100 watt dimmer circuit!!!...help

please help me with this hardware design. A circuit is to be designed such that the incandescent bulb(100 watt) connected to it reaches its maximum intensity and then dims off as slowly as possible. It should be taken care that that the circuit should not be manipulated once the switches connected to it are on.
i thought of solving this problem using a comparator. a refernc voltage is set and the capacitor slowly charges and once it reaches the max, o/p of comparator b'cums low which den disconnects the capacitor from vcc and the cap discharges & the intensity of bulb diminishes. but the problem i'm facing is that how can a 100 watt bulb be glowed using a simple dc voltage......can u give any alteration to my circuit so that the design is complete....plz help plz....

Resistors in series cannot drive a light bulb.
Your resistors and the lightbulb have such a low value that the capacitor would never charge and the light bulb would never light.

Your capacitor is in the wrong location in the circuit. It should have one of its wires connected to ground. Then the voltage will be divided by the charging and discharging resistors which will cause the output voltage to be half.

Use an emitter-follower transistor or a darlington transistor to drive the lightbulb. Their input resistance is very high (low current) so the capacitor will charge and discharge properly.

You forgot to say what is the voltage for the 100W lightbulb. If it is 120VAC then 120VDC will light it exactly the same.
But then the high voltage will need a divider so that it doesn't blow up the comparator.

the 100 watt bulb is to glowed using normal house hold ac supply. but if one end of the capacitor is grounded, then i think the capacitor will discharge very quickly during the discharging cycle. can u plz help me sir with ur modified circuit diagram. i would be highly obliged......

Hopefully you will find these links useful:

http://www.epanorama.net/documents/lights/lightdimmer.html
**broken link removed**
http://www.discovercircuits.com/L/lite-dimmer.htm

**broken link removed**
http://www.hobbyprojects.com/D/Dimmer_Circuits.html

Hope it helps!

I changed your circuit so the capacitor charges quickly to almost 120VDC then discharges slowly.
The darlington transistor drives the light bulb but it will dissipate a max of about 30W of heat during the dimming.

To my opinion, it would be meaningful to start with a specification of the voltage ramps. dims off as slowly as possible can mean seconds or hours. I also wouldn't expect, that the exponential capacitor charge/discharge accidentally gives the intended timing.

If the capacitor is big enough then the ramp will be so slow that you won't notice its exponential slope.

I would imagine that using µcu and fet or triac is out of consideration? This would be perfect job for something like pic 10F series... There could be even solution with standard logic that I would recommend over any analog one for this app. There are dimmer circuits that utilize UJT's for ramping or Vdc control. Is this schoolwork that requires specific solution or real project?

hi sinisia!!!.....this is not a mega project.....u can say its a mini skul project....what kind of logic do u suggest 4 me to solve the problem?.......

You could use kit like **broken link removed**and modify analog input to give you slow dimming down function. It is simple to implement and does not require too much experience. From circuit that you posted originally, i would venture out and say that you are absolute beginner in electronics. Because of that I would recommend keeping experimenting with circuitry on line voltage to bare minimum. Untill you master electronics better, I would recommend keeping your interest on voltages you can lick . This kit IS on line voltage, but because it is already proven schematic, you have better chance of finishing it without changing hair style.

If you would like to go that route, modification is very easy to achieve function you need. First you assemble unit as originally intended and test it. After that you modify control voltage input section by replacing couple of components and inserting a switch.