12V halogen light bulbs

[wahiba]

My Eumig uses a 100W 12V bulb. I have noticed the bulbs available for domestic use of 50W or 25W and only once 5W.

Do these bulbs have the same pin distance?

Do they work?

If they work I assume videoing the gate, direct, or through a magnifier can be attempted.

Anyone tried it?

Just trying to save buying a bulb and finding out for myself!

[S8 Booster]

Hi,

What about bringing your bulb to the shop to compare there?

Seems like physical outer size refelcts pin "order" but compare at shop.

[Pedro]

The pin distance of halogen bulbs with reflector may serve, when comparing a 12 V/100 W projection bulb with reflector with a 12 V/20, 35, 50 W domestic bulb with prism reflector.
The only problem is, that when inserting a bulb with less wattage than designed, the operation voltage can rise significantly, depending on the transformer the projector is using. A bulb with less power consumes less current, causing less voltage drop inside the transformer, resulting in a higher pin voltage, that may burn the bulb rather quickly.
So a diferent (weaker) bulb should be supplyed from extern, with an adequate transformer.
Pedro

[wahiba]

I always thought transformers were pretty efficient devices so that the reduction in current should make little difference. Unless anyone can prove otherwise?

I would take the old one with me, I only changed it recently, but like an idiot binned it. The local camera shop which I have not been in often has a load of bulbs so I will have look there.

[jessh]

if it is an MR type bulb(such as MR-16 or MR-11) you may even be able to find a version of it with a covering glass that provides UV protection, this could be good for giving your film extra protection, especially when doing slow or frame by frame transfers. I am working on building something similar to the workprinter and am planning on using these types of lamps, I am thinking of using a MR-11 20W/12V, the great thing about these is they produce less heat than a non reflector lamp that gives out the same amount of light, and it mostly is radiated out the back of it, away from the film path.

~Jess

[S8 Booster]

If the transformers were so bad as Perdo describes they would be at risk of catching fire at nominal load.

He has a point but slightly pushing it.

You can always use a voltage meter to check the system voltage with the original and other lamps. +/- 10% from "normal" lamp is usually acceptable.

[Pedro]

Sorry, but it´s the normal behavior of any rather small transformer, nothing "bad". The reason is, that the secundary coil never has a zero Ohm resistance, so there must be a voltage drop at the coil when any current is present. And we have a lot of current here, it´s 8,33 A for a 12 V/100W lamp. And if the internal coil resistance is only 10 Ohms, what is not so much, the voltage drop is 10 Ohms * 8,33 A = 8,33 V. So when disconnecting your lamp, the unloaded output will rise it´s voltage up to 12 V + 8,33 V = 20,33 V. (This is only considering the DC conditions, w/o the effects caused by AC)
Now, when connecting a weaker lamp, with less current consumption, the internal voltage drop is less, too, and the terminal voltage at the lamp becomes automatically higher, a quite normal situation.

F.e., take a 12 V/60 W domestic halogen transformer and connect 3 halogen bulbs, 12 V/20 W each. The math is correct: 3 * 20 W = 60 W. Now disconnect one ore two (simulating burning) and watch the third. What do you recognize: It is becomming brighter! How, depends on the transformer design.

Only when using a rather bigger transformer than required for a certain application, with a lower internal coil resistance (thicker coil wires), the voltage becomes more independed of the load. But most consumer devices use only what really is neccessary, to be cost effective. Projector transformers even are mostly located into the air flow of the fan in order to be cooled!

I only wanted to call the attention of this technical fact according to Mr. Ohm and would measure the effective voltage at the desired load anyway.
Pedro

[S8 Booster]

Hi.

Did not dispute your calcs Pedro. Just that I have been dealing with larger powertransformers for 25 years or so and I am confident that if transformers are designed the way you describe and intended for consumer use I am pretty sure those projectors would not pass any IEC/VDE/CE classing today and thus not allowed to be sold commercially.

Imagine: If you buy a lamp that is slightly out of spec (more power - less resistance) and install it in the projector there might be a real risk of fire due to internal overheat of the transformer, unless it is designed short curcuit protected.

I really hope that the tranformers are not that bad anyway?
Other factors for the transfomer:
Dirt outside tranformer = less cooling / overheat
Insufficiant cooling capacity (if fan cooled) Check dirt on fan, fan function too?


:?

[S8 Booster]

Just struck my mind that with some skill the lamp might be fed by an external adjustable powersupply to adjust the optimal setting.

Be aware that improper managing of electrical systems are lethally dangereous so never do this unless you are familiar with it or authorized for it.

RGDS

[Pedro]

Hi S8Booster,
no danger at all, only the lamp burns earlyer, and no reason at all not to pass security tests. The only effect is, that without or wiht less load, the pin voltage rises. With smaller transformers, the pin voltage rises even higher, when reducing the nominal load to "less load" or "no load". Try it out and you will see. Take off your projector lamp and measure the AC voltage. Then connect it again and measure again. The result you´ll get with a different wattage, will be somewhat inbetween nominal load and free pins. That is really the normal behaviour of transformators, unless they are pretty over-sized. And this is (one of) the reason(s), why a transformer produces heat. Other reasons for heat belong to magnetic fields and inductivity and are more complicated to understand.
So the efficiency of a transformer is never 100%, the values are somewhat around 70% for small transformers. The rest is heat and the described losses in the coil resistances, resposible for a not constant output voltage. Heavier transformers also have a better performance and can even go close to 90 %.
I thought, the simple calculation of Mr Ohm was simple enough for everybody.
Pedro