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Thread: Low Voltage Dimmer????

  1. #1
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    Low Voltage Dimmer????

    What is the difference between an electronic low voltage dimmer and a regular dimmer. I don't understand why you need an electronic dimmer to dim a transformer.

  2. #2
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    A regular dimmer, by virtue of how it chops the AC sine wave to do the dimming action, imposes a DC current on it's output which can cause transformer-based devices into core saturation or severe overheating. (As in letting out the magic smoke.)

    An electronic type dimmer uses a different method of "modulating" the basic AC waveform so that it will have little or no DC current on its output.

    If I could find them, I would post a picture of the two different waveforms and you can actually see the difference. But a regular dimmer basically flattens the tops and bottoms of a sine wave, and if you draw that out on a plot of amplitude and voltage, you'll see that the flat tops portion maintains a steady-state value (read: DC) over a much longer time that a "straight" sine wave.
    Never fear, Slick is here.....

  3. #3
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    Quote Originally Posted by mxslick View Post
    A regular dimmer, by virtue of how it chops the AC sine wave to do the dimming action, imposes a DC current on it's output which can cause transformer-based devices into core saturation or severe overheating. (As in letting out the magic smoke.)

    An electronic type dimmer uses a different method of "modulating" the basic AC waveform so that it will have little or no DC current on its output.

    If I could find them, I would post a picture of the two different waveforms and you can actually see the difference. But a regular dimmer basically flattens the tops and bottoms of a sine wave, and if you draw that out on a plot of amplitude and voltage, you'll see that the flat tops portion maintains a steady-state value (read: DC) over a much longer time that a "straight" sine wave.
    A couple of dimmer output voltage waveforms.

    Leading edge:


    Trailing edge:


    Unless the dimmers are faulty, there should no significant dc component in the voltage.

  4. #4
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    Quote Originally Posted by Besoeker View Post
    A couple of dimmer output voltage waveforms.

    Leading edge:


    Trailing edge:


    Unless the dimmers are faulty, there should no significant dc component in the voltage.
    Thank you for you reply. That's what I was looking for.

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    I would classify a Variac or some variable impedance device as a non-electronic dimmer. Anything else is probably an electronic dimmer.

    A Variac or variable resistor (impedance) will not contribute a DC component to the output if there is no DC component in the input.

    Now consider a phase shift type of dimmer, whether leading or trailing edge. If the turn-on or turn-off angle is not the same for the positive half cycle vs the negative half, then you will produce a DC component.

    A DC component in an AC waveform applied to a transformer on either the primary or secondary side will add a DC bias to the hysteresis curve and produce a greater peak magnetizing current on one half of the AC cycle than the other half cycle. The effect is greatly dependent upon the shape of the hysteresis curve, and magnetic excitation level. The magnitude of excessive heating from the peaked current is a function of many factors.

    In addition to magnetic saturation is the heating from higher frequency components in the non-sinusoidal waveform.

    Note: a half wave rectifier and capacitor input filter on the output of a transformer produces a large DC component and peaked charging current. With appropriate design this can be made a reliable product. So DC in a transformer primary or secondary does not mean it won't work. It is design dependent.

    The volt-time integral of the voltage applied to a coil with a ferro-magnetic core will determine how far into saturation the core is driven. To avoid an unbalanced hysteresis curve steady state operating condition the magnitude of the positive and negative volt-time integrals must be equal.

    I have looked at the DC component of an inexpensive phase shift dimmer and it was not very large in comparison to full rated current of the dimmer.

    Can you provide information on what a so called "electronic dimmer" is in comparison to a standard inexpensive phase shift dimmer (probably meaning leading edge because these are cheap to make with a Triac).? Even a Triac dimmer with added circuitry could dynamically adjust over a cycle or so to achieve an average DC output of zero.

    I see no inherent reason a trailing edge dimmer is less likely to produce a DC component than a leading edge dimmer. However, it may be easier to design a circuit for zero DC output on a trailing edge dimmer. Here one could simply control the turn off point of the positive half cycle from the input control knob, and on the negative half control turn off to occur when the integral of the positive and negative half cycle currents become zero.

    .

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    Using Besoeker's great graphics :smile:

    As I understand it the above waveform would come from a standard inexpensive dimmer and if used with a transformer the fast rise on the leading edge creates an inrush current each time possibly overheating the transformer.

    Trailing edge:

    The above trailing edge waveform would be what you would want to use with a transformer as the rise time in 'normal'.

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    iwire:

    A step change in voltage to an inductor does not cause a step change in current. Instantaneously you can not change the current in an inductor.

    If I connect a battery thru a switch to a series L R circuit (inductance resistance), and the initial current thru the inductor at the time of switch closure is zero, then the current will be of the form (1-e^-tR/L). An exponential curve starting at 0 (1-e^0 = 1-1 = 0) and gradually rising to 1. The lower the resistance the longer the time constant.

    In contrast if there is current flowing in an inductor at the time of switch opening, then a very large voltage is generated upon opening because you can not instantaneously change the current thru an inductor. This voltage will be limited by something breaking down or some means of voltage limitation. For example a reverse biased diode across the inductor will provide a current path for the inductor current upon opening the switch.

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  8. #8
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    Gar, while I fully believe what your telling me, sadly I have no idea what your telling me. :confused:

    Was my post wrong? Was it right? I can't tell from your post.

    What is your take on the reason for a specific dimmer for use with transformers?

    My take was based on what I have read on the Internet but that is always suspect. :smile:

  9. #9
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    Bob:

    If a load has a series inductive component you can not have an instantaneous rise in current.

    Transformers themselves have a inrush problem due to being driven into saturation as the result of the residual flux state from the last turn off. For a mechanical switch controlling the input to the transformer the resulting inrush current is random and can be very large. This is somewhat unrelated to the Triac dimmer application.

    I will try to do an experiment with a Triac dimmer and a transformer and see if I can determine what may be the problem with such use.

    .

  10. #10
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    Here's some more information.

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