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Dimmer switches

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sram

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I was just reading the howstuffworks article on dimmer switches. I think I understood 95% of it but I still have a question. In the modern dimmer switches, the idea is basically when( Or how quickly) to turn the circuit back on after it was turned off. Like you see in the article, if the dimmer switch is set to very bright, the circuit will turn on almost instantaneously after turning off. If the opposite is true, then it will wait till later in the cycle of the AC current sine wave. Now, why will the light be bright if the circuit is turned on quickly? It doesn't have to do with the amount of voltage applied. The peak voltage is always the same and it doesn't change(110 or 220 volts). Is it just because of how much of the time the circuit is on during a cycle?

So, if the circuit is on 90% of the time, the light will be bright because there is enough time for it to reach the necessary heat.

And if the circuit is on only 5% of the time, the light will be dimmed because there isn't enough time for it to reach the necessary heat to be very bright?

Is this what they are trying to tell us?

Also, is this triac thing common in many other circuits?


Thanks.
 

dougy83

Well-Known Member
The light brightness is related to the amount of power it receives; more power means a brighter output.

If the power is applied for a longer duration every AC cycle, then there is more power being delivered to the light and it will consequently be brighter.

If the power is applied for a lesser duration every AC cycle, then there is less power being delivered to the light and it will consequently be less bright.

TRIACs are generally only used for switching AC loads. This includes switching heaters, lighting, motors, etc.
 

sram

Member
The light brightness is related to the amount of power it receives; more power means a brighter output.

If the power is applied for a longer duration every AC cycle, then there is more power being delivered to the light and it will consequently be brighter.

If the power is applied for a lesser duration every AC cycle, then there is less power being delivered to the light and it will consequently be less bright.

TRIACs are generally only used for switching AC loads. This includes switching heaters, lighting, motors, etc.

Thanks buddy.

Oh my God I can't believe I forgot to put the howstuffworks link! Here it is anyway:

https://home.howstuffworks.com/dimmer-switch.htm


Okay, what you explained is just like what I said. But still!! I mean, the frequency of the AC current is 60 Hz. That is fast relatively, so it is like the circuit is on all the time, right?
 

dougy83

Well-Known Member
The 60Hz frequency is relatively fast, compared to what your eyes can discriminate. There's also the effect of the lamp filament having a thermal mass and therefore not cooling instantly - it certainly isn't able to cool enough to stop emitting light at 60Hz. This means the light output will be somewhat constant (with some ripple, which you won't see).

If you use a lamp that has a faster response, e.g. an LED, then you may be able to notice the oscillation by viewing it out the corner of your eye (the rods in your eye have better frequency response than your cones); alternately if you move your head or the lamp quickly you will see discrete pulses from the LEDs distributed across the path.
 

sram

Member
Oh wait, I think I got it. The tungsten filament or whatever it is takes time to heat. So, even if you apply normal AC current that is not chopped at all, it will gradually heat and be brighter consequently. The process is so slow that it can even be remarked by the naked eye, hence the modern dimmer switch solution is working properly.

In other words, if -somehow-a different filament is used which can heat and reach its maximum brightness instantaneously, the solution could be rendered ineffective. Am I right?
 

dougy83

Well-Known Member
Yes, the tungsten filament takes time to heat and to cool. You'll notice it when you turn a light on or off, e.g. car headlights or halogen bulbs will seem especially slow. Yes, the naked eye won't see the ripple (there will be ripple, but not as much as for a faster responding light source).

Using a faster responding light source will make running at 60 (actually 120Hz) less comfortable for humans due to the possibility of detecting the constant flickering. You will still be able to increase or decrease the average brightness using this method though. LEDs are usually dimmed (or multiplexed in the case of e.g. digital clocks or other multi-digit displays) at a much higher frequency than this; I believe the rate is generally in excess 300Hz.
 

sram

Member
The 60Hz frequency is relatively fast, compared to what your eyes can discriminate. There's also the effect of the lamp filament having a thermal mass and therefore not cooling instantly - it certainly isn't able to cool enough to stop emitting light at 60Hz. This means the light output will be somewhat constant (with some ripple, which you won't see).

If you use a lamp that has a faster response, e.g. an LED, then you may be able to notice the oscillation by viewing it out the corner of your eye (the rods in your eye have better frequency response than your cones); alternately if you move your head or the lamp quickly you will see discrete pulses from the LEDs distributed across the path.

I wrote my previous post before seeing this post of yours which is quoted. Okay, I think we are on the same page! I thank you very much.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
A tungsten filament is like a current dependent resistor. it's also about 15x less resistance in the cold state than operating. S, if you apply 10V (lets use DC for now) to the 120 VAC lamp no light might be emitted. Onece you heat the bulb at full power and THEN go back to 10V, you may have light.

Triacs turn off at zero current without a gate drive. For a resistive load, zero voltage and zero current occurs at the same time. They don't with an inductive load.
 

dougy83

Well-Known Member
S, if you apply 10V (lets use DC for now) to the 120 VAC lamp no light might be emitted. Onece you heat the bulb at full power and THEN go back to 10V, you may have light.
Can you explain how this is possible? If you apply 10VDC to a filament, its temperature will reach the same steady-state value irrespective of whether 120V was applied previously or not.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
The tunsten bulb has a really nasy Resistance Temperature curve. https://www.techlib.com/electronics/magic_lamp.htm The curve in the article doesn't even reach the lamp operating temperature.

Some temperature controllers actually have to be rated for tugsten wen used as a heater, so the resistance change can be taken into account.

I can't seem to mathematically show what's going on, only observationally. There is a good chance that the dimmer may be current limiting.

I just did an experiment with the dining room light, I turned on the lamp to full, ten reduced it until I could barely see the filiment. I turned it off, waited a few minutes and turned it on again and waited and waited and waited, Like 5 minutes. The filament never glowed.
 
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