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Thread: Contactor

  1. #1
    NER
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    Contactor

    Can anyone direct me to a source for a contactor capable of handling an inductive load of 1000 watts? I need this in order to run my 8x10 enlarger with ANY digital timer. Thanks very much.

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    Quote Originally Posted by NER
    Can anyone direct me to a source for a contactor capable of handling an inductive load of 1000 watts? I need this in order to run my 8x10 enlarger with ANY digital timer. Thanks very much.
    I don't know what sources that you have locally. I would check with WW Grainger, or Johnstone supply if you have either of those available...any electrical wholesaler would have it. You do need to determine what coil voltage the contactor will need to be...They come in various voltages from 24 volt to 480 volt. The amperage that you will require for 120 volt load will be 8 amps. For 240 volt it would be 4 amps...not a terribly large load. I don't know why you are calling out an inductive load since incandescent lighting (including halogen) represents a resistive load and not inductive. The only type of lighting that would be inductive would involve a ballasted lamp such as flourescent, mercury, or carbon arc.

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    NER
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    I wish I knew more about electricity, but I don't. It's probably fair to say that when it comes to that subject, I am the most stupid person in the world. I mentioned inductive load because I spoke and exchanged e-mail with a representative from Gralab today and she told me that is what I have. Specifically, I asked if the Gralab 900 timer model would work for my enlarger and she said no, my enlarger would fry the timer because the timer has an inductive load capacity of only 600 watts. (I have that part in writing too.) The enlarger in question is a Superchromega F Dichroic II model. Do you believe she was incorrect? Thanks for your help. By the way, I run the thing on a 120 circuit now so I guess that would be the 8 amp coil I should look for (although I could never explain why except to say that you told me so). Oh yes, do you happen to know if the Gralab 900 will work for this enlarger? I thought it would but decided to check with Gralab today before buying one. As you can see, I'm now only more confused about this. I do appreciate your help. Thanks again.

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    Dear Norman,

    Put a relay in between the resistive source (lamp) and keep the transformer live.

    Neal Wydra

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    Quote Originally Posted by Neal
    Dear Norman,

    Put a relay in between the resistive source (lamp) and keep the transformer live.

    Neal Wydra

    Be careful. If the transformer is producing a lower voltage, then the current will be very high and finding such a relay will be difficult. Also, you don't know how sensitive the timer's sold state relay is. The contactor could be (depends on design) inductive enough to frap your timer as a coil on a relay is quite inductive. Get the timer folks to recommend a contactor and put the worry on them.

    Aristo used to make contactors. I don't know if they still do or not. A timer with a mechanical relay that can handle the load would work if you are not set on a specific timer.

    Although you have a transformer in the system, your load may not be purely inductive. I'm not saying to ignore the warnings about using a certain timer with this transformer, I'm merely pointing out that the load is not purely inductive, probably only partially. The relay will "see through" the transformer to the load on the transformer, i.e. the resistive lamp. It can still kill a solid state relay that is not properly protected.

    Take it slow.
    Watch for Loose Gravel

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    The purpose of a contactor is to carry the electrical load of your enlarger rather then having the timer carry the electrical load of the enlarger. When a two pole contactor is installed it acts as an electrically operated switch. The timer only acts to provide the signal to operate the switch. The coil of the contactor is an electromagnet and it causes the switch to close and to open. When energized by the timer the coil will pull the switch closed and send voltage to your enlarger and by the same token when the timer is no longer energizing the coil the switch will open to stop the supply of electricity to the enlarger lamp.

    The installation would amount to this...you would have the two leads(black and white wires) presently going to your enlarger lamp connected to one set of the two open contactor contacts. You would then have 120 volts (black and white wires) from a wall outlet connected to the other opposing set of two open contacts using 14 guage wire. Next you would connect two wires (black and white wires) to the contactor coil and these would then be plugged into your timer enlarger output. The black wire that is connected to the timer enlarger output should be fused with an inline fuse of 3 amp capacity. This will provide the safety for your timer. The three amp fuse will blow if at any time a short circuit develops and a load of greater then 345 watts occurs. Be sure that all of your ground (green wire) is connected and continuous all the way from the enlarger lamp input to the wall outlet.

    There is no need to worry about anything other then the contact rating of the contactor...a 20 amp double pole single throw normally open device with a 120 volt coil will serve your purpose. The 20 amp rating is a measurement of the current carrying capacity and provides a safety factor for your purposes. No contactor coil will draw anywhere near what the current rating of your timer is. I have worked with electricity for over forty years in all types of control circuits and I know what I am talking about. By the way there is no transformer in this circuit...so where the heck did that come from.

    I have done this with my own enlarging equipment. It will work and it is consistant with national electric code.

  7. #7
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    I see old lighting contactors on Ebay from time to time for pennies on the $. That's where I would look. Here's one with 120V coil with a BIN of $20! Be careful.
    He is no fool who gives what he cannot keep..to gain that which he cannot lose. Jim Elliot, 1949

    http://tonopahpictures.0catch.com

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    This is what will work for you. This particular device is available from WW Grainger...It would need to be mounted in an enclosure as would any device purchased on the open market. The enclosure needed would also be available from the same source.
    Attached Files

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    Norman,

    Aristo markets a contactor rated at 1200W at 120V. See http://aristogrid.com/Tech03.htm . Should be what you need.

    Cheers,
    Will

    Quote Originally Posted by NER
    Can anyone direct me to a source for a contactor capable of handling an inductive load of 1000 watts?
    My Verito page

    Anyone can appreciate a fine print. But it takes a real photographer to appreciate a fine negative.

  10. #10
    NER
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    Thank you everyone for your advice and assistance. I'm sorry that I'm such a dunce when it comes to this material. Reading some of the above makes me understand how the beginning photographer (i.e., the one who doesn't yet know the difference between an f-stop and a bus stop) must feel when reading through some of our discussions of HD curves and so forth. I think I can work out my problem with the help you've all given me. Again, many thanks.



 

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