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Thread: TTL/Analoge drivers?

  1. #21
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    Thumbs up TL,DR: Yes, you understand it. :)

    Quote Originally Posted by myself View Post
    Show controllers provide a voltage output for each color channel, which indicates the intended brightness of that channel. It's called "analog" because the voltage varies throughout its range (probably 0-5v in most cases) according to the requested brightness at any given point in time, so for instance, 2.5v would mean "half power".
    This is absolutely correct. If you want to get into the gritty details, you may want to read through the ILDA standard for projectors, which talks about all the signals sent to a projector, not just the color modulation signals.

    This isn't useful for driving lasers directly, because it has weak drive strength, and because diodes are current-driven. It's just a signal.
    Yup. You've got this down pat.

    Diode lasers, like other diodes, have a "forward voltage drop" when they're operating. Varying the voltage across them is nonsensical, the voltage is basically flat even across wide variations in current. (This is the flat region after the "knee" in the I-V curve.) So the function of a laser diode driver is to take that voltage signal from the controller, and use it to control a current with some chunky drive strength behind it.
    Correct again. Laser diode drivers are constant-current drivers. However, in the end the only way to change the amount of current flowing through the diode is to change the voltage across the diode. (Ohms law, basically.) The difference is that a tiny change in voltage can have a dramatic effect on the current. So the driver is designed around maintaining a given current output that is proportional to the input signal.

    At its heart, a laser diode driver is probably a big fat transistor and sense resistor for feedback, with an op-amp varying the drive current according to the signal voltage.
    Pretty much. Often mosfets are used, but I've seen darlington transistors used in higher-current drivers. The current sensing resistor is often a .1 or .2 ohm precision resistor, and it needs to be purely resistive; a wire-wound resistor will wreak havoc with the driver due to inductive effects. Beyond that there are tricks to tweak the feedback circuit to eliminate ringing, and some drivers also include safety circuits to protect against excessive input modulation, reverse polarity, and so on.

    neither the analog color signals nor the diodes themselves are ideal. The color signal may not go fully to zero during times when the laser should be off.
    Surprisingly the color signals from the controller are pretty close to ideal. You can run into issues where you can't get the full 5 volts to the driver due to low input impedance on a poorly designed driver (this makes the losses in the long signal cable more significant), but it's very uncommon to not see zero volts when the laser should be off.

    A bigger problem is the linearity of the diode. This can be compensated for in software (assuming it's a supported feature), but it is tricky.

    Another issue is what to do about the lasing threshold. Most diodes will begin to emit light at very low current levels, but you won't get a coherent beam until you get above the threshold current. Some drivers try to improve the modulation response time by "idling" the diode at just below the threshold current. This results in a diffuse output all the time, which is visible if your projector doesn't have a shutter. The alternative is to completely kill current to the diode below a given input signal level, but this introduces a delay when the diode turns on. Different drivers follow different schools of thought when deciding how to handle these two problems. (Some allow you the flexibility to set it up however you want, idle or no idle.)

    So a good driver probably has adjustments (trimpots?) to scale both the top and bottom of the input signal to match what it gets back from the sense resistor.
    You *need* the high-end adjustment for any driver, or else you run the risk of blowing the diode from over-current. Having the low end adjustment is a nice bonus that many drivers include, and being able to set the cut-off voltage on the modulation signal (anything below this level = no output) is another plus, although it's less commonly found.

    It hopefully has some sort of temperature feedback, which down-regulates or outright inhibits the drive current with increasing temperature.
    This is a high-end feature that is not normally found on hobbyist-level drivers. Some commercial drivers include a thermal feedback input from the diode can, however.

    It might have thermal protection for itself, too, since that big pass transistor will be operating in its linear region and dissipating a bunch of power when the laser is on.
    Some of the nicer drivers have an internal thermal shutdown, but most of them just warn you to do the math ahead of time to ensure you heat sink the output transistor appropriately.

    When we buy the cheap RGB TTL laser modules, there's a tendency to think of the whole package as a monolithic unit. Four mounting screws, a power connector, 3 color connectors, done. But really, there's nothing special about this cheap 3-in-1 TTL driver board, it can be removed and replaced with better drivers. The lasers themselves are just 2-wire devices, and can be driven as we please.
    Yes, assuming that you know which diodes are used (so you can set the current limits on your analog drivers properly). Barring that, you'd need to measure the current first, before you replaced the drivers. But yes, several people have upgraded their TTL-only projectors with better drivers to get analog performance. Note that you need to be extremely careful when connecting and disconnecting the leads to the laser diode, as laser diodes can be killed instantly by static electricity. In particular, low power single-mode diodes (*especially* low power red diodes) are super vulnerable to this failure mode. (If you don't mind going down the rabbit hole, look into the Pangolin "Lasorb" device.)

    As for knowing where to set the current, that information is readily available for nearly every modern diode on the market. A conservative approach would be to use the data sheet limit, but most people simply go to DTR's Laser Shop and look at the test results he's posted for each diode. (He posts current draw vs power output in fine steps from threshold all the way *past* the maximum.) And of course, you could always measure it yourself by applying a steady 5 volt signal to the TTL driver and measuring the actual current to the diode...

    Also, once you know where you need to set the current you can use a dummy load to set it ahead of time (before you connect the laser to the driver), or you can dial the driver down low, connect the diode, and then ramp it up while you monitor current in real time. Either way works. Just don't get carried away or you'll blow your diode.

    Bottom line: you have a very good understanding of how this all works.

    Adam

  2. #22
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    On making first adjustments to modest laser DIY LED drive currents I can recommend the easiest and cheapest way so that one doesn't destroy the diode is to build a simple current limiting-voltage divider setup as shown in the attached photo. Diodes are 1N4007's.

