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Cut Constant Glow in LED

Discussion in 'LEDs and Optoelectronics' started by Aadhithya M Sivalingam, Feb 3, 2017.

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  1. Aadhithya M Sivalingam

    Aadhithya M Sivalingam

    Feb 3, 2017
    The Case of the Frequency Generator:

    The frequency generator is coupled to an amplifier. And it is a whole setup. Even if one wants to use the frequency generator, the amplifier also needs to be switched on; though the knobs are kept at zero.

    Usually this frequency generator is used to modulate lighting of halogen lamps: Sine wave modulated and in the offset of 2.5 V. If the offset is reduced there is very less (than even one half cycle) ON period. The amplitude is always set somewhere in the range of 5-7V. There is a trade-off between the amplitude and the offset that can be set. This relationship is as given as below

    x + |y| = 10 where ‘y’ is offset and ‘x’ is the amplitude

    (All these information are hands on and first.)


    I tried modulating a 70W 220V LED lamp. It did modulate, but there is NO off state. To put it in other words, the off state is: a constant less intensity glow. Even when the offset is set to zero the lamp glows. No below-zero offset can be achieved. No matter how low (negative) the offset can be taken the lamp continues to glow (don’t forget the trade-off I mentioned above).

    I have been looking into possibilities that might be the cause for it.


    1. The output of the function generator (and amplifier) was connected to multimeter leads. With no output, it gives out a 70V (±5) output (RMS multimeter).

    2. On connecting a CRO: initially I viewed 50Hz AC which was moving up and down like a skipping rope whip. On zooming out we could see the wave become a line and then act accordingly as I modulate and set offsets.


    How to modulate the LED source with a proper negative half cycle (OFF STATE EXISTING)?
  2. duke37


    Jan 9, 2011
    I do not uderstand your description. Please provide a circuit diagram (schematic) so that we can work out what is happening.

    How can the amplifier with no output be giving 70V AC.
  3. Aadhithya M Sivalingam

    Aadhithya M Sivalingam

    Feb 3, 2017
    Sorry I do not have a schematic to provide. The reason being, I am working with packaged products, and not raw circuits which I can see. I will try to explain the circuit to you:
    A box packed with both Amplifier and Function Generator, and the amplifier is closer to the supply and the next step is the function generator. (I know the order because, the function generator cannot be switched on without switching on the amplifier first. And of course the reverse is not true)
    The amplifier has sockets to which light sources (or any equipment) to be modulated can be put in series with.

    And the 70: The amplifier is ON and only the function generator is OFF. There also happens to be a constant leakage
  4. davenn

    davenn Moderator

    Sep 5, 2009
    well show us photos of your set up
    show a BLOCK diag if you cant show a schematic

    you have to help us help you

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Better yet provide us with links to the devices you're using. We will be able to make much better sense from the manufacturer or supplier's descriptions.

  6. Aadhithya M Sivalingam

    Aadhithya M Sivalingam

    Feb 3, 2017
    Heat Source : LED
    This is a setup for Lock In Thermography (A non destructive method to analyze material for defects)
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