Another rms post

Discussion in 'General Electronics Chat' started by inwo, Oct 6, 2014.

  1. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    I'm measuring input power in a cheap led bulb.

    I can't post the complete circuit here.

    However 48 leds are in a series string with a 51 ohm limit resistor. (after power supply)

    A ball park calculation is all I need.

    Scope across 51 ohms shows ~ 50% duty cycle 5.1 volts peak.
    So its a 120 hz strobe light? MVC-011F.JPG

    Do I need to measure wave form across led stack?

    Average voltage is ~66

    When I work backwards 100ma X 66volts = 6.6 watts. For 50% duty cycle that would be 3.3 watts.

    Exactly = the measured input power with wattmeter.

    ps.
    These are 13 watt bulbs.
    Scope shows a little higher, but I'm playing with input voltage.
    5.1 volts is ok to use for purpose of discussion.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    It sounds like you are already there. What more are you looking for?
     
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  3. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
    315
    I think I see the light.

    With 5.1 peak volts across 5.1 ohm being 100 ma peak, and the peak voltage during conduction across 48 leds being 144 volts.

    The 13 watt rating is peak.

    Very deceptive.
    Now to decide whether to let it slide with bad feedback or dispute.


    "
    Voltage: 110V

    Consumption: 13W

    LED Brand: CREE
    LED chip model: 2835 SMD
    LED quantity: 48 pieces
    Light color: Warm white/ white
    Lumen: 1550-1750L"
     
  4. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
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    Sorry, just looking for any major errors, as seller disputes my claim.
    Didn't know if guessing at 50% was even close, from the wave form.
    I think I now understand where seller is coming from.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    Oh I get it, you bought a "13W" bulb that draws 3W?

    FWIW, I agree with you that it is a dirty trick to use peak power. It is not conventional to quote peak or instantaneous power, since the perceived output and cost of operation both depend on the average power. Nobody cares what the peak is.
     
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  6. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
    2,433
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    Thanks,
    At first, I was asking for math check.

    Now, an honest opinion of my perceived scam was helpful. :)

    Actually I'm looking for high lumen per watt bulbs for a continuing application.

    I got samples of 20 or so.
    I'm learning what to look for. However some still outright lie. :( IMO
     
  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,519
    if you ever care to convert an amp signal with an arbitrary waveform to its true rms, consider using this chip. It will give you a linear analog output that you could later feed to an ADC. It's an expensive chip, but it's worth every cent.
     
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