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    Diode Laser Power Supplies

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    Laser Diode Drive Requirements

    The following must be achieved to properly drive a laser diode and not ruin it in short order: Note that the damage from improper drive is not only due to thermal effects (though overheating is also possible) but due to exceeding the maximum optical power density (E/M field gradients?) at one of the end facets (mirrors) - and thus the nearly instantaneous nature of the risk.

    The optical output of a laser diode also declines as it heats up. This is reversible as long as no actual thermal damage has taken place. However, facet damage due to exceeding the optical output specifications is permanent. The result may be an expensive LED or (possibly greatly) reduced laser emission.

    I accidentally blew one visible laser diode by neglecting to monitor the current but it wasn't the sudden effect some people describe - the current really had to be cranked up well beyond the point where the brightness of the laser beam stopped increasing. It did indeed turn into a poor excuse for an LED. One data point and you can conclude the world. :-)

    Another one was blown by assuming that a particular driver circuit would work over a range of input voltages when in fact it was supposed to be powered from a regulated source. At first the degradation in brightness appeared to be reversible. However, what was probably happening was that damage to the laser diode was occurring as soon as the brightness appeared to level off. The natural tendency was then to back off and approach this same point again. Not quite as bright? Crank up the current. Finally, once it is much too late, the realization sets in that it will *never* be quite as bright as it was originally - ever again. This one still lases but at about 1/10th of its former brightness.

    If you then try to power this damaged laser diode with a driver circuit using optical feedback, further instantaneous damage will occur as the driver attempts to maintain the normal optical output - which is now impossible to achieve and only succeeds in totally frying the device as it increases the current in a futile attempt to compensate.

    Also see the section: How Sensitive are Laser Diodes, Really?.

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    Alternatives - Diode Laser Modules, Laser Pointers

    Laser Pointers and Diode Laser Modules - The Low Stress Approach

    Where what you really want is a working visible diode laser, a commercial laser pointer or diode laser module may be the best option. Both of these include the driver circuit and will run off of unregulated low voltage DC. While the cost may be somewhat higher than that of a bare laser diode, the much reduced risk of blowout and built-in optics may be well worth the added cost. It doesn't take too many fried laser diodes to make up this cost difference!

    Believe me, it can get to be really frustrating very quickly blowing expensive laser diodes especially if you don't really know why they failed. This will be particularly true where the specifications of the laser diode and/or driver circuit are not entirely known - as is often the case. Helium-neon lasers are much more forgiving!

    Buy one that accepts an unregulated input voltage. Otherwise, you can still have problems even if you run the device from a regulated power supply. All laser pointers and most (but not all) modules will be of this type. However, if you get a deal that is too good to be true, corners may have been cut. A proper drive circuit will be more than a resistor and a couple of capacitors!

    To confirm that the driver is regulating, start with an input near the bottom of the claimed voltage range and increase it slowly. The brightness of your laser diode should be rock solid. If it continues to increase even within the supposedly acceptable range of input voltage, something is wrong with either the laser diode (it is incompatible with the driver or damaged) or driver (it actually requires a regulated input or is incorrectly set up for the laser diode you are using). Stop right here and rectify the situation before you blow (yet another) laser diode!

    See the chapter: Laser and Parts Sources for a number of suppliers of both diode laser pointers and diode laser modules.

    If you still aren't convinced that someone else should deal with laser diode drive design issues, the remainder of this chapter provides suggestions for integrated drive chips, sample circuits, and complete power supply schematics. But don't complain that you haven't been warned of the sensitive nature of laser diodes.

    Power Regulators in Laser Pointers

    Power regulators? What power regulators? Some laser pointers are so cheaply designed that there are none.

    "My laser pointer requires those little button cells which are really expensive and hard to find. I was wondering if I can instead connect 2 wires and make a battery pack for it using 3 AA batteries. Do all pointers have power regulators?"

    One would think that they all have some kind of power regulator but that apparently isn't the case. Given the downward pressure on prices of these things, it isn't surprising that regulator circuitry will be sacrificed. The manufacturer can sort laser diodes by current versus output power and select a current limiting resistor to be safe for the voltage from a new battery.

