power consumption of light bulbs
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  1. #1
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    Default power consumption of light bulbs

    i have these "cornknob" led bulbs. they draw 0.045 amps at 231v. so 10.4 watts. measured with multimeter. if i plug them into a socket type watt-meter it shows the same amps and volts but 4.5 watts. what does this mean?

    i assume it might have to do with the crude capacitor power supply. but how much power do the leds consume in the end?

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    Possibly a very dumb attempt at an answer, but could it be one reading is peak when driven from an AC source, and the lower one is reading average power (reduced due to the actual "area under the curve" of the sine wave)?

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    No way to know. There's some sort of switching supply that has a <1 power factor. A multimeter can't tell you power. A true watt-meter can, but only the total power of the thing. Does it have a power factor readout too? Hopefully the power supply is reasonably efficient, maybe 85% or better, so if you can get total power, assume 85% goes to the LEDs and the balance is lost as heat in the power supply.

    See power triangle- What is a Power Triangle? - Active, Reactive & Apparent Power - Circuit Globe

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    Quote Originally Posted by dian View Post
    . what does this mean?
    It means you need to understand power factor.

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    The inverter power supply is hungry for some power too. The test probes would have to measure the output but then the package would have to be opened, and probably ruined.

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    Quote Originally Posted by jim rozen View Post
    It means you need to understand power factor.

    True!

    "An LED bulb" is not a "known device".... there are lots of types, with all sorts of internal power supplies.

    Yours probably has a step-down SMPS. It likely has some form of capacitor and rectifier as the first part of it, which will have a "leading" power factor lower than 1.

    Just going by the numbers you give, it appears to have a power factor of about 0.45, which would be considered "low", but not extremely low.

    Some have enough LEDs that they use a constant current IC to drive them (sum of voltages is high enough to make losses reasonable). That type probably would have a higher power factor.

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    as i mentioned its just a cap reducing the voltage. i didnt realize this could have such a low power factor. so do i understand correctly, the power meter is right and 4.5 watts are consumed by the device? then lets assume an efficiancy of 80% roughly 3.5 watts goe to the leds. so 350 lumen would be a realistic expectation?

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    Ah, I missed the part about series capacitor, was not sure what "crude capacitor power supply" referred to.

    The entire point of the capacitor instead of a dropping resistor is to use "lossless impedance" instead of resistance, to drop voltage.

    So the capacitor is of a value to insert significant impedance in series. That means it is "acting like a capacitor" (rather than being large enough that it is more like a short to the 50 Hz) and that characteristic will be the main thing presented to the power line as a load.

    Capacitors have the current "leading" the voltage, inductors have it "lagging". So the capacitive power factor of the lamp can be helpful, since most loads on the power line are inductive. It cancels out a small portion of that.

    Essentially, none of the voltage drop across the capacitor "counts" when figuring power used. Your power meter is probably correct (to the degree that is properly made and calibrated).

    The lumens depends on the efficiency of the LEDs, which varies wildly depending on the LED.

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