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5 volt supply straight from 240v AC mains

Discussion in 'Embedded' started by techie_alison, Jul 13, 2006.

  1. Hi,

    Please may I ask what the arrangement is when you see a single LED powered
    straight off of a mains power supply without any transformers or switch mode
    circuitry? In other words, totally uninsulated or regulated.

    I have an old computer with an external hard disk which needs about 30
    seconds to spin-up before the computer. With a small timing circuit, 555,
    or using a PIC even (have dozens) after a set time a relay would be set,
    thus powering on the computer. A 7805 could be introduced to take into
    account the voltage swing. Half wave rectification could result in 120v

    This doesn't need to be insulated from the outside world, safety is not a
    concern, just that roughly 5v should be available for the small circuit and
    the 3amp relay.

    Any ideas?? Just interested to hear of how this is done. Or would it be
    easier to just buy a small tordial TX and make the box a bit bigger?


    techie_alison, Jul 13, 2006
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  2. techie_alison

    Tom Lucas Guest

    I think you'll find that that is a neon lamp and not an LED. LED's
    connected directly across the mains will give light in the form of fire.
    Why not buy a 5V supply to run the electonics off rather than mess with
    mains yourself?
    SAFETY IS THE PRIMARY CONCERN! Mains is not something to be fooled with
    or it WILL kill you. Insulate everything or when you (or somebody else)
    is not at full concentration you will touch something and die. There
    should be fuses and other protection in the circuit as well. This is why
    you should seriously think about buying a supply and letting someone
    else handle the high voltage design - and the legal concerns that go
    with it.
    If it were me then I would use 5V from the Hard drives supply to trigger
    a PIC (but a 555 or an RC circuit would be just as good) to control the
    PC's power switch. If the PC has an ATX supply, then great because you
    can stay low voltage and use a simple relay to replace the PC power
    switch (remembering that only a pulse is required to simulate a buton
    press). If the PC supply is AT then the power switch is mains and you
    could use a mains relay switched by 5V but you need to be sure that the
    terminals are properly insulated on the mains side.

    Seriously, your tone doesn't sound like you are giving mains electricity
    the respect it needs. I know enough people who have died from electric
    shocks and each time it was because they thought safety wasn't a
    concern. Be careful.
    Tom Lucas, Jul 13, 2006
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  3. techie_alison

    Peter Guest

    Half wave rectification would NOT result in 120V and a 7805 can't drop
    anywhere near that much voltage. If you don't know this then it would be a
    very bad idea for you to make any live circuit. I recommend that you use a
    wall wart for your 5V.

    Having said that, a mains rated capacitor in series can be used as a sort of
    voltage dropper followed by a rectifier to get low voltage. I actually found
    a circuit like this in a Russian battery charger from the 60s. Yikes! You
    get small size at the expense of safety and I don't think that its

    Peter, Jul 13, 2006
  4. Typically a series capacitor is used with a capacitive reactance
    sufficiently large at the mains frequency to limit the current to a
    desired value. Due to harmonics and other high frequency noise usually
    present on the mains, the capacitor reactance at these high
    frequencies are quite low, thus quite large peak currents could flow,
    thus, it is a good idea to also include a series resistor.

    These kind of circuits only makes sense with very low current demand,
    since at higher currents, the capacitors become quite large.
    In order to minimise the load current and hence the required capacitor
    size, it would be a good idea to use a relay with a large coil voltage
    (and hence low coil current) driven by a high voltage open collector
    output or a separate high voltage transistor.

    Paul Keinanen, Jul 13, 2006
  5. Mmmm, hi Paul

    Thanks :)

    I think I'll stick with a small toroidal. Much more in line with what I've
    done before, just had seen the LED concept out there before and wondered how
    applicable it was.

    techie_alison, Jul 13, 2006
  6. techie_alison

    Don Guest

    <standard disclaimers re: HVAC apply>

    Put a bridge across the mains (rated at the appropriate
    peak to peak voltage plus margin ~300V+).

    From the + output of the bridge, feed the anode of an LED.

    Connect the cathode of the LED to the collector of a high
    voltage NPN. Put ~22K from collector to base.

    Tie the base of this first Q to the collector of another
    NPN. Tie the base of that second Q to the emitter of the
    first Q.

    Connect base of second Q to second Q's emitter through
    ~330ohm. Connect emitter of second Q to bridge's - output.

    Make sure all your components can handle the rectified
    line voltage. And, you'll have that full rectified line
    across the first Q with Iled flowing through it so make
    sure it can dissipate the power.

    Adjust the 330ohm for Iled/brightness.

    If you don't know what you are doing, then don't do it! :>

    Don, Jul 13, 2006
  7. I still have a US made battery charger from the 60s that is composed
    * 2 safety interlock switches to disconnect both sides of the line when
    the lid is opened.
    * a rectifier.
    * a 4 watt night light bulb.

