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High schooler here reading on thyristors.

Videos online and books say they can be used as light dimmers. I assume that's because we can control the amount of current that can enter a load. But what got me thinking was: won't the light bulb continuously flicker because there's this part in the output signal where the voltage is just zero?

Or do text books assume that the frequency is so large that you hardly notice the lightbulb flicker? I've been searching for an answer everywhere in the internet, but none of them mention anything about it.

(I made a drawing here showing the normal signal. There's this tiny moment when it's flat zero, and I wonder if that is noticeable when connected to a light bulb)

Example waveform

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    \$\begingroup\$ yes, it will and does flicker. most incandescent bulbs cool so slowly it's hard to notice, but cheap LED bulbs make it visible. Look at a cheapo "LED filament" bulb, aka antique. Often just ~38 (or 75) LEDs in series, no other rectification; flickers like crazy. \$\endgroup\$
    – dandavis
    Nov 19, 2020 at 5:14
  • \$\begingroup\$ Ah, so they do flicker! Haha, thank you very much. The book failed to mention that concept of flickering at super fast speed, but thankfully I can put that question to rest. \$\endgroup\$ Nov 19, 2020 at 5:33
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    \$\begingroup\$ You really can't see the flicker especially because the glow in incandescent bulbs isn't due to current directly, but to temperature, which doesn't change nearly as quickly. \$\endgroup\$
    – Hearth
    Nov 19, 2020 at 5:38

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Here is few factors come together. Flickering frequency is 120 or 100 Hz. Human eye can see it only by side view, not when you look straight. Second, voltage changes, but current amplitude doesn't follow voltage exactly, because resistance of filament not constant, it depends of temperature. So, filament heats, current decrease. Wattage amplitude smaller. And third, the filament still hot and glows some time after voltage became zero. LEDs fixtures is different story. They fed by DC and brightness regulated using PWM on higher frequency.

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    \$\begingroup\$ " voltage changes, but current amplitude doesn't follow voltage exactly" From OP's waveform, Ohm's law is true in every single point. The time constant involved in filament heating and cooling is too long for it to care about the thyristor, just the RMS voltage applied to it and the current will match the voltage waveform in every point. \$\endgroup\$
    – winny
    Nov 19, 2020 at 11:29
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    \$\begingroup\$ @winny Ohms law can be applied to that circuit only in differential form. The resistance of filament in incandescent bulbs made from tungsten alloy and it is not linear. Cold filament has lower resistance and then heats up , resistance up and current decrease. \$\endgroup\$
    – user263983
    Nov 19, 2020 at 12:41
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    \$\begingroup\$ Does not matter. The instantaneous voltage/current relationship is linear. You are talking about the long time constant temperature dependency. \$\endgroup\$
    – winny
    Nov 19, 2020 at 13:31

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