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Laser current modulation issues

  1. May 19, 2008 #1
    I'm working with a couple different diode lasers, attempting to automate control of them (on and off mainly) by modulating the source currents externally. One of them is a green laser that draws ~1.4A, 3.3V but has no modulating chip in the power line.

    I've got another smaller red laser module (draws ~50mA @ 3.3V) which does have a transistor in the line with a 0-5V analog input to modulate the current. I figured that since the company I got this module from assures me that the transistor can handle a couple amps through it, I should be able to cut the wires on either side and splice the transistor into the green laser power supply.

    However I'm in the middle of the splice job and have a (maybe stupid, maybe major) concern: The wires used for the low-power laser (coming out of the modulation control transistor) are very small and I'm not sure that they'll be able to handle the higher current for the green laser (1.4A).

    Will I have problems with the modulation working reliably as a result of this?? If this modulation scheme sounds silly, could anyone suggest a good transistor or switch to use for regulating current with a 0-5V input?
  2. jcsd
  3. May 19, 2008 #2


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    What gage wire is it?
    Why not just connect the new wires directly to the module?
  4. May 19, 2008 #3
    I believe it's around 22-26 gauge wire, and after checking a bit more I've found a website which suggests I should have no problem with 1.4 A on this system.

    My main motivation for not just ripping the small wires off is because as-is, the module is shrink-wrapped a couple times and I was hoping to not have to start from square-one with the components.

    The more I think about this, the more I'm concerned that the modulation control unit won't actually handle the higher current load in a similar fashion: if the red laser draws 50mA and the module regulates the output linearly with a 0-5V input, would it really be able to linearly regulate 1.4A with that same 0-5V input while achieving the full range?
  5. May 20, 2008 #4


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    Probably not, it sounds like it was designed to pass 50mA when the input is at or close to 5V.

    But you can test this. Instead of connecting to the green laser, connect it to a test load instead. Either a 1 or 2 ohm resistor, or a few standard diodes in series (that can handle 1.4 A) should work for testing purposes. See what the max current is and go (or not) from there.

    Whether you use resistors or diodes to test, make sure they can handle the current you plan to drive them at. If not, then you can at least test them up to their max rating and decide if 1.4A might be feasible.
  6. May 21, 2008 #5
    Yarr. I made a HUGE initial assumption that now changes my problem completely.

    The "modulation control" output has 3 wires: red, black, and yellow. After probing voltages while playing with the input, I've determined that the red and black are standard dc power, whatever is input before the black box "modulation control". The yellow, quite sadly, is simply whatever voltage is input at the modulating input.

    This means that the device I thought was a small transistor wrapped up is not at all the modulation control circuitry, but in fact voltage references. The sneaky company has tricked me: The modulation control must be built-in to the red diode laser that I do not care to dismantle at this point.

    And so my problem morphs back to a much more basic question:
    I'm looking for an item that will regulate (modulate) the current delivered to a load based on an analog 0-5V or digital 0/5V input. I need it to be able to wire to a USB DAQ device output so I can use Matlab to automate turning a laser on and off. It must be able to handle ~1.5A through it. I'd prefer not to use a transistor that needs to be bread-boarded. Are there any relatively easy and CHEAP solutions???

    Thanks for the advice so far!
  7. May 21, 2008 #6


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    Can you just use a 5V reed relay?
  8. May 22, 2008 #7
    I guess I could use one.... but:

    I thought that reed relays are for low-current, slow switching applications and aren't particularly durable due to the moving contacts.

    I need to be able to switch my laser on and off at ~20-40Hz, and I figure that a transistor would be better for that. Suggestions on transistors, circuits, or alternative prepackaged devices?
  9. May 22, 2008 #8


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    Ah, no a relay is not useful at 20-40Hz. Their settling time is on the order of a few ms of bouncing. So you need to use a solid state switch like a transistor.

    You can look at the various IO devices available from National Instruments, and see if they have a useful PC-driven solid state switching device. 1.4A is pretty beefy, though, so you may need to add an external transistor driven by that PC-driven IO.

    You can also search for other simple PC IO devices, to see what capabilities are out there.




    If you just want the solid state relay itself, go to www.digikey.com and search on PVDZ172N-ND (solid state relay stand-alone). I'm not sure if a SSR will work for you -- check out the datasheet and let us know.

  10. May 22, 2008 #9
    A power mosfet could do the trick aswell.

    I've used an IRFZ24N before and they can handle a maximum of 12 A, also they can switch fast with turn on times on the order of ns.

    Sounds fine for your application. Good luck :)
  11. May 22, 2008 #10
    Since I already have a USB I/O device for the 0-5V analog control (measurementcomputing*dot*com USB-1208LS) capable of sourcing 30mA, I figure that a solid-state relay should work.

    Berkeman, the one you've recommended is unfortunately not an in-stock item and I'm betting I'd need to order more than one. I did, however, find an alternative photorelay that i think will work:

    Toshiba TLP3542. Since I can't link to it, here are relevant specs:
    1.33VDC input
    10-20mA forward current
    0-2.5A load
    0-60V load

    Attached are a couple diagrams of the chip and testing circuit from the datasheet. So am I right in thinking that in order to use this chip, I would need to:
    1) attach the 0-5V analog output from the PC I/O board to pin 1
    2) attach the ~3V, 1A source to the red wire to the diode laser
    3) attach the black wire from the diode laser to pin 6
    4) Tie pins 2 & 4 together, and ground them to both the ground on the PC I/O board and the power source for the laser

    Sorry to be asking again, but I just want to make sure that I have this right before I attempt!

    Attached Files:

  12. May 22, 2008 #11


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    Looks like a better choice than the one Berkeman found.
    The TLP3542 has a higher current rating and your 1.4 amp requirement was kind of close to the max rating of the other part.

    Edit: Verify that the max turn on turn off time will fit your requirements. It should but it's somewhat slower than the part Berkeman suggested.
    Last edited: May 22, 2008
  13. May 27, 2008 #12
    To wrap up this issue, this is my implemented successful solution:

    I had a power MOSFET sitting around the lab, and did a bit of research on it (IRFZ44N). It has a lot more capability than I need:
    +-20V gate voltage
    +-50V drain-source voltage max
    49A max drain-to-source current

    Perfect. Since I'm running a variable current & variable voltage power supply, I simply set a max. of ~3V, 1.2A and used the analog output (4.5V to overcome the threshold voltage) to switch on and off.

    Absolutely perfect for a very fast, low leakage current voltage controlled current source. Plays nicely with Matlab :-p
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