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Thread: Pull down resistors - Help Please

  1. #1
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    Default Pull down resistors - Help Please

    Hi all

    I have a problem !

    When I unplug the db25 from the back of my projector (with the projector switched on), the Blue and green DPSS lasers emit a beam. This doesn't happen on Marconi's dual red.

    I believe that this could be due to the 0-5v analogue modulation input being left 'floating'. Would a pull down resistor sort this problem and if so, what resistance should I use?

    Anyone who knows me must realise by now that i'm a complete and utter nervous wreck when it comes to electronics. I have a total lack of confidence and am always worried in case I blow something up. I guess this comes from too many bad experiences

    Any help is much appreciated, and sorry if i'm in the wrong part of the forum with this numpty question but I wasn't sure where else to put it.

    Cheers

    Jem

    P.S. The new avatar is of my (now working) RGB projector

  2. #2
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    Jem
    A 10k to 0v (earth) should do the trick - one on each input as well but im sure you know that much.
    Also its an idea to use the ILDA interlock pins (4 and 17) to close the safety shutter or kill the beams by other means (such as pull down to 0v or power removal via a relay) if the ILDA connector is removed.

    Rob

  3. #3
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    Thanks Rob

    You're a star

    I'll get it sorted this next week and it should then be nice and safe for the UK meet, I don't want to blind you all

    Will anyone be taking a good (calibrated) power meter to the meeting. I could just do with confirming what my meter's reading is correct.

    Cheers

    Jem

  4. #4
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    I will bring my lasercheck but its only good for single wavelengths maybe James Stewart could be good enough to bring his fieldmaster. I wouldnt mind seeing how accurate the panel meter on my SP168 is.

    Rob

  5. #5
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    Sorry, I realise I didn't make it quite clear.

    Single wavelengths is exactly what I want to measure. I'm sure my meter is reading what it should be as all the lasers in my RGB are reading what they should be.

    What's got me slightly confused though is when I take a reading off my(beautiful SLM 532nm) 100Mw Coherent Compass 315m it seems to read on the low side. The digital controller is telling me it's outputting 99.5mw but my meter reads quite a bit less than this, to the eye it certainly looks like 99.5Mw - Strange.

    Cheers

    Jem

  6. #6
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    Hi Jem

    Yes a pull-down resistor may help extinguish the reds if they are floating
    usually they are OFF in this condition, However they may still appear ON with a very low (below a mw) output, This is because the the diode is still putting out some light even tho it is below threshold. acting like an led.
    This could be caused by two things..one is of course the output of the DAC is not completely turned off according to the diode. and we use a high impedance input circuit ) around 10k ohm. therefore depending on the boards
    drive it may not be pulled-down to off completely. Altho this high of an impedance it should be easy to to drive.
    The other may be, that we purposely try not to toggle the diodes from completely OFF to full ON as this is not good for the diode facet.
    Each driver is the same but diodes are not electrically. so some will do what you are describing, while some will not do this. I have found that some diodes will still appear to output some light even tho the current is way below the threshold, the threshold is around 85ma to lase while a few milliamps will make some light. Thats why we suggest the use of a shutter.

    Hope that helps.

    As far as the 315's power, P6 is the calibration pot on the head.
    with age it will change from the factory setting.
    Your software still thinks its at that setting as it was calibrated there.
    You will need to measure with a calibrated power meter to make the decision
    whether to change that setting. However, that setting is interactive.
    meaning the power out the head will change as well.
    Perhaps there is a change that could be made in the sotftware for calibration purposes, I dont know.
    I am not fimiliar with the digital sw as very few people actually have it and you are the first that we are aware of that actually got that digi controller.
    Very lucky to be sure..the rest of us have to use the analog controller as-is.
    I have acquired heads that were set at 100mw from the factory that have been anywhere from 85mW to as much as 130mW, depends mostly on the heads age and condition.
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  7. #7
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    Quote Originally Posted by Jem View Post
    Thanks Rob

    You're a star

    I'll get it sorted this next week and it should then be nice and safe for the UK meet, I don't want to blind you all

    Will anyone be taking a good (calibrated) power meter to the meeting. I could just do with confirming what my meter's reading is correct.

    Cheers

    Jem
    Hi Jem
    I will have a calibrated Scientec meter, good for 20 watts, curtesy of Marconi ( it arrived today, thanks Chuck !! )


    Regards

    Fluff
    The light at the end of the tunnel. Its' a white laser.
    www.rocknite.co.uk

  8. #8
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    Bonza!

    Hope you dont mind sticking it in front of my whitelight

    Rob

  9. #9
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    Nice !

    Jem

    Hmm... Just tried posting the above and got told my message length was too short, apparently it should be at least 15 characters... That should do it

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