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Thread: 110 VAC LED circuit

  1. #1

    Default 110 VAC LED circuit

    I found this circuit years ago and wondered if anyone has an alternative and/or comments

    I know it is not the safest circuit but it does allow you to drive an LED from 110 VAC without a transformer and with very few parts.


    Click image for larger version

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  2. #2
    PJ Allen's Avatar
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    Default Re: 110 VAC LED circuit

    You don't need a resistor (make it all about reactance) or a bridge.

    Circuits are only as safe as you are.

    Click image for larger version

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    The cap should be rated > 200v

  3. #3

    Default Re: 110 VAC LED circuit

    I'd put two 250V caps in series in case one of them fails short. But other than that, PJ's circuit is simpler. The difference in performance is that his flickers at 60 Hz, whereas the bridge circuit flickers a 120 Hz. But even that could be mitigated by replacing the 1N4007 in PJ's circuit with second LED, oriented the same as the 1N4007. That way the forward voltage of each limits the reverse voltage the other one sees. DOING SO MAY INTRODUCE A FAILURE MODE, HOWEVER, THAT THE HIGHER-RATED 1N4007 IS THERE TO PREVENT.

    NOTE: Don't be fooled by the low voltage present across the LED(s). The voltages present ANYWHERE in this circuit compared with earth ground (i.e. your body) can still be LETHAL.

    -Phil

  4. #4

    Default Re: 110 VAC LED circuit

    Quote Originally Posted by Phil Pilgrim (PhiPi) View Post
    NOTE: Don't be fooled by the low voltage present across the LED(s). The voltages present ANYWHERE in this circuit compared with earth ground (i.e. your body) can still be LETHAL.

    -Phil
    My original circuit with the full wave bridge isolates the LED connections so it is probably the safest from the standpoint of contacts with the LED leads - correct?

  5. #5

    Default Re: 110 VAC LED circuit

    I have always liked to put a large, polarized, electrolytic cap at the positive and negative of the full wave rectifier, this will smooth out the voltage yielding a higher average voltage.

  6. #6

    Default Re: 110 VAC LED circuit

    Quote Originally Posted by RonCzapala
    My original circuit with the full wave bridge isolates the LED connections so it is probably the safest from the standpoint of contacts with the LED leads - correct?
    ABSOLUTELY NOT! Rectifiers provide zero isolation. If you want isolation, use a transformer.

    -Phil

  7. #7

    Default Re: 110 VAC LED circuit

    Thanks for your insight!

    - Ron

  8. #8

    Default Re: 110 VAC LED circuit

    Since this can be done so simply and inexpensively, it makes me wonder why manufacturers of wall switches and outlets still use neon lights for lighted switches.

    I have some high-end ground fault outlets and fan timers that use LEDs but they are pricey. Of course they require a neutral wire while a neon is fed thru the load.

    The darn neon bulbs always end up flickering. I'd much rather have a bluish LED wall switch than the orange glow...

  9. #9

    Default Re: 110 VAC LED circuit

    I've read that it's bad form to use an LED as a rectifier, that it's wise to install a series rectifier such as a 1N400x.

  10. #10

    Default Re: 110 VAC LED circuit

    Quote Originally Posted by PJ Allen View Post
    You don't need a resistor (make it all about reactance) or a bridge.

    Circuits are only as safe as you are.

    Attachment 83735

    The cap should be rated > 200v
    Hi PJ,
    could you give me a bit more detail about the current calculations involved in your circuit and the role of the rectifier diode?
    Fred

  11. #11
    PJ Allen's Avatar
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    Default Re: 110 VAC LED circuit

    FB,
    in re. my calculations:
    In ac circuits, capacitors and inductors have impedances (resistance in the time domain), capacitive reactance and inductive reactance.
    Capacitive reactance is XC
    The formula (elementary electronics):
    XC = 1 / (2 * pi * Hz * C)
    The line freq = 60 Hz
    XC = 1 / (6.2832 * 60 * 0.27uF)
    XC = 1 / 0.000102
    XC = 9803 Ω

    --> 120vac / 9803 Ω = 12ma

    in re. the recifier diode:
    It's there to divert the current from the negative alternation (half-cycle), keeping the LED from getting "back biased" into oblivion.
    As others have noted, LEDs make poor rectifiers; their PIV is in the range of... nil, practically speaking.

  12. #12
    prof_braino's Avatar
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    Default Re: 110 VAC LED circuit

    Quote Originally Posted by PJ Allen View Post
    You don't need a resistor (make it all about reactance) or a bridge. Circuits are only as safe as you are.
    How unsafe are LED christmas light strings? In January, I was able to buy strings of 25 to 70 LEDs of various colors in various configurations.
    A common configuration was similar to post #2, except there was only a single resistor and two strings of many LEDs in series biased opposite each other. .

