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Thread: Projector construction - laser time delay?

  1. #1
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    Default Projector construction - laser time delay?

    For any of you folks using a time delay device or circuit for the lasers in your projector -

    Would you be willing to post an example of the circuit and/or relay you used? I'm getting very close to the heavy wiring stage of projector construction, and would like to implement all CDRH requirements up front.

    Thanks!

    Randy
    RR

    Metrologic HeNe 3.3mw Modulated laser, 2 Radio Shack motors, and a broken mirror.
    1979.
    Sweet.....

  2. #2
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    Hi Randy,

    You might poke around electronic supply places (actually, electrical supply places like Graybar and the like) and look for something called a "Time Delay Relay". Using one of those would be an easy way to accomplish the time delay.

    Bill

  3. #3
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    bill-

    my concern (in my mind. i never tested) with using a relay to switch the mains power for the laser is that of the initial high voltage spike off of the relay? would this cause a problem??

    i just know that a relay opening and closing has (potentially) thousands of volts instantaneous spikes. could these power supplies for the lasers handle that potential?

    -Marc
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  4. #4
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    Cool

    Marc;

    I think you're referring to the spike that you get when the coil de-energizes and the magnetic field collapses. That can generate a brief high voltage spike. But it's easy to protect against that - you simply solder a diode across the coil terminals on the relay so that it blocks current from flowing around the coil (forcing the coil to energize when the power is applied). When the power to the coil is extinguished, the collapsing field will tend to try to keep the current flowing in the same direction, thus the output (or ground) terminal will have a higher voltage on it. This will short through the diode back to the input terminal, effectively dissapating the HV pulse.

    I have a diode like this on my main switching relay for my projector. As far as I know, it's recommended for nearly all relays.

    Also, this pulse shouldn't affect the lasers, unless you're using the same power supply to power the lasers *and* the relay. In my projector, the 12 volt rail powers the interlocks, while the 5 volt rail powers the lasers. The two are completely separate.

    Adam

  5. #5
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    The high voltage pulse will be on the relay control circuitry, not the laser side! Just add a diode like buffo said and you'll be fine!

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    When the going gets weird, the weird turn pro.


  7. #7
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    Hi Marc,

    A time delay relay automatically has all of the timing stuff inside it. You just give it power and... some time later... it energizes.

    As for any spikes, remember, whatever you do to turn on your projector, such as the power switch or even power plug would cause some kind of spike, if I interpret your definition of "spike" correctly.

    Bill

  8. #8
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    He's talking spikes from the coil and they are very serious if not handled properly, as buffo explained. I worked on some nuclear instrumentation at one time and one of the diodes blew on one of the relay coils. Everytime the relay would click it would generate a huge spike that would reset the intrumentation. Nasty. I avoid relays where possible due to that.

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