    Click image for larger version. 

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    I had bought an RGB bundle from DTR's Shop that was a total output of around 600mW, and so to get a feel for how much drive current was needed at what voltage I used this jig. Mind you, one still needs to use good anti-static practices since this does not use diode protection like Lasorbs.

    Once I had a safe ballpark idea for what voltage and current each color diode would begin lasing I then knew how to setup my FlexMod P3 drivers on each color diode with dummy loads.

    I eventually bought a LaserBee 3.7W USB power meter since it was fairly inexpensive ( although in past days and time I had use of a Coherent 210 power meter good for 10's of mW to 30W) and used the LaserBee to get some measurements of my Chinese 1W RGB projector. The combined measurements seemed to support the 1W claim but I longed for a second opinion. Some months later, while searching Ebay I found the meter portion of a Coherent 210 power meter in great condition and very cheap, but without a sensing head. Just a few weeks later I found a separate seller of a 210 power meter sensing head very cheap with high power heatsink. I put two 9v batteries in the meter base, plugged the head in and IT WORKED PERFECTLY on all ranges.

    A few days ago I ran the same power measurement tests on the 1W RGB projector and the power levels for each color matched the LaserBee readings. The Coherent power meter/head granted has a much faster acquisition time for registering optical power than the LaserBee, but still, the LaserBee only took about 10-14 seconds to stabilize on each reading, plus it generates a graphic log of its readings. I paid just about the exact same amount for ye' ol' Coherent 210 meter and head as I did the new LaserBee 3.7W USB.

    I heartily recommend either one since they are under $200US.
    ________________________________
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  3. #23
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    ... or even more simple - a LM317 and a resistor (excerpt from the datasheet):
    Attached Thumbnails Attached Thumbnails LM317 - constant current regulator.png  

    Aufruf zum Projekt "Müll-freie Meere" - https://reprap.org/forum/list.php?426
    Call for the project "garbage-free seas" - https://reprap.org/forum/list.php?425

  4. #24
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    Quote Originally Posted by buffo View Post
    This is a high-end feature that is not normally found on hobbyist-level drivers. Some commercial drivers include a thermal feedback input from the diode can, however.
    The very affordable SimpleDrives linked earlier include a thermistor which is optional to use as well as a heck of a lot of other features normally associated with higher end drivers. The only thing I can think of that it doesn’t do is run a TEC.

    David
    "Help, help, I'm being repressed!"

  5. #25
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    [QUOTE=dkumpula;347656]The very affordable SimpleDrives linked earlier include a thermistor which is optional to use as well as a heck of a lot of other features normally associated with higher end drivers. The only thing I can think of that it doesn’t do is run a TEC.[/'quote]

    The SimpleDrive is the lone outlier in that regard. Although I'm pretty sure the thermistor is meant to be installed on the heatsink of the output FET to ensure it doesn't overheat, rather than being installed on the diode mount. But yeah, it's a good driver (and affordable, as you said).

    Adam

  6. #26
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    [QUOTE=buffo;347658]
    Quote Originally Posted by dkumpula View Post
    The very affordable SimpleDrives linked earlier include a thermistor which is optional to use as well as a heck of a lot of other features normally associated with higher end drivers. The only thing I can think of that it doesn’t do is run a TEC.[/'quote]

    The SimpleDrive is the lone outlier in that regard. Although I'm pretty sure the thermistor is meant to be installed on the heatsink of the output FET to ensure it doesn't overheat, rather than being installed on the diode mount. But yeah, it's a good driver (and affordable, as you said).

    Adam
    And am I correct that the SimpleDrive ships from overseas?
    ________________________________
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  7. #27
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    Comes from Moscow, Russia. Great product, I’ve bought a lot of them.
    Runs with Lasers

  8. #28
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    Quote Originally Posted by clickamouse View Post
    Comes from Moscow, Russia. Great product, I’ve bought a lot of them.
    Thanks, I thought I had read they were overseas, heard great reviews on them, just wondered how fast shipping was.
    ________________________________
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  9. #29
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    Quote Originally Posted by buffo View Post
    The SimpleDrive is the lone outlier in that regard. Although I'm pretty sure the thermistor is meant to be installed on the heatsink of the output FET to ensure it doesn't overheat, rather than being installed on the diode mount. But yeah, it's a good driver (and affordable, as you said).
    Hi Adam. I agree with you that this is an outlier. I went back and double-checked to be sure, but the thermistor is meant to be installed on the diode mount/heat-sink. It is set to shut down the driver if the temperature reads 50C or more. Here is a copy of the instructions if anyone is interested.
    Click image for larger version. 

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    To be honest, I haven't used the thermistors to date, but plan to incorporate them on my latest build. Heat isn't much of an issue on my builds are they are all single mode.

    Quote Originally Posted by lasermaster1977 View Post
    Thanks, I thought I had read they were overseas, heard great reviews on them, just wondered how fast shipping was.
    To the US, delivery times vary widely. Using cheap/untracked post, I've seen an average of 2-5 weeks with one out of a couple dozen orders lost in the post. Tracked postage is more, but the delivery time is closer to one week and I haven't lost any packages shipped this way.

    Happy shopping!

    -David
    "Help, help, I'm being repressed!"

  10. #30
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    Hi, i've been away from this for a while, however, i've been reading all the post and replies.
    I have a few question now, since i didn't tought of this for a while .
    I will decide to buy the SimpleDrivers, do i need any good heatskink for them?
    Can i make a show only from the Sdcard in the projector?
    Lat but not least; If in a show, i have the diodes permantly going off and on, doesn't that affect the life of the diode? Since the first current can cause peaks?
    Greetings, and Happy new year!
    Last edited by Shysk; 01-07-2019 at 07:31.

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