers can (and do) short out.

    2) Switching power supplies do not necessarily have transformers, (more and
    more wall warts are switchers, they are far more efficient)

    3) Transient voltage protection can be added to virtually any circuit.

    4) With proper circuit design, there can be something to limit current when a
    capacitor fails.

    5) With proper circuit design and component selection, the danger of a capacitor
    failing can be infinitesimal.

  8. RoughRider

    RoughRider Guest

    See response point by point....
    "Vaughn" wrote in message A few points of data:
    They fail and usually the primary blows open. But short from primary to
    secondary is rare.... I've never seen it with wall-warts.... I have seen
    this with pole top distribution cans after a lightning strike.
    You are thinking about buck or boost designs that are used with DC-DC
    voltage conversion. While many of these also use a transformer for
    isolation, cheaper ones where the ground doesn't have to be inverted or
    isolation isn't required don't have a transformer, just an inductor.
    Anything connected to a wall outlet will have a transformer for isolation
    and to provide a ratiometric voltage reduction for the output side. And
    since it is switching at 100kHz or higher, the transformer has a ferrite
    core and is very small (unlike an iron core operating at 60 Hz).
    Sure. But that will require a fuse as well to open and protect the
    conducting MOV from self destruction. For the low current application
    described in this thread, a (diode) or other semiconductor based OVP would
    be more appropriate to be used with a resistor.
    True. This entire concept is a throw back to the
    transformerless/batteryless Rogers tube radio. Remember even old TV's had a
    HOT chassis. Every home with a TV or tube radio had one of these. I used
    to play a game with my sister by holding the metal TV tuner dial and an
    adjacent speaker lead from a radio. The harder one pressed, the more
    current would flow. The winner was the one that could make their muscles
    visibly twitch! But standards, liability, insurance, experience being what
    it is, we are a lot smarter these days not to make products this way.
    Of course. But many of us get components from a surplus store, so who knows
    why the caps are there in the first place. ??

    Bottom line is that the concept is electrcially sound. Is it safe? No. Is
    anything likely to go wrong? No. Remember, it takes a collection of
    mistakes to create a catastrophy.

  9. George Ghio

    George Ghio Guest

    Nick Pine + Tools = Danger.

    This is just another example.
  10. Vaughn

    Vaughn Guest

    You made a collection of pretty good points, which collectively fail to
    logically lead to your conclusion that the circuit is inherently unsafe.

    Bye yourself
  11. Guest

    And it has battery backup :)
    I doubt that. The diodes will probably open, and I used really skinny wire.
    Or diode damage. So... I added a 1K series resistor. This would also work:

    0.022 uF @400 V*
    || |
    || | | |
    | | 3V
    120 VAC --- --- to clock
    ^ _
    | |
    10K | |

    *Digikey's $29.95 M400-KIT-ND 400 V capacitor assortment contains
    10 of these, along with 10 of 14 other values from 0.001 to 0.1 uF.

  12. Clark

    Clark Guest

    Looks like a shock hazard to me.

  13. Guest

    My smoke detector draws 7 uA = 60x370C, which makes C = 315 puffs, eg 2
    0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
    sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.

  14. Bill_M

    Bill_M Guest

    Of course you don't have enough current for the audible device so your smoke
    detector should sit quietly by while you toast!
  15. Guest

    0.001 0.001 uF @400 V*
    || || |
    || || | | |
    | | 9V
    120 VAC --- --- to smoke detector
    ^ _
    | |
    10K | |

    My smoke detector draws 7 uA = 60Hzx2xsqrt(2)x120VxC, which makes C = 344 pF,
    eg 2 0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
    sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.
    No. That current would come from the 9 V trickle-charged battery.

    We might replace it every 6 years or so, if the alarm
    fails to shriek with the test button.

  16. Don K

    Don K Guest

    You only need 20mA of AC current to cause ventricular fibrillation.
    If the capacitor fails, someone could get killed touching whatever
    your circuit connects to.

  17. Guest

    0.0047 uF @400 V*
    || |
    || | | |
    | | 1.5V
    120 VAC --- --- to clock
    ^ _
    | |
    10K | |

    can trickle-charge a Seth Thomas quartz clock with hands and a AA battery
    ("Replace every 12 months or in case of suddenly inaccurate timekeeping.")
    The trickle-charged battery might last 10 years.

    The battery current jumps when the second hand ticks, but a 1K resistor
    and a 10K microfarad smoothing capacitor reveal an average 95 microamps
    = 60Hzx2xsqrt(2)x120VxC, which makes C = 0.0047 uF, one of the 150 caps
    in Digikey's $29.95 M400-KIT-ND. Their P10KBACT-ND 10K resistors cost
    $15.86/1000... 1N5231BD1CTND 5.1V zener diodes cost $13.65/100. Their
    number is 1-800-344-4539. Most orders are shipped in 12-14 minutes.

    This circuit uses about 1.5Vx95uA = 142 microwatts, ie 1.2 Wh/year worth
    $0.000125 at 10 cents/kWh. It's probably illegal. Don't try this at home.

  18. Guest

    Only with constant house fires.

  19. phatty mo

    phatty mo Guest

    Why not a small dc-dc converter,from your main 12/24/48V battery bank..
    Safer,and probably more efficient. (takes the inverter out of the
  20. Guest

    I don't have a battery bank.
    If your main inverter doesn't run 100% of the time, you might make the cap
    larger to charge the battery more when the inverter does run. This circuit
    itself is unlikely to make the inverter come out of hibernation.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day