    You would have to check inside to be sure or make sure you batteries are exactly the same maximum voltage. Even that isn't totally guaranteed as really dreadful designs could depend on the internal resistance of the batteries to limit current.....

    To be reasonably safe, you would have to measure the current using a fresh set of the recommended button cells and then add enough series resistance to make sure the current can never exceed this value even with brand new AAs (or whatever you are using).

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Integrated Circuits for Driving Laser Diodes

    Laser Diode Drive Chips

    Many semiconductor manufacturers offer laser driver chips. Some of these support high bit rate modulation in addition to providing the constant current optically stabilized power supply. Other types of chips including linear and switching regulators can be easily adapted to laser diode applications in many cases:

    Note: Free samples of ICs like laser diode drivers may be available for the asking even if you won't be buying a million parts in the future. Manufacturers often provide some means of requesting free samples at their web sites. Just be honest about your needs - they consider it good PR and you might just tell a friend or colleague who WILL buy a million parts!

    Comments on Some Commercial Drivers and Detectors

    (From John Sojak (trax@allways.net)).

    As far as modulation is concerned, the Analog Devices driver is hard to beat for three bucks. Couple that with a 555 and a battle proven LM317 front end and cry 'BINGO'. Maxim used PECL inputs ... arrgh! I don't need to spit photon packets at 150 mhz! Linear Tech IR receiver looks good, although the $7.00 price tag + a handful of linear doesn't really appeal to me. Too bad you can't get inside the epoxy covered die in the Sharp TV/VCR consumer IR receiver modules (apx $1.50/100 pcs). Not everyone in the world wants to decode bursts of 40 kHz back into data!

    Oh, by the way - an Optek BP812 Optologic sensor performs quite well at at 760 nm. It's an active device available in either totem pole or open collector outputs. The applications guy at Optek says the device won't work at 760 but looking at the response curve, I disagree. It's response is only down about 10% in the reds! Most silicon photo stuff is down about 60-75% at 760ish nm. From what I have seen, the device is very usable at 760 nm. Useful part for red diodes and HeNe stuff.

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    More on Laser Diode Characteristics and Drivers

    Comments on Driver Education for Laser Diode Driver Designers

    (From: Jonathan Bromley (jsebromley@brookes.ac.uk)).

    This is getting a little scary. Laser diodes have been around for a good few years now, and I thought it was fairly widely known how you make them go and (harder) keep going for a long time; but there have been several postings recently from folk who are busy making themselves poorer by driving lasers inappropriately. Here are the rules on how you do it right:

    1. Just because it isn't hot doesn't mean you didn't already fry it.

    2. Unlike most other things, running them at the "typical" data sheet values won't work. I'm not talking suboptimal here; I mean that it won't work, not even a little bit.

    3. You must never, never, never exceed the full rated *optical* power output of the laser, not even for a fraction of a microsecond. If you do, your laser will be degraded or dead. This means LOTS of careful design to avoid nasty switch-on and switch-off transients, for example.

