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Dissapointed from a LED light bulb...

Discussion in 'General Electronics Chat' started by Externet, Dec 31, 2015.

  1. ronv

    ronv Well-Known Member Most Helpful Member

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    Wow, That's a thing of beauty .
    :cool:
     
  2. spec

    spec Well-Known Member Most Helpful Member

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    Wow again ... nice :cool:

    (don't want to be a kill-joy but have you checked to make sure you are not over driving the LEDs)
     
  3. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    Just a quick note.

    When modifying a commercial unit you should take a lot of precautions. There could have been a lot of hours of engineering spent to make sure it was safe, and there could be some unknown safety issues.

    At the very least i would try to find out what kind of LEDs they are for sure, then do some measurements.
    After the circuit is modified, check for overheating/hot spots to make sure it is safe, and the LEDs are not being over driven. LEDs that are under driven last a lot longer than those driven at their rated current. They dont burn out they becomes more and more dim as time goes on so it might be hard to tell if they are dying.
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Only in theory :D

    The specification for equipment changed, to a common 230V EU wide - however the tolerances allowed the UK to remain 240V and the rest 220V.
     
  6. mvs sarma

    mvs sarma Well-Known Member

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    MOst bulb manufacturers are just using a capacitor type ac to dc making and just adding a small series resistor as current limit.

    If real illumination efficiency is needed , perhaps led selection and pwm type efficient driver and proper orientation of the leds for a proper beam of light are main cafeteria IMHO
     
  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Here is a simple constant current LED circuit. Non isolated. 90-265Vac.
    I like this little PWM. More complicated than a bridge and current limiting cap!
    upload_2016-1-4_7-53-35.png
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Are commercial light bulbs safe when they are made cheaply "over there"?
    To promote saving electricity, my utility company gave away compact fluorescent light bubs for free. Of course they were made cheaply "over there" but they were all recalled and replaced because some of them dripped burning plastic. The Chinese company that made them used a copy of a certification label from a competitor so their light bulbs were actually never designed and certified to be safe.
     
  9. chemelec

    chemelec Well-Known Member

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    I Believe the Only Reason that LED Light uses a 1u5 Capacitor, Is for Safety.
    Even if the LED's should SHORT OUT, the Capacitor will Prevent an Over Load to the power line.
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    Hmm, interesting. I haven't checked the mains supply voltage in years.
     
  11. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The capacitor is the current limiter. When run at 220VAC 1/2 the power line voltage is across the cap. Changing the value of the cap directly changes the current flow. A cap is used in place of a resistor is to have no power loss while limiting current.
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Here it varies a lot: 80v to 120v rms.
     
  13. chemelec

    chemelec Well-Known Member

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    YES, I Believe that is What I Said.
    Just in a Different way!
     
  14. spec

    spec Well-Known Member Most Helpful Member

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    Hell- that's a wide range!
     
  15. Externet

    Externet Member

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    The capacitor drops the voltage the LEDs series does not, at the current that same capacitor allows to flow. If the LEDs series drop 110V, there will be 10V across the capacitor for a 120VAC mains. And the current depends on the capacity value.
    The capacitor must be calculated to provide its Zc as if it was calculated for a resistor. It just does nor heat-up.

    Vc + Vd = Vmains.
    Zc acts as a current limiter of the load.
    For Zc = 0; LEDs series must drop all the mains and be within LEDs specifications to avoid the smoke.

    At 60Hz:
    C=0.1 uF = 26.5KΩ
    C=0.5uF = 5.3KΩ
    C=1uF = 2.7KΩ
    C= 5uF = 530Ω
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    That would be true if the capacitor were feeding a pure resistive load, and thus feeding the load with a pure sine wave current, but that is not the case due to the non linearity of the rec diodes and the effectively 170V zener diode formed by the string of 51 LEDs. The reason why the capacitor does not dissipate any power, if it is a perfect capacitor that is, is that the current and voltage are 90 degrees out of phase, so to calculate the 'loss' caused by the capacitor you need to use vectors in the maths.

    Mention has been made of the safety aspects of the capacitor. In fact this method is not that safe- the capacitor is under great stress, especially when suppling the large gulps of current as there is effectively no resistance or inductance in series with the capacitor by virtue of the 120V supply low Z. Capacitors used for applications like this have to be the right type to stand the stress (polycarb rather than polyester for example) and have to be X rated types to comply with the safety rules.

    The vendor claims that the LED light complies with safety standards- Oh yeah. Compliance with safety standards is a long and expensive process, so I just can't see it. The light may have been designed with the intention of meeting the safety standards, but that is a differnt matter alltogether. The vendor also says the light will work from 85V RMS: theory and practice shows this to be complete nonsence.
     
    Last edited: Jan 4, 2016
  17. ronv

    ronv Well-Known Member Most Helpful Member

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    Even at 240 volts you would have been disappointed.
    At 240 there is about 50 ma average per string (about right) and about 7.5 watts. So 15 watts total. That would make it kind of like a 100 watt regular bulb. A lot better, but probably not what you were expecting.:(
    EDIT:
    About the same as 120v & 3 Ufd.
     

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  18. spec

    spec Well-Known Member Most Helpful Member

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    Nice sim.

    Presumably R2 & R6 represent the ESR of the capacitor, but why a value of 10 mili Ohms for the 1.5uF cap and 1 Ohm for the 3uF cap?

    The individual LED diodes are characterised at 20 mA or 30 mA depending on the actual type used, which is not known, yet you say that 50 mA would be dissapointing. Why is that (genuine question)?

    Out of interest, could you do a run on my second circuit to see what that would do? Why my circuits were not tried I can't understand- probably because of all the inuendo and lack of understanding. :mad:
     
    Last edited: Jan 4, 2016
  19. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    Yeah, it's just plain nuts. More usually it is between 90 and 120, with 90 more in the summer but it can go as low as 80 for a few hours. That's not all however, because it can drop even lower than 80 for a couple seconds, which causes air conditioners to shut off or go into their fan-only mode (no compressor) so you have to wait for the thing to start up again,which can take 60 seconds or more, and on a hot day that's a long time especially when it will end up shutting down again 5 to 15 minutes later.
    Worst electric power line regulation i've ever seen, ever.
     
  20. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    What third world hell hole do you live in?
    Do water buffalo turn the generator?

    LOL I think I know where you are. My comments still stand.
     
  21. spec

    spec Well-Known Member Most Helpful Member

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    We are spoiled in the UK: typically 235V varing by around +-2%, but some domestic wiring, which does not conform the the regulations, gives large voltage drops at the wall socket.

    Also, the mains plugs and sockets vary a lot in quality, not to mention the workmanship of the connections. It is common to find live and neutral swapped and the earth terminal screw not tightened sufficiently. Our wall sockes are rated at 13A but many are only good for around 5A. You often find plugs on electric fires steaming hot and turning brown.
     

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