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Thread: Ground issue

  1. #1
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    Default Ground issue

    Ok so here is the problem, I have a small projector in a box all bolted to an aluminum base. I have a 100mw green bolted to it and the power supply for the green isolated. The galvo's and there power supply and the transformer for them are also bolted to the base. Here is the clincher ive already killed 4 burner diodes and i really don't want to make it 5. the burner diode is powered by a die4drive module which is all bolted to the same heat sink. It seems to be an issue when I connect the ilda cable which I have connected to this module and the green blanking. should there be a optoisolator between the dac and the powersupply, not too sure how fast optoisolators are and the drive circuitry will have to be to do this blanking.

  2. #2
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    If you use die4drive you can't ground the laser diode case or you will blow it. It is a nice circuit but it is very unsafe in regards to accidentally shorting the diode to ground and blowing it. So make sure the laser diode electrically isolated from ground and stays that way. I mounted my laser modules on a sheet of mica using nylon screws to solve the problem.

  3. #3
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    Indeed. The Die4drive has a MOSFET, then a current sense resistor on the ground side of the diode. It would actually be fine in a positive ground system, which is odd given that most diodes driven with it are likely to be in negative ground cans.

  4. #4
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    Quote Originally Posted by The_Doctor View Post
    Indeed. The Die4drive has a MOSFET, then a current sense resistor on the ground side of the diode. It would actually be fine in a positive ground system, which is odd given that most diodes driven with it are likely to be in negative ground cans.

    Not odd, actually.
    There is a very good reason why Robin and I and many other manufacturers choose this approach.
    First, it has to do with modulation speed..it is mostly the choice of devices used. using a P channel FET may fix that problem but I'm not sure it will.
    First you have to find a good P channel FET that will do the job.
    Second, When I first started making drivers, and I've made many . I started with the opposite polarity cans as this was the norm for a long time so this was the way to go, then these new cans came out changing everything, but no matter what circuit I used I never could get the switching speed up high enough to suite my needs or at least work up to a flat 30khz..I was very disappointed by the results altho I did get close it was not good enough.
    Finally, After much digging and experimentation I stumbled on the culprit.
    I found that input capacitance of the switching device was way too high and that the op-amps liked switching to ground better rather to rail.
    Bi-polar devices could not do this properly so FETS were the answer.
    However, Finding suitable devices were easy to find but were always out of stock for some strange reason. Then I found what I was looking for in a device which I had to commit to. both in op-amp and FET I found that they had to work together in concert rather than just throwing a circuit using off the shelf devices in order to achieve the desired results. And with some added work I found what I was looking for.
    Yes, You can make a driver that just gets the job done but when you are catering to lasershow people who use TOP end equipment you cannot scrimp on the driver as this will effect the speed and quality of the projected image. These things show-up as lack of blank spots, delay problems and dim edges to bright spots. Not to mention overspikes that kill diodes or the reverse in which I have seen from many others as lack of a clean edges caused by smoothing or cheating by over dampening to try to cure a fault or problem with the circuit.
    I have tried the P channel FETS that are available but so far I do not like the results, Perhaps thats why you seen others using this opposite methode,
    For a long time I often wondered why manufacturers used this approach until I found out for myself. Because it is easier and it works. and rather well I might add.
    One thing that I did find funny was I just seen on another forum where someone decided to change from NPN bipolar darlington to a PNP darlington output driver so that they may use the grounded cans, He quoted that the capacitance was lower this way and would increase the speed of the circuit and also eliminate static damage to the diode..this is a false statement.
    Static will kill the diode just the same. Capacitance is still high and speed is sacrificed.
    But I remember doing the same thing when I started..so.
    yes, it would make mounting much easier as the machining and assembly would be easier and less prone to accidents but so far the best methode is the isolated approach.
    .
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  5. #5
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    Quote Originally Posted by carmangary View Post
    If you use die4drive you can't ground the laser diode case or you will blow it.
    Which is excatly how I blew my diode up last week. When the new ones get here the lens holders will all be firmly wrapped in black tape before I switch it on.
    A lovely childhood. Just me my mother and the voices.