    Marc Guardiani, Jul 13, 2006
  8. techie_alison

    Jim Stewart Guest

    Yeah. Sounds like a good idea.

    In my youth I had an early model airplane R/C
    set. The battery pack was charged with an on-
    line charger.

    I can painfully remember touching the wrong part
    and doing the chicken walk.
    Jim Stewart, Jul 13, 2006
  9. Go to Microchip's website and look at the AP note TB008 "Transformerless
    Power Supply". The example is for 115 VAC, but the concept is the same.

    Donald Harris, Jul 13, 2006
  10. techie_alison

    Chris Jones Guest

    The relay will draw a substantial amount of current and it is challenging to
    make a circuit that can supply this much current from the mains without
    wasting (and having to get rid of, as heat) more than a watt (and that's if
    you use a 48V relay too, low voltage relays will be worse.) A small
    transformer would be a nicer solution. By the way, a "solid-state relay"
    will draw less current than the coil of a mechanical relay though it may
    not like charging the capacitors of a switched-mode power supply.

    If you must have something cheap, would it suffice to get rid of the relay
    (which I guess was to apply power to the computer) and then open up the
    computer and insert a circuit with a PIC or whatever to hold the computer
    in reset for 30 seconds after the computer's power comes on? That way
    there is no new mains wiring, just a bit of logic. If you can't open the
    computer for whatever reason, is there a connector on the computer which
    provides +5V from the computer power supply, and also a reset signal which
    can be pulled low to reset the computer? I am guessing that this is a
    *really* old computer, correct me if I'm wrong.

    Chris Jones, Jul 14, 2006
  11. techie_alison

    John Perry Guest

    Tom Lucas wrote:
    Actually, the neon lamp will explode :). Unless you're careful to get
    a neon _indicator_, which will have the necessary current limiting
    resistor built into the package.

    John Perry
    John Perry, Jul 14, 2006
  12. techie_alison

    Simon Guest

    If you must have something cheap, would it suffice to get rid of the relay
    Or add a capacitor/resistor combination across the reset jumpers that will
    pull the reset signal low for however long you need to keep the PC from
    Simon, Jul 14, 2006
  13. techie_alison

    Neil Guest

    Actually Some power strips use and LED. And a large resistor.
    Neil, Jul 14, 2006
  14. techie_alison

    jasen Guest

    there's 5V on the disk drive power connector plug (inside the external case)
    there's 12V there too which is probably better suited to driving a relay
    with mains contacts. 555s will operate off 12V, and can drive many relays

    jasen, Jul 14, 2006
  15. techie_alison

    default Guest

    Piece of cake if you know the current consumption of your circuit. I
    run LED's and small circuits directly from the 120 VAC 60 HZ mains all
    the time.

    Use a capacitor to drop the voltage. - no heat and no energy wasted.

    You take your circuit - measure the current and voltage it needs.
    Subtract the voltage from the mains supply voltage and calculate a
    "dropping capacitor" for the voltage you want to drop with the current
    you need.. Use the formula for capacitive reactance.

    Put the cap and a 100 ohm 1/2 W resistor in series with one side of
    the mains - the resistor is to limit inrush current as the cap charges
    and works as a fuse if the cap shorts. That goes to your four diode
    full wave bridge rectifier.

    Here I use ~.47 ufd/250 Volt caps for lighting a LED or two or three
    in series from the 120/60 mains

    If you are just interested in using an LED on the mains supply - you
    can just bypass the LED with a small diode to keep the reverse voltage
    from getting over .6 volts instead of using a rectifier or use back to
    back LED's and light two with no rectifier.

    The cap should be a non-polar type with an AC rating for the mains
    voltage or 3X the DC rating.

    Don't use the circuit with no load - the output voltage will be high.

    When powering circuits you have to take into consideration your load -
    if you're driving a relay, for instance, the current will be much
    higher when the relay is energized. So in addition to solving the
    capacitive reactance for the relay current + circuit, you also have to
    give it some means to prevent voltage overshoot when the relay is not
    energized - a simple shunt regulator with a single 1 watt zener may be
    all you need -

    I posted the data from the Siemens 1990 Optoelectronics data book
    "Operating LEDs on AC Power, Appnote 6 " on
    alt.binaries.schematics.electronic a couple of months ago

    Another way to steal a little bit of power that may come in handy for
    your disc spin up timing - a current transformer. Just add a few
    turns of wire to a transformer with an open core (like a toroid or
    side by side bobbin transformer) and put the load on the low voltage
    windings - A trick we frequently use to turn on solid state relays
    when an X-ray machine or other dangerous piece of equipment is running
    - to sound horns or work warning lights. A little trial and error
    involved - but that ties the output to a load - whenever the load is
    drawing current the output is there.
    default, Jul 14, 2006
  16. techie_alison

    Byron A Jeff Guest

    First off it's a dangerous one. A non isolated, non stepped down, non regulated
    circuit connected to 240V is a terrible accident waiting to happen.
    Why? Any old wall wart you have laying around can safely bring the voltage
    down to a safe level.

