halogen lamp & PWM

[Thunderchild]

can I use a pulse width modulated signal to control the brightness of a halogen lamp?

can anyone tell me a bit about halogen lamps why are they used are they more efficient than incandecent ?

I have a slide projector and i want to control the brightness of the lamp.

oh its run on AC so does that mean I can use a simple triac circuit ? I think I need a 1 KHz signal though so that the fliker is not visible so I would need the PWM generator wouldn't I ?

 

[Sceadwian]

Halogen lamps have a filament made out of some kind of alloy that evaporates in the halogen atmosphere in the bulb and redeposits evenly over the lifetime of the bulb allowing the filament to run at a much higher temperature then standard incandescents which would eventually develop hot spots and blow at the same temperatures a halogen runs at. Yes, you can PWM them. As it's an incandescent bulb flicker shouldn't be too much of a problem. It takes a certain amount of time for the filament to cool down and even at 60hz if you use a triac to do phase control of the dimming you shouldn't notice any flicker, it'll smooth itself out, although I'm not sure about extremely low duty cycles. I don't know about efficiency but the color temperature is higher and it produces a broad spectrum so they're really good lights to use when you want good colour reproduction and high output. AC or DC it doesn't care so you could run one from a 555 using a FET to drive the bulb. The only disadvantage I know of is that they run really hot and the bulbs have to be kept immaculate because even natural finger oils will cause enough extra heat to be dissipated in the bulb itself to shorten it's lifetime. Startup current may be a problem and should be considered.

 

[Hero999]

Halogens are incandescent and are more efficient than normal incandescents because the fillament runs at a higher temperature.

Dimming halogens (although often done) isn't a good idea as at lower temperatures the halogen cycle doesn't work effectively enough and the bulb life is shortened.

 

[Thunderchild]

thanks for that yes so now i understand why my dad always told me not to touch car light bulbs, I don't need too low a duty cycle as it has to run the projector but I want to reduce the power a bit as its a bit bright and makes a lot of heat it has damages the fan which now slips causing it to slow down and let it all overheat.

 

[RODALCO]

Thunderchild,

You have to check that your fan still runs at the correct speed and runs not to slow or is slipping.
Projector fans assist with the forced cooling to dissipate the heat from the lamp.
Because of that slipping more heat is trapped in the projector by the lamp, which will eventually fail prematurely.
It can also damage your slides.

Regards, Raymond

 

[Thunderchild]

yes I intend fixing it I think the heat made the fan plastic soft and the central peice that goes over the motor shaft has "ripped" every time it runs the plastic softens again and the fan starts to wobble I can here is shaking about because it is now "off centre" (once hot) it starts to slip slows down and the games over, I want to control the lamp power because in a not dark room the light is sufficient so in a totally dark room its plenty and can be turned down using less power and making less heat I'm not to happy with this sucker using 280 Watts and 250 of that is just the light its running on the "economy" setting at 22 V instead of 24 but i think it can come down further.

I want to put glue on the fan spindle to repair the rip and rebond it firmly to the spindle and i suppose I had better put some sort of sheild over the spindle tin fiol I guess, I also think there is some kind of guard missing that leat air in from where it should not come rather than coming from the lamp causing a lack of efficienzy in sucking out the air its a centrifuge fan by the way

 

[Sceadwian]

You should probably completely replace the fan, if it ever fails completely the unit will self destruct, and the fact that it's been run when it's slipping hints that it's been run over temperature, a safe bet is replacing it won't hurt and will likely help, even if you decide to implement a dimmer on the main bulb. Mind you this being on a projector with other frequencies being produced flicker may occur. The flicker of a halogen not being noticeable is only from direct human persistence of vision. You might see subtle banding depending on how the LCD of the projector is updated.
I didn't think about the lower temperature of dimming a halogen would shorten it's life, but after Hero's post it seems obvious, because the 'regeneration' of the filament doesn't occur at a lower temperature when the element isn't 'gassing' to redeposit itself. I don't know if the reaction is actually evaporation or sublimation but I always found halogens an elegant upgrade from traditional incandescents.

 

[Thunderchild]

this is an old slide projector not a pc projector,

the bulb is set onto economy by means of a plug which has the appropriate terminal shorted, the manual says that 24 volt is full power and the bulb lasts 50 hours but there is also a 22 volt tap on the ransformer for the economy setting and the manual rekons that on this setting it will run at 70 % power and last 200 hours.

I guess todays halogens are cheaper than when this was made in 1977 (I think), the problem with replacing the fan is a) getting one and b) having to strip all the slide changing mechanism out to get the motor out and get it all back together all those lovely springs and leavers to get back in exactly as they came out !

the other thing is if this fan went a new one will too I think the issue is that the centre of the fan goes nowhere it just spins on itself and is the point at which airflow is 0 so it takes all the heat, the edge of the fan is spinning fast so gets cooled down. its not totaly wrecked yet so I will fix it firmly and then sheild it with foil so that the foil will block the heat and help dissipate it, its one of those centrifuge fans shaped like a cup

the other thing is I could just use a lower power bulb ! I suppose it would still not last longer but would advoid making alterations to the projector as its 30 years old I'd rather keep it in its original condition