    My Kill-O-Watt meter measures 0 watts for a single string, and measured IIRC 60millamps for 5 strings. This that many strings, it was approaching usable amount of light.

    I was thinking this might be an option for low power lighting, if one could get used to the weird colors.

    Is this circuit too dangerous, for say, under counter lighting? Would it be any more dangerous than incandescent lights using 120v?

  13. #13
    PJ Allen's Avatar
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    Default Re: 110 VAC LED circuit

    "How unsafe are xmas light strings? "
    If they're UL listed then that's pretty safe, about as safe as anything gets.
    [I checked the boxes of mine, many manufactures, all marked UL listed.]

    Incandescent (halogen) under-counter lighting is based on 24vac (because there's no code restriction for that), there's a step-down in the mix.

    You could still use the capacitive reactance approach with a 24vac step-down transformer. Try that with a 1uF (NON-polarised.)

  14. #14

    Default Re: 110 VAC LED circuit

    Here is a link to a thread I posted regarding harvesting cheap LED's from the Christmas light strings... Thread #6 has the actual schematic used in the "approved" off-the-store-shelf Christmas lights.

    http://forums.parallax.com/showthrea...ght-white-LEDs

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  15. #15
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    Default Re: 110 VAC LED circuit

    There have been lots of fires caused by christmas lights here is Oz. This is often due to faulty circuits coming from China. Just because they stamp compliance on something doesn't mean anything here. Of course, in Oz we have 240Vac and it is more lethal than 110Vac.
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  16. #16
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    Default Re: 110 VAC LED circuit

    Quote Originally Posted by Cluso99 View Post
    Of course, in Oz we have 240Vac and it is more lethal than 110Vac.
    My one trip to Europe so far was on business to install a product we sell stateside. Said product has a switching power supply, so compliance with British power was a non-problem; just change the plug. The power supply doesn't care about either input voltage or frequency.j

    So I'm working with the British liaison tech and I start to open the enclosure with the power on, and the guy FREAKS OUT and literally rips the plug out of the wall before I can get the enclosure's back off.

    ME: What's the big deal? I do this all the time.

    BRITISH TECH GUY: There's a heat sink on that power supply that's hot. Very easy to touch accidentally.

    ME: Well, I know that, it's bit me a few times too. No biggie.

    BRITISH TECH GUY: Yeah, but we have 240 volt power here. *EVIL GRIN*

  17. #17
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    Default Re: 110 VAC LED circuit

    (BRITISH TECH GUY: Yeah, but we have 240 volt power here. *EVIL GRIN* )
    '
    I would have replied "Yes but its only 50 Hz, The dwell time at zero crossing is a lot longer than at 60 Hz. Plenty of time to let go !!!"
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  18. #18

    Default Re: 110 VAC LED circuit

    $WMc% - Yes, but at 60Hz we have 20 more 'opportunities' to let go. :-)

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  19. #19
    $WMc%'s Avatar
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    Default Re: 110 VAC LED circuit

    @ Beau
    '
    This is true
    '
    Another reason to live here and not their!
    __Walt McDonald__


    It's not rocket-surgery
    I see why we don't have any water,All of the pipes are full of wires!
    E=WMc2
    Now with WiFi
    Not in the Spin Bunch
    import RPi.GPIO as GPIO

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  20. #20
    prof_braino's Avatar
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    Default Re: 110 VAC LED circuit

    Quote Originally Posted by PJ Allen View Post
    If they're UL listed then that's pretty safe, about as safe as anything gets.
    Incandescent (halogen) under-counter lighting is based on 24vac (because there's no code restriction for that), there's a step-down in the mix.
    I didn't realize you were such a trusting soul. Due to past experience, I assume all markings on all products from China are only guidelines as to what they hope you think are getting, until proven otherwise.
    RE: 24vac, good information but I was asking about regular 110-120v incandescent that has just chord-plugged-into-the-outlet type lights.

    I have two circuits for LED's. One is the same LEDs from the string and the same single resistor as on the original string, only mounted on a PCB project board.
    The other is similar to the circuit in #1 without R1and C1. There is a resistor (under 1K) in serial with the LEDs. The idea was to have enough LEDs so the voltage is as close to 110v without going over, and so a minimum value for the current limiting resistor could be used. The idea was that the least power would be dissipated as heat.

    The first circuit has been operating happily since xmas. The second is my own concoction. It appears to work but I didn't build it onto a board yet, I could not determine if I missed important. This is my context for asking.

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