    There's no apparent reason as to why you wouldn't want to take the much
    more safer route.
    Unless you have a cost concern here, there's no justification for throwing
    safety out the window. None whatsoever.
    If you are determined, Microchip has a Technical Brief for a transformerless
    power supply. You can find it here:


    This is dangerous. The brief says that it's dangerous. I'm telling you that
    this is dangerous and that you shouldn't do it. It's a safety hazard and a
    fire hazard and there's no good justification.

    My suggestion: DON'T DO IT!

    'nuff said.

    Byron A Jeff, Jul 14, 2006
  17. techie_alison

    Byron A Jeff Guest

    Absolutely. BTW you can connect an LED directly to the mains under a
    couple of conditions:

    1) You use sufficient current limiting.
    2) You make sure to wire your LED with another diode (which can be another LED)
    in antiparallel configuration. This limits the reverse voltage for each of the

    I rigged up a cheap 120VAC motion sensor by tying the LED of an optoisolator
    directly to the 120V light fixture of motion sensing lights. I used a 10K
    2W resistor and a reverse LED as a local indicator. The optoisolator converted
    the dangerous 120VAC into a safe optoisolated 12VDC.

    BTW the current limiting applies to neon bulbs also.

    Byron A Jeff, Jul 14, 2006
  18. techie_alison

    Mark Fortune Guest

    If memory serves, I have seen this done once with an LED using a pair of
    resistors as a voltage divider - it might have even just been the one
    resistor. Personally I think it was cheap workmanship though. This will
    work with an LED, although it will flicker as it's only illuminated for
    less than half the time, I think it can work with only one resistor but
    only when you know the resistance of the device it is powering.
    For hard disks this is a non option - as thier resistance varies wildly
    as it spins up, settles, and does all its internals stuff, and the
    resistors would have to be big mothers as hard disks draw a hefty
    current. No no no no no
    forgive me if I seem a little dense here... but if it's an external hard
    disk, does it not have its own regulated power supply? could this not be
    utilised in some fashion? but what sort of interface is it? ide, scsi,
    parallel? that might give us some clues as to what would be a good solution.
    safety is always a concern.
    I need a bit more information on your set up really, but I can envisage
    two possible scenarios:

    scenario 1) you have a true external hard disk, in an external hard disk
    caddy with all the wiring and gubbins and some sort of parallel
    interface whotnot. The drive is powered independantly of the computer.
    If this is the case I would utilise the regulated 5v line powering the
    hard disk

    however, I think scenario 2 is more likely:

    scenario 2) You have a hard disk that is external of the computer -
    connected via an IDE lead and the computers power supply. You need the
    hard disk to power up before the computer - although I dont know why at
    this stage, but i'll pretend for now there is good reason for it. If
    this is the case, you NEED a good regulated power supply for the drive,
    and not just the 5v line if its a 3.5" drive - 12v will be required as
    well or it wont spin up. 2.5" disks generally only require the 5v line.
    Your best bet is a regulated 5v/12v power supply - which should be
    fairly simple to construct. then simply run the timer circuit off that
    Mark Fortune, Jul 14, 2006
  19. Check out this:

    Step-down rectifier makes a simple dc power supply

    Link was picked from http://www.epanorama.net/
    Tomi Holger Engdahl, Jul 14, 2006
  20. You could use a 12K (12000 ohm) limiting resistor in series the the LED to
    limit the average current of the LED to around 20ma. The 12K resistor will
    need to be at least a 5 watt unit and will get pretty warm during operation.
    You will also need a bypass diode hooked across the LED with the anode of
    this diode to cathode of the LED and vise-versa to limit the reverse voltage
    of the LED.

    The avoid the loss of power and subsequent heat generated by the resistor
    you can use a 0.22 uf capacitor in place of the resistor. Make sure the
    voltage rating of the capacitor is at least around twice the peak value of
    the AC or around 600 volts. It must be a non-polar capacitor (most
    electrolytics are polar).

    C is calculate from the following formula: C = 1/2*Pi*f*Xc
    this formula is an algebraic variation of the capacitive reactance formula

    We use to use this technique to limit current for indicator lights in 440
    volt power distribution switchboards.

    NOTE: You can still get shocked by this arrangement if either or both of the
    diodes open. 20ma of current across your heart is enough to kill you. Don't
    even try this if your not familiar with the proper electrical safety

    The safest technique, by far, is to use a small, step-down transformer
    because of the increased level of isolation it offers.

    Dorian McIntire, Jul 15, 2006
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