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Thread: zener diode question(s)

  1. #1
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    Default zener diode question(s)

    hi.
    i'm going to try to organize this post so the thread doesn't get messy.
    ewww,i hate that messy thread

    preface.
    i'm planning on powering the modulation input on my cni 100mw 532 with a "wall wart" dc power supply(5 volt).

    i found a local supplier that can provide only a 5.1 volt 5 watt axial lead zener diode.

    questions.

    1.i have heard you can go over 5 volts if it is not
    by much. is that true?

    2. does the watt rating of the diode make a difference if it is over(perhaps well over) what is needed based on the current from the dc supply? i.e is the five watt diode dangerous to use and will it perform properly at this 5 watt value?

    if the answers to either of these questions is yes,i am thinking of looking for a 4.5-4.8 or 4.9volt supply to keep from frying the pump diode,whatever that is exactly

    peace,
    wes

  2. #2
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    This is what I have done with the diodes as per the posts that were on here a few months ago. If this is in any way wrong please correct me...But I have used as this diagram and tested with no problems...It did shunt the power supply on a bench test and popped the fuse I had in the test devices. I have not applied these to the modulation lines. This is on all the lasers now...had problems with a spike in the not so distant past....dead greeny and power supply.
    Attached Thumbnails Attached Thumbnails laser protecter.bmp  

    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  3. #3

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    Hi,

    Be careful using wall-warts. They have a tendency to float high when they are not pulling the full rated current. I have had 5 volt 1A wall warts float 3 or 4 volts high when the load is light. Also they have AC ripple which will increase the peak voltage, by 10-20% over the measured DC Voltage depending on wall-wart quality and any filtration circuits you implement.

    Most electronics can handle a .1Volt overage no problem, but Laser Diodes are one of the exceptions. The question is weather the circuit will pass on that extra .1volts to the diode. I don't have one of these or even a related one so I can't tell you with any confidence.

    As for the diode worry about the current not the wattage. If your within current spec you won't have a problem with heating.

    Have a good one.
    Shawndoe

  4. #4
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    wow,thanks for the warning re:wall warts.

    i think i'll try what buffo suggested and use the terminal inside the beambox that is currently running the original greenie.
    i will test it with a voltmeter but i'm hoping it will provide what i need.
    i can live with the wires running out to the outboard power supply/driver if it means saving the life of even one innocent laser.

    i'll look at the protector circuit posted here too and try digikey for the diode.

    buffo sent me a link to 5v 1watt zener on digikey but it expired and i am a little baffled when i look at their page.
    guess i'll pm him for the link again.

    thanks so much.
    just the warning about the wall wart points to the value of this forum and the quality of it's members..
    peace,
    Last edited by wes; 07-06-2007 at 16:13.

  5. #5
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    Quote Originally Posted by MechEng3 View Post
    This is what I have done with the diodes as per the posts that were on here a few months ago. If this is in any way wrong please correct me...But I have used as this diagram and tested with no problems...It did shunt the power supply on a bench test and popped the fuse I had in the test devices. I have not applied these to the modulation lines. This is on all the lasers now...had problems with a spike in the not so distant past....dead greeny and power supply.

    ok,two questions.

    1. in the diagram,the driver and the supply are shown separated.
    my driver/supply is integrated in one box"lab style"and has two leads coming out for the modulation.
    if i place the diode across these leads will that be correct?
    i still will consult w/ my buddy re:reading the diagram,not familiar enough to discern everything,but it looks like there are two diodes in that circuit.
    what i assume is that cni has put protection between the supply and the diver and i need to place just one diode on the modulation input leads.
    is that correct?

    2.it is a 5.6v diode but i don't want to go over 5v at all.
    what am i missing?

    edit.ok.i got a link from buffo for a 5.1v 5watt diode.
    still curious about the first question in this post.

    thanks.
    Last edited by wes; 07-06-2007 at 17:56.