    4. Use the built-in monitor photodiode to regulate the light output. This monitor diode looks at the leaked light from the back facet of the laser (a few percent of the useful front-facet output). The current through it is nicely proportional to light output, if you have a reasonable reverse bias voltage on it. Anything from 2V to 15V reverse bias is usually OK (on the photodiode; *never* reverse bias a laser diode!)
    The basic problem comes from the characteristics of the laser device. They are a bit like LEDs, so you will see a forward voltage of about 2.2V for almost any reasonable forward current (just like an LED, but the voltage is somewhat higher). Voltage drive is therefore an exceedingly bad idea. Current drive is a bit more predictable. Up to a certain current - the laser threshold current - you will get the device acting like a feeble LED. Above the threshold current, laser operation starts properly and the light output rises very rapidly as a function of current. Something like this:
             ^ Light Output
             |- - - - - - - - - - - - - - -* ---- Maximum Rated Light Output
             |                            *|
             |                           *
             |                          *  |
             |                         *
             |                        *    |
             |                       *
             |                      *      |
            -|*--*--*--*--*--*--*--*-------+--------------> Forward Current
             |                     |       |   
                                   |       |<-- Maximum Current
                                   |<-- Threshold Current
    The snag is, the difference between threshold and maximum current is usually quite small; no more than 10% or 20% of the threshold. The threshold current varies greatly from one device to another (even within the same type number) and also varies with temperature. Result: setting a fixed current value is doomed to failure. For some lasers, and on some days, it will be under the threshold and no laser action will occur; on other days, it will be over the maximum current and your precious laser will turn into a useless LED (like the original posting in this thread). The only safe way is to use the monitor diode current to servo the light output. Even this isn't ideal because the monitor current is different for different lasers, but: But BE CAREFUL. Transient overdriving, even for very short times, can seriously damage the lasers. Transients commonly occur: Above all, remember that it is excessive light output that destroys lasers. The heating effect of the drive current is not a big problem except that it has the effect of pushing the threshold current down. Excessive light levels, on the other hand, can damage the tiny end mirrors of the lasing crystal.

    Sharp (one of the big suppliers of laser diodes) also make some nifty 8-pin drive chips that are pretty good if you don't need to modulate the laser rapidly. For modulation, consider setting the light output close to 50% of full output using a really slooooowww-responding feedback circuit, and then impressing a fixed-amplitude modulating current on the laser. This is OK because the gradient of the light/current graph is reasonably predictable for any given laser type, so it's possible to calculate a suitable safe modulating current from the data sheet.

    Good luck to all - and don't forget the eye safety regulations.

    (From: Paul Mathews (optoeng@whidbey.com)).

    Laser diode structures are usually so small that damage thresholds are very low on every dimension. The general approach to protecting them is to series AND shunt filter (and/or clamp) supply voltages to limit the voltage compliance of current source driving circuits. Also, consider having some of the current limiting be by means of an actual resistor rather than just active circuitry. The parasitic capacitances in active driving circuitry can interact with dv/dt on supply lines to turn on the drive circuit (e.g., drain to gate capacitance with MOSFET drive), so the resistor limits current even when this happens. Using bypass capacitance local to the pulse current loop has the dual benefit of absorbing residual transients and avoiding any effects of upstream series filter components on speed.

    (From: Mark W. Lund (lundm@physc2.edu).

    You can blow out the laser in nanoseconds if there is enough voltage and/or power in the pulse. Two methods: electrostatic discharge type damage which punches holes in the cavity; brief high power which damages the front facet.

    Make sure that the power supply to the modulation circuit is filtered to prevent surges, isolate the signal circuit to prevent surges on the input line from getting to the laser.

    There are an infinite number of ways to get a damaging pulse. Most common is the power supply. It helps to have a scope capable of capturing transients for this. The other ones that I will admit to: using a circuit that wasn't grounded to the metal optical table--brushing the table with one line of the circuit and oops; a commercial laser diode power supply that was clean until we used it in computer control mode when it sent out very fast (anhard to see) spikes; hooking the laser up backwards; using a power supply that had a big capacitor across the output which had enough charge in it to do damage; and forgetting to put a peltier cooled laser on a heat sink (the more current I gave the cooler the hotter the laser got....oops.)

    Well, that was embarrassing, but I hope it encourages others to save a few (laser diode) lives.

    (From: K. Meehan (meehan@srvr.third-wave.com)).

    Semiconductor lasers are very sensitive to power spikes. The level of current that is a problem depends on the laser structure and how much of the current is converted into optical power vs. heat. In general, reverse current spikes are very damaging, no matter what level. Make sure that you are modulating the diode so that you go below laser threshold but not below 0V. In the forward direction, very short overshoots (<1microseconds) in current can be handled until you blow the facet off of the device (catastrophic optical damage - COD). Longer pulse overshoots aren't any better. The current level that damage occurs varies from device to device. I tend to recommend less than 10% overshoot in all cases. COD is very easy to note, just look at the laser (while it is not operating) under a microscope. The facet coating is damaged near the emission region, if there is a coating. Otherwise, you will see an enhanced region (darker area) when looking under Nomarski - maybe not so easy to see.

