The Effects of Reflector Design and Lamp Orientation

[Dazzzla]

I don’t think powder coating will stand up to the heat from a MH lamp, unless you mean enamelling.

DJ

[juice_e]

I have a question about ballasts. I can get 2 x 250 watt mh ballasts for cheap. Could i wire them together in series and hook up 400 watt bulb? Is that do-able?

[Guy Grotke]

There was a thread on here about using multiple fluorescent ballasts and starter to run MH lamps. Not a good idea, but you might get it to work. If I was trying to do that, I would use a digital scope to capture the volatge and current of a standard 400 Watt ballast, cap, and igniter circuit, then see if I could match it. Just hooking them up with no knowledge of what you need is asking for trouble. These are not just simple inductors; They are constant power autotransformers with a specific behavior to the igniter actions.

You should also know that MH lamps are very particular. Get the voltages and currents wrong, and they won't start or won't keep running. They are power-limited by the ballast, so one that supplies too much power will burn the lamp out quickly, or even make the lamp explode! => =>

This is why the moderator killed the thread.

[juice_e]

aaaaa... good idea that i dont do that then .... ... is a MH bulb that is 11 " long ( M59) too big for a light source?

[Guy Grotke]

There are two critical lamp dimensions for a DIY projector:

1) Arc length. 25 mm is about the longest you should use. Lots of small double-ended lamps have 16 mm arcs. Really expensive 5 mm arc lamps work great but don't last very long.

2) Outer envelope diameter. Any more than 2" in diameter and you won't be able to get a pre-condensor lens close enough to the arc. Smaller is better because you can then use a smaller reflector and pre-condensor lens.

Total Length just limits how small your box can be.

[bodgy]

can i get away with 40mm arc length? i've seen people do it... I'm just sick of trying to find a perfect lamp (hard in aust), and im thinking bout this one:

The Phillips HPI-T PLUS 400W
(from: www.lighting.philips.com...subtab=dimension )
Length: 285mm
Width: 46mm
Arc: 40mm
Bulb Shape T46
Bulb Finish CLEAR
Watt (W) 400
Volt (V) -
Cap Base E40
Color Temperature (K) 4300
Lumen (Lm) 35000.0
Beam Angle (o) -
Line Frequency (Hz) 50
Color Rendering Index (Ra) 65
Color Descriptions 643 COOL WHITE
Average Life Hours -

Does it look like it is capable?

[Me2!]

your link is dead.

if its really long you cant just paste it.

[Guy Grotke]

If you use a lamp with a 40 mm arc length, most of the light will be coming from the two bright spots at the ends of the arc. It is difficult to get the light from both ends into a projection lens, so you may waste a lot of that light.

I think you can make it work reasonably well, if you have a good quality lens that is at least 100 mm in diameter. If you have a smaller diameter projection lens, or a long-throw lens and matching field fresnel, then you might get better results with the lamp arc just a bit off center so you get all of the light from one end. That would be better than losing all of the light from both ends. Experiment!

[juice_e]

i seen a stainless steel bowl in the dollar store yesterday, do you think it will work or is it too cheaply made? I looks pretty solid and smooth. I didnt measure it so im not sure what size it is.. maybe 10-12" ish.

[Me2!]

Hold it in both hands with your finger tips under and thumbs at the sides of the rim. Look into it. If you can rotate it around the axis made by your thumbs without your reflection warping all over it is a fairly good sphere and will work.

If your reflection is distorted and moving it will send light at uncontrolled angles.