Dimmer
- Thread starter JdoubleU
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By clicking here. :grin:
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Really depends on the kind of dimmer- resistance, autotransformer, mag-amp, sct/thyristor, etc.
If you mean a generic wall dimmer, Google is your Friend.
If you mean a generic wall dimmer, Google is your Friend.
justdavemamm
Senior Member
- Location
- Rochester NY
Most work by reducing the voltage by taking out part of the sinewave voltage applied.
Power electronic devices like triacs, SCRs and IGBTs are commonly used.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
A typical dimmer works by delaying the triggering of a TRIAC (a special type of power transistor) during each half-cycle. The knob or slider varies the length of the delay. The bulb filament averages the pulses into a continuous glow.
Looking at the drawing above, the missing chunks of the sine wave are caused by the delayed triggering. This is what the waveform would look with the dimmer around 75 to 80% of full on.
Looking at the drawing above, the missing chunks of the sine wave are caused by the delayed triggering. This is what the waveform would look with the dimmer around 75 to 80% of full on.
Yes, it's a 45 deg delay angle which would mean the voltage was applied for 75% of the time. It reduces the applied RMS voltage to 95% of fully phased on value.
How this translates into actual light and perceived light level is altogether more complex.
cadpoint
Senior Member
- Location
- Durham, NC
Most work by reducing the voltage by taking out part of the sinewave voltage applied.
Power electronic devices like triacs, SCRs and IGBTs are commonly used.
My question is what not chop the top of the sine wave off or even lower it with the use of the sine wave ! ?
Well let me answer myself, you can't because the lamp needs at least the constant pressure of the voltage to be maintained, even though its a sum of Root Mean Square is still needed at some level for the lamp to glow.
Thanks for Posting that Bo !!! Thats' cool !!! I get it, I might not have described it right, but I get it...
Shoot, you just reduced the RMS, now what ?
What now?Shoot, you just reduced the RMS, now what ?
Conventional triac and SCR dimmers have a few drawbacks.
The waveform I gave is for that. It's sometimes referred to as a leading edge dimmer - you vary the timing of the leading edge.
This leads to:
- Poor (lagging) power factor
- High harmonic current content
- Audible noise
- They work only with particular types of luminaire, typically incandescent
Harmonics are the same but pf is leading.
Some dimmers have the capability of operating as leading or trailing edge.
And this can give benefits on systems where a number of dimmers are installed.
Imagine just two of them. The first conducts over the quarter cycle from 90deg to 180 deg. The second, from zero to 90deg. The second half cycle, 180deg to 360deg is similar. So you now have a complete cycle of sinewave current.
With judicious use of a combination of leading and trailing edge, problems 1 and 2 can be mitigated to some extent. But it can get messy.
This something I did a few years ago....
Sine wave dimmers are now becoming more widely used for more critical applications. They use PWM control, much like variable frequency inverters.
They solve issues 1, 2 and, to a large extent, 3 with minimal filtering and a switching frequency above the audible range.
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