    Another problem that you might be having is spiking during start-up or shut-down of the device. Current supplies that look lovely during operation sometimes have spikes in the output when you turn them on or off. You might want to short the device, making sure that there is no bounce during the shorting, before turning your supply on or off. There are several laser diode driver companies out there that make current generators with slow starts and minimal overshoots. Avtech, Melles Griot, ILX, etc.

    Variations in Laser Diode Monitor Photodiode Current Sensitivity

    It would be nice if the monitor photodiodes associated with all laser diodes had the same sensitivity - or even were consistent for a given model. But, unfortunately, this is not the case.

    "I am designing a driver circuit for a laser diode (NEC NDL3220S). The problem is that the spec sheet says the output of the monitor photodiode at rated power is max: 0.5 ma, typical: 0.3 ma, min 0.1 ma, at 5 V. This is a huge range! If I set for 0.3 ma and the actual output is 0.1 mA I will burn out the laser. I do not have equipment for calibrating the laser output directly."

    (From: Alan Wolke (74150.451@CompuServe.COM)).

    Welcome to the wonderful world of laser diodes! You'll find that a 5:1 range in monitor current is typical, with even a full order of magnitude being common! This is one reason why most laser diode based applications have a provision for trimming/tuning the driver circuit to the particular laser.

    Your safest bet is to design the feedback loop to operate with less than the minimum monitor current, and provide the ability to actively tune it to the appropriate operating point. Thankfully, the relationship between output power and monitor current will remain reasonably constant over the lifetime of each particular device. So, once it is properly set, you're done.

    Response Time of Laser Diode and Monitor Photodiode

    (From: Derek Weston (derekw@alphalink.com.au)).

    For those of us who have performed the infamous LD to LED (LD->DELD) conversion more often than we'd like, there's an interesting item mentioned near the end of the article: Visible-Laser Driver Has Digitally Controlled Power And Modulation regarding LD drivers. It points out two important characteristics of LDs:

    (From: John - K3PGP (k3pgp@alltel.net)).

    For high speed data and very high frequency RF subcarrier/video work I've always biased my laser diodes to 1/2 laser power then modulated them near 100%, much the same as a standard AM radio transmitter. This does result in a faster response time rather than cutting the LD completely off. It's also probably a bit easier on the laser diode especially if it's a high power unit. (Mine draws 1 amp when putting out 500 mw.)

    I never tried biasing it down to BELOW laser threshold at the 'LED' level. Although this would be an improvement over cutting it off completely, I would think this would be slower than biasing to 1/2 laser power.

    (From: Sam).

    Also see the section: Digitally Controlled Laser Diode Driver which has a bit more on the circuit mentioned above.

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Testing Laser Diode Driver Circuits

    If you do build these or any other circuits for driving a laser diode, test them first with a combination of visible (or IR) LEDs and one or more silicon diodes (to simulate the approximate expected voltage drop) and a discrete photodiode to verify current limited operation. To accommodate the higher current of laser diodes compared to LEDs, use several identical LEDs in parallel with small balancing resistors to assure equal current sharing:
        COM o--------------+-------+-------+-------+---------+
                         __|__   __|__   __|__   __|__      _|_
                   LEDs  _\_/_   _\_/_   _\_/_   _\_/_ ---> /_\ Photodiode
                           |       |       |       |         |
                           /       /       /       /         +----o PD
                         5 \     5 \     5 \     5 \ 
                           /       /       /       /
                           \       \       \       \
                1N4002     |       |       |       |
         LD o-----|<|------+-------+-------+-------+
    Note that the sensitivity of this photodiode to the LED emission will vary considerably depending on its position and orientation. Tape the photodiode and one of the LEDs together (sort of like a homemade opto-isolator) to stabilize and maximize the response.

    Using this 'laser diode simulator', it will really only be possible to confirm that the laser driver current regulator is functional, not to actually set it up for your laser diode.