  6. #6
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    Cool

    Wes;

    Yeah, the zener diode goes across the blanking leads. You don't need a zener on the PSU inpu to the driver board because on your laser the PSU and driver board are integrated into a single unit. (So you'll just have to trust that the folks that built the thing knew what they were doing! )

    MechEng3's circuit does show a 5.6 volt zener... I think 5.1 would be safer though.

    Re: wattage. The wattage rating of a zener diode lets you know how much heat it can handle when it's shunting. Since power = volts x amps, and we're talking about a 5.1 volt diode, a 5 watt zener can pass 1 full amp of current at that breakdown voltage without overheating and melting. Since you're not going to be able to source anywhere close to an amp of current from wherever you pull your blanking signal from, you'll be just fine. (You'll notice that MechEng3's circuit only spec'd half-watt zeners, which in my opinion are a tad low for the job - they'll start to get mighty warm with just 100 ma of current.) Then again, they shouldn't be conducting for very long, if at all. You ought to be able to see something is wrong and shut it down...

    Adam

  7. #7

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    The wall wart will supply enough voltage that at full load the IR drop across the warts internal resistance will drop the output voltage to what it is rated for.

    As for going over 5 volts I would not exceed 5% or 5.25 volts.

  8. #8
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    thanks clutch.
    i've ordered some 5.1v 5watt zeners from digikey.
    i'm also going to go with buffo's first suggestion to me and run the modulation off the beambox circuitboard.probably from the same terminal that now runs the greenie i'm replacing.
    this will give blanking and should be more consistent than a wall wart.
    i'm a little concerned that a 5.1v zener diode may not be enough overhead if the 5v from the board wavers at all.

    so far, all i've found are 4.3v,5.1v and 5.6v zeners.

    mecheng's diagram shows 5.6v diodes.
    i'm still wondering ,does anyone else has confidence is using 5.6v zeners for protecting a 5v modulation circuit?

    peace,
    wes
    Last edited by wes; 07-08-2007 at 21:01.

  9. #9
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    o.k.,problem solved.
    my 5vdc source is running at 4.85v.
    i like that
    so my 5.1v zener diode will be perfect and very safe being such a low overage.
    i get a few hundredths difference on multiple readings w/ my voltmeter but the great preponderance of readings come in at 4.85v plus or minus .03v.
    i'm going to test it many more times to confirm but i think i will take a peek at my new laser in action today.
    i'll add the zener diode as soon as it gets here but if i keep getting consistent readings i won't be able not to take a look at the new cni.
    thanks everyone who responded.
    thanks for helping me learn.
    it's so cool!

  10. #10
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    Hey,
    Thought Id clear something up about using the zeners for protection.
    My suggestion was to use them both for the power input to the laser drivers (if not run from ac) and for modulation overvoltage.
    I have seen many power supplies and drivers from all over and many do not offer much protection from either overvoltage or reverse polarity.
    My choice of selecting a 5.6 volt zener was so that there would be absolutely no conduction in normal use..where-as a 5.1 may conduct if the supply chosen was not adjusted or checked to be absolutely 5.0v or below..
    Many supplies may be within 5 percent of spec which means 4.55 to 5.5 volts
    which shouldnt be a bother..but if you do use a 5.1v zener be sure your supply is adjusted or measured to be at 5.0v.
    However using a 5.1 for the modulation input will probably be ok..
    I use 5.6 as all my units are tested very close to the 5.0v limit so there is no conduction. a 5.1 may conduct under these circumstances.

    Whatever value you chose just be sure to check the operating parameters.
    They are really for your protection.
    I have seen lasers destroyed accidently by overvoltage from PC supplies and wallwarts and wondered what could be done to help.
    Ive also heard of stories where the mains were also accidently applied by mis-wiring or floating around.
    I just selected a value that was 5 percent in the SAFE ZONE .
    As all zeners are not created equal...they to have a tolerance of accuracy.
    Last edited by marconi; 07-09-2007 at 10:07.
    "My signature has been taken, so Insert another here"
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