  6. #6
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    one more question, how should this be connected to the ilda port ive found that my green only blanks when connected to the - rgb or - intensity and groun which i beleve are 16 17 18 19 on my alphalight dac? how should this die4drive controller be wired now that i have it isolated by mica.

  7. #7
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    Connect to + not - for the Die4drive, referencing to it supply gnd. Take care not to exceed 5V. There is a gnd for the mod but it connects to the supply ground also. Not sure what ILDA pin that would be for + Red or + intensity though.


    Marconi, that thing about working two parts together as one unit reminds me of a zero crossing switch I designed. I wanted one to remove the clicks that would affect audio recording when lights were switched on or off. All designs I saw were too big to fit in a light switch space, and too expensive, and consumed around 10 WATTS just on standby! I made mine micropowered (literally, standby consumption of microwatts, even the on-time tax on power was <1&#37; of load power), very tiny, cheap. I didn't use a triac because they're all grossly insensitive so would need a complex gain stage between zero sense and gate. So I rectified it for an SCR, and used a single PNP transistor as a comparator to contol the SCR, no other active parts. Choosing those two parts was a bitch, but I settled it after testing enough standard parts to find the two that worked with all samples I bought. Only those two parts worked with the extremely low power consumption I was after. I have three of them running in my flat, not one has failed to date.

    Was thinking of selling them, but I wouldn't know how, that stuff needs VAST manufacturing to make it viable as a business, on the same scale as domestic light fittings and such, and I don't even want to think about setting up for that, and I don't know how to sell my idea to those who do, as it's so simple in hindsight I couldn't tell them how it was done without giving the secret away. So I'd rather give it away anyway, and I would, if I could be sure of preventing anyone profiting from it.

    (So if anyone reading this figures it out from this text, this written statement binds you to non-commercial use).

    I had found it saved incandescent light bulbs, I don't think any ever blew while switched by this thing, because there's never any surge capable of breaking a filament. The only way bulbs left those circuits was to replace bulbs blown in others when I had no others spare, and they were usually put back when I did get some. One's been run in a hallway for about 15 years, switched many times per day... I used to think that if a big firm bought this they'd do it to bury it so they can still sell lots of light bulbs. All moot now though, soon none of that will matter. Thing still works wonders for heating elements though, I think it was good for 400 watt resistive devices as it stands, and easily more, as the limiting part was a small bridge rectifier.

    -----

    Back to topic, I can see the point about modulating on the low side but I won't settle for that without trying for myself first. I might find a way to get a common ground and a few tens of KHz mod. That zero volt switch probably wasn't supposed to be possible either, and certainly nothing I ever encountered came close to the reduction in size, cost, and power demands that I acheived with that. There's a huge amount I don't know, but if I set to a task, and there is a way, I'll usually find it. I start with looking at what's already done, but also in other situations that might adapt, and that's often rewarding.

    Edit: In one design I want to do, with a TEC, isolation will be easy anyway, TEC's make that the natural state.

  8. #8
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    Boy you guys are great, fast and helpful responses to the difficult questions, great to have a place to converse with knowledgeable people.

  9. #9
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    What about the Sam's laser faq driver that is being sold with the 405nm PS3 replacement pieces...? I bought 2 of those drivers. And I DON'T want to blow a 405nm diode!

    -Max

  10. #10
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    mixedgas is online now Creaky Old Award Winning Bastard Technologist
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    Red face

    You dont have MOC30XX series zero crossing optical triac driver chips in the UK? You poor child! In the future, this may be easier!

    http://www.fairchildsemi.com/pf/MO/MOC3041-M.html

    These really do help cut down light dimmer noise.

    Kudos on doing it yourself !

    PS we need a smily face blaster, I'mmm getting tired of them.


    Steve

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