Standard lamp dimmer has a threshold at low light levels..why?

This is something I have benn curious about and I should probably know but I don't so I have Googled but don't find anything...

I'm talking about an ordinary household lamp dimmer connected to an ordinanary incandescent light bulb.

Starting with it all the way off, if you slowly turn it up, nothing happens at first, then you get past the threshold and the bulb suddenly comes on at say 25% brightness. Now you can turn it back down from there to say 10% brightness. Why does the system exhibit this threhold effect? In other words, you can't simply turn it up to 10%, you have to first go up past the threshold and then back down.

Is it related to the resistance of the bulb changing?

Would the same thing happen with a constant resistance load?

Mark

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Mark
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The extremely simple circuitry in cheap dimmers consists of an RC delay (the potentiometer and a capacitor), with the capacitor voltage across the series combination of a diac (or similar threshold-with-hysteresis device) in series with the triac gate. Often, the diac is integrated with the triac, so you'll just see a 3-terminal semiconductor device, the pot and the capacitor. Anyhow, this circuit retains a different amount of voltage on the capacitor from half-cycle to half-cycle, depending on whether the previous half cycle fired the triac. This 'snap-on' behavior was first described, IIRC, in the old GE and RCA thyristor manuals, along with slightly more elaborate trigger circuits that prevent it.

Paul Mathews

Mark wrote:

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Paul Mathews

It might have something to do with the "fact" that initially the filament is "inactive" and once you turn it on it requires less power for the filament to stay "active". Basicaly the heat generated by the filament increases the avg electron energy so that once you get the electronics on that higher energy state it doesn't require as much energy to keep them there. I'm not sure if this is the real reaosn but makes sense to me ;)

Jon

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Abstract Dissonance

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Please don\'t top post.

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Reply to
John Fields

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No.
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John Fields
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the key is in the diac

thanks John thanks Paul

Mark

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Mark

I found that top post quite informative and brief. The quoted post wasn't particularly long and only one general point was being addressed so it would've been useless to do otherwise!

Tim

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Deep Fryer: a very philosophical monk.
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Reply to
Tim Williams

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No, the key is in the additional circuitry.  Notice that the same
DIAC is used for the circuit which exhibits hysteresis and for the
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John Fields

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