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Thread: Is it possible to find replacement for an individual module form a cheap RGB laser?

  1. #1
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    Laser Warning Is it possible to find replacement for an individual module form a cheap RGB laser?

    Two years ago I started working on a DIY laser projector. I have ordered a set of galvos and the cheapest RGB laser set that I could find (300mW, 50 euros from ebay.de, I think). Today I decided to pick up the project again, but I was disappointed to find out that the red module doesn't work. By measuring some of the pins, I found that it gets 3.7V, just like the green and the blue diodes, so it's unfortunately not a power/control board issue. I can see a very faint red glow in the red diode, but it's so dim that it doesn't even project a dot on my wall. Obviously it should be much stronger, I've been able to light matches with it in the past.

    I could buy a replacement set, but 50 euros isn't very cheap to me, and it seems like an overkill to replace the entire thing if only one laser is broken.
    Is this perhaps some sort of standard diode that I could buy ebay or didgkey or a similar place? Based on what I can see, it is 638nm and it's supposed to run on 3.7V. It should also fit in the enclosure that I already have.

    I have uploaded some images here about my laser kit. You can see how dim it is, because it is turned on on all the images except the first one. For more specs about my lasers, this is the closest product I could find on british ebay. Different enclosure, but same control board, wattage and wavelengths.

    I found this diode that seems to be the right wavelength, and it looks the same as my broken one, but I'm not sure if it's gonna be compatible with the control board and lens that I already have. The only thing that probably doesn't match is the wattage, which on the linked diode is 500mW, but I'm pretty sure that the one I already have was only 120mW. Not sure if this would be a problem with the control board.

  2. #2
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    Actually I just looked at the specs for the diode I've linked, and it seems to be 2.6V max so it probably won't work...

  3. #3
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    Do the diode drivers have potentiometers? If so, you may be able to dial up or down the current. Voltage doesn't really matter as long as the driver is capable of supplying what is needed.

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    Quote Originally Posted by JohnYayas View Post
    Do the diode drivers have potentiometers? If so, you may be able to dial up or down the current. Voltage doesn't really matter as long as the driver is capable of supplying what is needed.
    Yes, it has three glued-down potentiometers. I will see if I can break the glue and adjust it to the correct voltage.

    Other than the voltage, do you think the higher wattage will be a problem? (the broken diode was somewhere between 100mW and 200mW, the replacement would be 500mW)

  5. #5
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    Diode drivers don't regulate voltage, they regulate current. There will not be an output voltage adjustment at all unless there's a step-down converter before the constant current stage, which there is on your module, but it's not adjustable. Since you only have one pot per channel, that's going to be a TTL driver (on/off only, no intensity control) and the pots will be your on current adjustment.

    Most likely your diode is just dead; low output even at nominal current is a reasonably common failure mode. It's possible that it's a problem with the driver, like perhaps poor connection between the wiper in the pot, in which case MAYBE tweaking it will get you somewhere, but more than likely trying to turn the current up will just make it more dead, especially if you try to do so without any idea how much current you're already giving it.

    You can monitor the diode current by measuring the voltage across the current sense resistor (circled below). Since that's a 2R resistor, divide the voltage by two and that's your current. (The blue also uses a 2R resistor, so same goes for it, the green has a 1R, so the voltage reading is the current reading.) If it's more than, mmm, 150mA? then the diode is dead. If you choose to replace the diode, then be sure to set this current well below the maximum indicated for the new diode before hooking the new one up.
    Attached Thumbnails Attached Thumbnails Annotation 2020-08-18 163905.jpg  

    Last edited by aberry; 08-18-2020 at 13:03.

  6. #6
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    Quote Originally Posted by aberry View Post
    Diode drivers don't regulate voltage, they regulate current. There will not be an output voltage adjustment at all unless there's a step-down converter before the constant current stage, which there isn't on your module. Since you only have one pot per channel, that's going to be a TTL driver (on/off only, no intensity control) and the pots will be your on current adjustment.

    Most likely your diode is just dead; low output even at nominal current is a reasonably common failure mode. It's possible that it's a problem with the driver, like perhaps poor connection between the wiper in the pot, in which case MAYBE tweaking it will get you somewhere, but more than likely trying to turn the current up will just make it more dead, especially if you try to do so without any idea how much current you're already giving it.

    You can monitor the diode current by measuring the voltage across the current sense resistor (circled below). Since that's a 2R resistor, divide the voltage by two and that's your current. (The blue also uses a 2R resistor, so same goes for it, the green has a 1R, so the voltage reading is the current reading.) If it's more than, mmm, 150mA? then the diode is dead. If you choose to replace the diode, then be sure to set this current well below the maximum indicated for the new diode before hooking the new one up.
    After poking around the board with my multimeter, I also found that only the red channel is 3.7V, the other two headers are outputting 6.8V. So it might be a dead diode and a dead board... Also, I measured the current, and it is 250mA (0.5v/2).

  7. #7
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    No, that makes it sound like the driver is fine and the diode is dead. The driver does not control the voltage across the diode, that is determined by the diode's electrical characteristics and is primarily a function of the current being applied to the diode. It sounds like you measured those voltages with the diodes disconnected, in which case you are measuring the output of the step down converters on that board, which just tells you the maximum forward voltage the driver can deal with (although the driver transistor and current sense resistor will take a bite out of that). Red diodes typically have lower forward voltages than green and blue so your numbers sound entirely reasonable. But you're measuring 250mA through the red, and you're getting some glow, so clearly the driver is working. The only way a bad driver would cause the behavior you're seeing is if there's a partial short across the diode output which would be a very unusual failure and could be ruled out by measuring across the current sense resistor with the diode disconnected. 250mA may be too high for the red diode and that may have contributed to its demise, but it's hard to say without know what specific diode it is.

    Also, FYI, do NOT disconnect or reconnect the diode while the driver is powered on. That is a very easy way to kill a diode (although your read seems to already be dead, so...).

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