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

M

Mark

Jan 1, 1970
0
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
 
P

Paul Mathews

Jan 1, 1970
0
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
 
A

Abstract Dissonance

Jan 1, 1970
0
Mark said:
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

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
 
J

John Fields

Jan 1, 1970
0
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

---
Please don't top post.

From:

http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

"Summarize what you're following up.

When you click "Reply" under "show options" to follow up an existing
article, Google Groups includes the full article in quotes, with the
cursor at the top of the article. Tempting though it is to just
start
typing your message, please STOP and do two things first.
Look at the quoted text and remove parts that are irrelevant.
Then, go to the BOTTOM of the article and start typing there.
Doing this makes it much easier for your readers to get through your
post. They'll have a reminder of the relevant text before your
comment, but won't have to re-read the entire article.
And if your reply appears on a site before the original article
does,
they'll get the gist of what you're talking about."
 
J

John Fields

Jan 1, 1970
0
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?

---
Yes.

See page 4 of:

http://www.littelfuse.com/data/en/Application_Notes/AN1003.pdf
 
T

Tim Williams

Jan 1, 1970
0
John Fields said:
Please don't top post.

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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