Special timer required for cold cathode head???

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Tom Stanworth

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Hi,

I was lucky enought to get a kamm 8 x 10 enlarger off e-bay for £31. It is in sound mechaninical condition, requiring only some cosmetic work for me to be happy with it! I cannot wait to use it, but am concerned about the cold cathode head. I have read somewhere that normal timers cannot handle such heads for voltage elctrickery type reasons that I dont understand. Is this true?

How should I best deal with warm up/light output issues to get best consistency between prints (cheaply)?

I'm still grinning at £31, but could be looking at a bigger bill for the back and hernia operations which will be required after moving it....with two of us lifting!

Thanks,
 

Donald Miller

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Cold light heads represent an inductive load as opposed to a resistive load. Typically an inductive load will have a higher inrush current then a resistive load. Thus the problem that can occur is that the relay in the timer that is responsible for switching the power supply to the lamp can be overloaded by this initial peak demand.

I don't know the power demands for the lamp in your enlarger. I have typically seen the initial inrush current on inductive loads run to six times the operating current in some devices.

What can be done in event the timer is incapable of supplying the necessary current is to install a supplementary higher load capacity relay to carry the load of the lamp and to have the timer lamp outputs utilized as the coil voltage supply for the supplementary relay.

Good luck with your purchase.
 

Les McLean

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There are timers available that can deal with the power required to drive a cold cathode enlarger. I have used cold cathode enlargers for most of the 28 years I've made prints and in my opinion the biggest problem you have to overcome is the variation in the illumination as the enlarger warms up and cools down, in addition to voltage fluctuation in the power supply to your darkroom. My early experiences with cold cathode were that after making a test strip the print made could be much lighter because the enlarger had not reached the working temperature. I overcame the problem by leaving the lamp on and covering up the head while I was not exposing the paper. It was a pain but it worked.

The problem was solved when I purchased a Zone VI enalarger with a compensating timer. This works by placing a probe in the lamp houseing that reads the intensity of the illumination and adjusts the speed of the timer accordingly. In effect when the light is brighter the timer speeds up and when it is dimmer it slows down, and it works. I started with a Zone VI compensating timer and replaced it with RH Designs Stop Clock Pro timer which is also an f stop timer. I would suggest that you have a look at their web site and perhaps email them for advice, they are very helpful. The web address is rhdesigns.com
 

Donald Miller

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Les McLean said:
There are timers available that can deal with the power required to drive a cold cathode enlarger. I have used cold cathode enlargers for most of the 28 years I've made prints and in my opinion the biggest problem you have to overcome is the variation in the illumination as the enlarger warms up and cools down, in addition to voltage fluctuation in the power supply to your darkroom. My early experiences with cold cathode were that after making a test strip the print made could be much lighter because the enlarger had not reached the working temperature. I overcame the problem by leaving the lamp on and covering up the head while I was not exposing the paper. It was a pain but it worked.

The problem was solved when I purchased a Zone VI enalarger with a compensating timer. This works by placing a probe in the lamp houseing that reads the intensity of the illumination and adjusts the speed of the timer accordingly. In effect when the light is brighter the timer speeds up and when it is dimmer it slows down, and it works. I started with a Zone VI compensating timer and replaced it with RH Designs Stop Clock Pro timer which is also an f stop timer. I would suggest that you have a look at their web site and perhaps email them for advice, they are very helpful. The web address is rhdesigns.com

Les,

I have used cold light heads for about as long as you have. Speaking from the perspective of someone who has dealt with electrical design considerations for well over 35 years, I feel confident that an 8X10 cold cathode lamp requires a heck of a lot more current then any 4X5 or 5X7 lamp. That current inrush consideration is certainly different then lamp brightness fluctuations.

A supplementary relay is a lot less expensive then a new timer. Additionally, for someone on a budget there are light integrators available on the used market today. These are devices that were used to monitor and control time related to light output in plate burners in the graphics industry. The cost of these used devices (many in excellent condition) is a lot less then a RH designs device.
 
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