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

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:

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

--- Please don't top post.

From:

"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."

-- John Fields Professional Circuit Designer

the key is in the diac

thanks John thanks Paul

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