Anyone have an OHP with a FXL bulb?

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Hi Eebasist,

When you say power suply, what exactly do you mean?

Most OHPs with FXL bulbs (in the US) use a single diode to half rectify the AC to a DC(ish) power line, and use a resistor as ballast.

Some have a slow start circuit to ramp up the line voltage over a second or so in stead of hitting the bulb with a thermal shock pulse, but if you were really worried about that, you could get a cheap 500W variac which could be used to achieve the same effect manually.

If you want details, check out the schematics available here:

Most dimmer switches are not rated for the high wattage - and many use triacs to do the dimming. I don't know if that is good.

If you can get the bulbs cheaply, you probably don't need the variac - it might squeeze a few extra hours out of each bulb, but probably not much more. I have seen variacs for < $50.

i'm an idiot.... half wave rectification is .7*120=84 not 1.4*60. Wish i would have kept my variac when i had one......i dont care about bulb life that much.....i'll just go to MH eventually or something. What value ballast resistor would you reccommend? I tried doing a voltage doubler on AC mains a few times for a tube amp and was told it isnt reccommended since there needs to be an isolation xformer for when you want to hit 500+ volts. Do i have anything to worry about at 84V with this scenario?
How does a dimmer work? I've seen ones rated at 500W so i'd try that if it could work
On looking at the schematics, I would say that the resistor is only used to reduce power using their hi/lo switch. I always wondered why these bulbs are rated at 82v...

Basically the diode and fuse should be enough...

I started with the sunsplash 4123 (the lower diagram) which uses an FXL. I found the bulbs going cheap for about $6.50 on yahoo shopping:

I had the variac lying around, and used it to soften the hit to the bulb at turn-on: the bulb's resistance is lower when it is cold, and it draws a current spike for a fraction of a second until it heats up. In that fraction of a second, it gets much hotter than normal, and is most likely to blow at this time...

I didn't notice much of an improvement in lifespan, though, and at ~ $6 a pop, it probably isn't worth the effort.

Bill-----its been a while since i did solid state work. Is a half wave rectifier simply a diode on one end of the AC? Thats what the schematic looks like, but it cant be that it? If not, just tell me what i need to do to do the half wave rectification.
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