Turn the lamp On & Off gradually...

[Shahriar]

Hi

I Want to build a device which can turn on a 100watt lamp gradually and then turn it off gradually
and do this ever and ever. I emphasis that i DO NOT WANT To turn on or off the lamp suddenly,
But I want to turn it on and off GRADUALLY.

Any help is appreciated in advance.
Shahriar

 

[Kevin Weddle]

What you want to do is change the frequency. That is it. It is best to generate a new frequency based on a transitor with feedback. An oscillator if you will. Then just filter out the 60 HZ and you will be left with the frequency of the oscillator. This circuit is going to be used to control the lamp.

 

[audioguru2]

Changing the frequency of the AC feeding a lightbulb won't change the brightness one bit. Just like the variable frequency DC motor speed control project that we have that doesn't work.
A transistor doesn't work with AC that has ground as a center voltage.

You need a huge variable resistor in series with the lightbulb, or use a small variable Pulse Width Modulation circuit just like in a common light dimmer. Getting a dimmer circuit to slowly change its phase shift would be very difficult. It would probably be easier to rectify and filter the mains, and modulate the resulting DC with a slowly changing PWM circuit.

 

[surajbarkale]

What you need is a PWM modulated by a Traingular wave of low frequency. Feed this PWM to a TRIAC through sutaible optocoupler (e.g. MOC3041). Check out it's datasheet at http://www.datasheetcatalog.com/datasheets_pdf/M/O/C/3/MOC3041-M.shtml it has some cool circuits.

 

[audioguru2]

Hi Suraj,
Getting the externally generated PWM frequency sync'd to the mains frequency would be difficult and would need more isolation.
Forget having any mains isolation, nobody is going to touch the circuit.

I know what you mean but the PWM isn't a triangle wave, it is produced by the output of a comparator that switches at the coordinates of a triangle wave and a DC voltage.

How about using a light dimmer with its phase-control pot replaced by an LDR? A slowly dimming LED would control it nicely.

 

[sanpic]

Hi Suraj.
Do you have a sample of PWM with MOC3041 ?

Thanks.

 

[Alun]

Why don't you use a bridge rectifier, and run the lamp off the unfiltered DC, a MOSFET could be used to switch the lamp, and a standard PWM circuit could control the MOSFET. ;D

 

[prateeksikka]

hi audioguru!
i guess changing the frequency ideally wont have effect on the brightness but practically would have. I guess all of us know that on a 50 Hz mains supply a bulb turns on and off 50 times a second.it would turn of once a second if it were 1Hz.and turning on and off once a second cant be sudden!isnt it?
another approach would be to use a waveform which changes firing angle of a thyristor in every second,this way it would dissipate varying power in each cycle and hence glow with a varying intensity.
or a microcontroller controlled motor rotating a variac supplying the power to the bulb!
hey which one seems best?

 

[audioguru2]

Sorry Prateek,
A lightbulb turns-on at 100Hz on a 50Hz mains. It is trying to reach full brightness at each of the two peaks of the AC voltage, for each cycle.

An ordinary lamp dimmer changes the firing angle of a full-wave thyristor (triac) in each half cycle not each second. Early for full brightness and late for dimmed.

You wouldn't want a motor-controlled variac for dimming anymore. A thyristor dimmer circuit is smaller, lighter and operates cooler.
Have you ever seen the huge variable resistors used for dimming in old stage shows? Later they used huge manual variacs. Now they are all thyristors. ;D

 

[prateeksikka]

yes sir!
thats the age of miniaturization! ;D
certainly a thryristor controlled power supply would be better than a bulky motor for a variac.
we can certainly control thyristors through controllers (micro)
feeding power to the lamp!
this world is going to shrink one day!!!!1 ;D

 

[Ldanielrosa1]

I'd use a microcontroller with a TRIAC.  I'd have it periodically check the line frequency so that it could work in different regions (with different frequencies) and in areas with poor frequency regulation (or on a portable/emergency power generator). 

 

[Alun]

You could derive the clock frequency from the mains frequency using a frequency multiplier or even a PLL.