Configuration: Wall socket to Gra Lab timer, to Variac toEnlarger?

[Domenico Foschi]

I have finally completed my Durst Laborator g139 that was missing many parts. I will be using a 500w bulb that I want to use at no more than 300W. For this I just purchased a Variac, or Variable auto transformer. Now the challenge for me is to find out the configuration, the "sequence", on how to connect the enlarger, Variac and Gra Lab timer together. Is it even possible?
Thank you.

 

[Dr Croubie]

That looks good.
If you go wall-variac-timer-bulb then you risk the timer not working properly: the times may be off (if it's a voltage-controlled timer), the lower voltage may not hold the relay in, etc.
So stick with wall-timer-variac-bulb.
Just make sure you get the variac input/output around the right way and you'll be fine...

 

[Domenico Foschi]

Thank you, doctor.

 

[Dr Croubie]

That electrical engineering degree turned out useful for something then

(although, as a professional practising engineer about to embark on a CPEng qualification I should advise you to obtain independent advice from a licensed electrician in your area if you have any doubts blah blah yada yada etc)

 

[BrianShaw]

...So stick with wall-timer-variac-bulb.
Just make sure you get the variac input/output around the right way and you'll be fine...

That's how I was configured when I had a darkroom set up. I'm so glad to know that an expert confirms that it was correct. I always thought it was, and it worked... but never was 100% sure.

 

[kstudio]

Figured I'd mention something about this configuration. Although this works, the current inrush from energizing the variac may cause the relay contacts in the timer to pit and fail prematurely. A typical transformer (and variac) draws up to 6 times its rating when energized. A good snubber will help prolong contact life.

If your exposure becomes erratic, check the relay before pulling everything apart. Had experienced a similar problem on a Propak printer (Colenta converted it to 120 volts and never changed the relay) and we were constantly replacing the relays. Gave up and replaced it with a solid state device (made for Kodak ) that I found at a flea market in Rochester. No issues since then.

 

[fotch]

Or, just run the bulb through the Variac & use a red filter in front of the lens and manually move it out of the way for exposure while watching the clock or timer. Also known as the KISS method.

 

[nworth]

The Variac gives quite an inductive kick when switched on and off under load. That might cause some problems with the GraLab's switch contacts. A reasonable capacitor (say 0.5 uF, 600V) across the contacts should mitigate it.

 

[gleaf]

Old technology Ohmite ceramic wire wound resistor with the sliding voltage tap clamp and he can tweek it in.. (power off between tweeks for safety.) Remember while on your dumping 200 watts into the resistor. Cook finger prints off, and other fine things.