[pure-silver] Re: Enlarger Lamp Life

The voltage at a transformer output is always higher without load.
Once you put the load, it (should) drops to its nominal value.
The voltage drop depends on the load and transformer respective impedances.
You should measure the voltage with a new bulb fitted.

Several factors may affect the bulb life:
- input voltage, you may be close to the power plant and the voltage may be on 
the high side. Measure the voltage at the mains and at the transformer output 
(loaded of course).
- the mains voltage has slightly increased these last 20 years. Here in Europe 
we've gone from a 220V to a 230V and maybe slightly more.
So if your enlarger is old then the transformer apply to the bulb this voltage 
increase. Same measures to be done.
- frequent switch off/on, which is the case with an enlarger, as switching on a 
bulb generates a current spike (a cold filament has a much lower electrical 
resistance than a hot one).

In my case, I stabilized the bulb voltage to the proper value (main goal was 
constant exposures) through a switching power supply that reduces also the 
current inrush which makes the bulb last longer.
The protection type of the supply must be a current limitator as the common 
"stop-and-retry" will not work with a bulb, it will make it blink (!).

More details on this page:
http://www.bonavolta.ch/hobby/en/photo/v35suply.htm
See "Switching Supply"

Claudio Bonavolta
www.bonavolta.ch

----- Message d'origine -----
De: Eugene Barrington <eandpbarrington@xxxxxxxxxx>
Date: Mon, 27 Oct 2008 15:31:33 +0000
Sujet: [pure-silver] Enlarger Lamp Life
À: pure-silver@xxxxxxxxxxxxx

>Hello - I'm a new poster to this list but I've consulted it regularly.
>
>I know that this topic has been dealt with before - but the enlarger  
>lamp I need to  use is is rather expensive (€21,£16.96,$26.17 and so  
>forth) and they have a nasty habit of blowing. My enlarger is a Meopta  
>with a colour head which uses  a Philips Focusline fibre optic lamp  
>type 6834 FO. I suspected a faulty transformer. using a multimeter I  
>got a maximum, but steady 13.1 volts output from the transformer. The  
>lamp is rated at 12volts 100w. Is it being overloaded at 13v? Could  
>this be the reason for the frequent blowouts or is there some  
>tolerance with the voltage?
>
>Hope somone can help with this -
>Eugene Barrington
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