Cold light with a Gralab timer?

Discussion in 'Enlarging' started by Donald Qualls, Dec 29, 2005.

  1. Donald Qualls

    Donald Qualls Member

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    I've just gotten a Gralab Model 300 darkroom timer for a good price, but note after receiving it that the data plate shows: 750 W, 6 Amps, 3 Amps "T". I presume "T" refers to the timer switched circuit. I plan to use it to switch a Zone VI cold light in my Omega D2.

    I'm a little concerned at this low rating, since the inductive load of a cold light gives it a very high startup draw relative to its steady state condition, as well as making it prone to arcing on switch contacts when switched off. I don't know the rating of the Zone IV unit I have, though, and it doesn't seem to have a data plate.

    Based on the size of the tube, it can't draw much more than 20 W; if that's the case, even allowing for 10x rating to handle startup load and inductive cutoff, the Gralab's relay should be fine. However, I don't know if the cold cathode tubes used in cold lights are really comparable to the common fluorescent lights I'm more familiar with; the light is bright, but not any brighter than a 15W fluorescent desk lamp I have here.

    So, my concern is, will I be overloading anything running the cold light from this darkroom timer? The heater circuit for the cold light will, of course, bypass the timer to remain on during the entire printing session.
     
  2. JHannon

    JHannon Member

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    Donald, I have a Aristo cold light Head on my Omega D2. I did the same thing checking to see if my timer would be OK. The only warning from Aristo was to make sure your solid state timer can handle inductive loads. I would think that a mechanical timer with a rating like yours should be OK. The smaller relays in solid state timers may be under rated. Mine was designed with the possibility of cold light heads and can handle it.

    I don't know if the Aristo is similar to Zone VI... maybe someone else can add to this.

    --John
     
  3. Donald Miller

    Donald Miller Member

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    Years ago I had an Aristo head on a D2 with a Gralab 550. I built up a relay circuit to handle the load. I figured that the relay was cheaper then burning out timer relay contacts.

    I let the timer switch the relay coil voltage and let the new supplemental relay contacts handle the cold light.
     
  4. PaulH

    PaulH Subscriber

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    I've tried it and it eventually fried the microswitch in the timer. I had to switch to one of the Beseler digital timers which has worked flawlessly.
     
  5. Nick Zentena

    Nick Zentena Member

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  6. Ronald Moravec

    Ronald Moravec Member

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    Aristo makes a contactor to isolate the timer from the cold light to solve potential problems. I use is in addition to the dimmer they make.
     
  7. Donald Miller

    Donald Miller Member

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    Building a relay circuit is quite simple if someone wants to do so. A simple 20 amp double pole normally open relay with 120 VAC coil, a little bit of wire, an enclosure, and terminals are all that are required.

    1. Install a male plug on one end of #16/3 SJ wire from the enlarging timer enlarger output and attach the other end to the new relay coil.

    2. Install a male plug to # 12/3 SJ cord to the common poles of the relay.

    3. Install #12/3 SJ cord with a female plug to the normally open poles of the relay.

    4. Install the green ground wire from the SJ cord to the enclosure chassis.

    5. Plug your cold light into the female outlet installed in step #3

    6. Plug the male plug installed in step #2 into a 120VAC power supply.

    7. Install the relay into the enclosure.

    This relay will handle over 2000 watts.

    All of the parts are available from a HVAC supplier such as Johnstone Supply or WW Grainger. Total cost of all parts should be approximately $18-$25
     
  8. lee

    lee Member

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    I dont think there is anything in this electro-mechanical timer to blow with the cold light. This is not the timer I would pick to use with an enlarger but sometimes we must do what we must do.

    lee\c
     
  9. craigclu

    craigclu Subscriber

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    Donald... I used my Gralab 300 with an Aristo rig for 10+ years without seeing any problems. I still use the timer for some things and it continues working fine.
     
  10. vet173

    vet173 Member

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    I used a greylab 300 with a 5x7 arisogrid for 15 years. No one told me it wouldn't work ok. I didn't tell the timer or light , so they didn't know either.
     
  11. Donald Qualls

    Donald Qualls Member

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    Lee, I presume you're referring to the fact I'll have to reset the timer for each exposure, but given I'm split filtering, I'd have to do that anyway for the blue and yellow filter exposures -- and I've been led to understand that the little Time-o-Lites that do reset are *not* suitable for cold lights, due to low rated contacts. Sure, what I'd like to have is a head with blue and green LEDs and separate timers for the two (then I could use Time-o-Lites, because the LEDs draw only a few hundred milliamps at the 125 V supply level), which would also eliminate filtering and such... Maybe in 2007...
     
  12. lee

    lee Member

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    Donald,

    you asked a question and I gave you an answer and judging from the others above here, my answer was ok. I have used Time o lites in the past also with aristo heads. Sorry you did not like my answer.

    lee\c
     
  13. Donald Qualls

    Donald Qualls Member

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    Lee, it wasn't that I didn't like your answer, I just wasn't sure I understood it.

    I now understand you to be saying Time-o-Lite's work fine with cold light? That's opposite what I had previously heard, but it'd be good news (though I still need to find a way to store two settings if I want to use the resetting feature with split filtering, since I can't reregister the paper to be able to do all the blue, then all the yellow for a series of prints).
     
  14. lee

    lee Member

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    sorry the internet is not a friendly place for understanding each other. My apologies. yes I have used the Time-o-lite with the Aristo cold light head. I dont think there is a way to store 2 separate times with the Time-o-lites I have used. I would suggest writing the separate times on a piece of paper and just making one exposure with one filter and changing the time and the filter and making the next filter. good luck on your quest.

    lee\c
     
  15. nolindan

    nolindan Member

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    The 'T' designation on a contact normally means 'tungsten'. In this case the maximum incandescent wattage is 360Watts [120v x 3A]. Lightbulbs have a very high current when they are turned on and can weld the contacts shut if the contact current is not derated.

    A cold light head's inductance causes a problem when the circuit is opened/turned off. The current in the head wants to keep going - think flywheel - and as the contacts open the current forms an arc accross the contacts causing pitting. A capacitor/resistor [called a snubber] accross the contacts will quench the arc. If you remember old car ignition systems there was a capacitor accross the distributor points and this was required because of the inductance of the ignition coil.

    If you are worried about the timer/head combination I would contact the manufacturer of the head.
     
  16. Donald Qualls

    Donald Qualls Member

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    Nicholas, I e-mailed Gralab a few days after I got the timer. Still haven't had a response. No idea how to contact Zone VI -- who handles that brand now?

    However, it's good to know what the T is for. Not sure I'd want to put a capacitor across the terminals of an AC device -- how do they quench the arc in a common light switch? Oh, wait -- you mean the capacitor in parallel with the switch, so it conducts for a fraction after the switch opens. Yeah, if I could get the case open I could not only easily check if there's a snubber in there, but solder one in if there isn't. Unfortunately, Gralab though blind rivets would be a nifty way to close the case quickly and cheaply, so I'd have to drill those out to open the timer. Still, I've got a setter for those, and the fasteners are cheapish...

    How would one calculate the value for a snubber?
     
  17. Curt

    Curt Subscriber

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    Donald, is the Gralab 300 timer the big square black box type timer you bought? I have three and I have used it with a Aristo cold light on my Beseler 45m, it's an electro/mechanical timer and can handle the inductive load. If you use it and burn out the timer let me know and I will send you one.

    Curt
     
  18. Steve Smith

    Steve Smith Member

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    Donald, Practically every snubber network I have ever seen or used has been 100 ohms in series with 0.1uF (100nF). I don't think the values would be very critical in this case (or most other cases).

    Regards,

    Steve Smith.
     
  19. Donald Qualls

    Donald Qualls Member

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    More to the point, Steve, I'd expect those values to be pretty much universal for a reasonable loads on 120-240 V service -- it's not that the capacitor needs to store much charge, it just has to take the current long enough for the physical contacts to separate beyond arcing range.

    Curt, yes, the Gralab 300 is a biggish black square box.

    I've gotten lots of opinions that the Gralab is fine with the cold light (and mine certainly seems to be okay), and some folks saying even the little Time-o-Lites are okay. BTW, I think I found a way to use two Time-o-Lites to do repeated split filter printing, assuming they have a "focus" setting that turns on the lamp. You'd hook them up in series (that is, plug one timer into the switched outlet of the other, then the enlarger lamp into the second timer), and set the blue and green times; for the blue exposure, set the "green" timer to "focus" and then swap to expose with the green filter. Lacking a focus setting, you could rig a box with a double-pole, double-throw switch to connect both supply and lamp hot wires alternately to the two timers, and just flip the switch back and forth from "green" to "blue" -- use a three-throw setup and you could add a "focus" position.

    However, since I don't do many multiples (yet), the Gralab with manual setting for each exposure is currently working fine.
     
  20. Donald Miller

    Donald Miller Member

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    If you want to do split filter printing with a single timer, provided the timer has a safelight output, you can wire the enlarger lamp in parallel to the two outputs. You could use a delay on break solid state timer on the enlarger lamp output only and use a delay on make solid state timer in series on the safelight output.
     
  21. Donald Qualls

    Donald Qualls Member

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    Donald, you're on the borderline between English and Greek with that post -- I *think* I understand what you're doing there, basically adding timers to the outputs of the timer, but that really doesn't help that much with a Gralab, because I have to reset the original interval for each exposure, while a simple switch would serve to trigger the electronic timers you suggest (I think). Time-o-Lite timers reset themselves after each run (the motor works against a spring, which pushes the hand backward until it hits the stop when the timer shuts off at the end of an exposure -- you set the exposure time by moving the stop), which would be a great convenience if either a) printing a whole roll of film to the same settings (which used to be a routine operation), or b) printing many copies of the same negative (also a routine operation at one time).
     
  22. Donald Miller

    Donald Miller Member

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    Yes, you are correct. I was thinking of a purely electrical timer rather then a mechanical/electrical timer. My suggestions would work with a purely electrical timer to control two outputs and two sequences.