    Once the circuit has been debugged, power down, and carefully install the laser diode. Double check all connections!

    Use the guidelines below in both cases (written assuming an actual laser diode is being used):

    Laser diodes are generally NOT very forgiving. However, if you take your time and make sure you understand exactly what is happening at every step along the way, you and your laser diode will survive to light another day!

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Schematics of Laser Diode Power Supplies

    The first five circuits are from published circuit diagrams or application notes, or were reverse engineered from actual devices. All use visible laser diodes though IR types would work with at most minor modifications to biasing points.

    Laser drivers (1) to (3) were from CW laser lights used for positioning in medical applications. Laser driver (4) was from a UPC bar code scanner.

    Errors may have been made in the transcription. The type and specifications for the laser diode assembly (LD and PD) are unknown.

    The available output power of these devices was probably limited to about 1 mW but the circuits should be suitable for the typical 3 to 5 mW maximum power visible laser diode (assuming the same polarity of LD and PD or with suitable modifications for different polarity units).

    Of the 5 designs presented below, I would probably recommend "Laser diode power supply 2" as a simple but solid circuit for general use. It doesn't require any special chips or other hard to obtain parts. However, I would add a reverse polarity protection diode (e.g., 1N4002) in series with the positive input of the power supply.

    In fact, funny that you should ask. :-)

    An enhanced version of this design including a printed circuit board (PCB) layout is presented in the section: Sam's Laser Diode Driver (SG-LD1).

    A very basic and a high power laser diode drive circuit are also included (both open loop - no optical feedback) as well as one that can be programmed for 1024 levels of output intensity.

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Toshiba Laser Diode Power Supply (TO-LD1)

    The actual laser driver portion of circuits (1) to (3) as well as the one presented in the section: Sam's Laser Diode Driver (SG-LD1) is very similar to the basic design provided in a Toshiba application note named something like: "Example Driving Circuit for TOLD92xx Series Visible Laser Diodes". The Toshiba Laser Diode Driver Schematic was scanned from the application note by Kent C. Brodie (brodie@fps.mcw.edu) who also provide a Circuit Description. The schematic is reproduced in ASCII, below:
      Vcc o-----------+------------+-----------------------+--------+------+
                      |            |                       |        |      |
                      |            |    Power Adjust      _|_     __|__    |
                      |            |       R2 10K      PD /_\  LD _\_/_    |
                      |            \     +----+            |        |      |
                      |        R1  /     |    |            |        |     _|_ C2
                      |       610  \     +---/\/\--+-------+        /     --- 1uF
                      |            /     |         |                \ R3   |
                      |            |     |         |                / 15   |
                    +_|_           |     |       __|__              \      |
                 C1  ---           |     |     E /   \ C            |      |
               22uF - |            +-----|------' Q1  '-------+     +------+
                      |            |     |      2SA1015       |    C|
                      |            |     |       (PNP)        |   |/ Q2
                      |           _|_.   |                    +---|  2SC1959
                      |     VR1  '/_\    |                    |   |\ (NPN)
                      |     2.2V   |     |               C3 +_|_   E|
                      |            |     |             10uF  ---    |
                      |            |     |                  - |     |
                      |            |     |                    |     |
      Gnd o-----------+------------+-----+--------------------+-----+
    This circuit lacks some of the protective features of the circuits, below, but is clearly the same core design. Complete datasheets for Toshiba laser diodes can be found by under 'Optoelectronics' and 'Visible Laser Diodes' at Toshiba's Datasheet Search Page.

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Laser Diode Power Supply 1 (RE-LD1)

    This one runs off a (wall adapter) power supply from about 6 to 9 V.
      Vcc o-----|>|-------+-----------+-------------------+--------+-----+
               1N4001     |           |                   |        |     |
              Rev. Prot.  |           |    Pwr Adj       _|_     __|__  _|_ C4
                          |           /   R3 10K (2)  PD /_\  LD _\_/_  --- .01uF
                          |       R2  \     +----+        |        |     |
                          |      560  /     |    |        |        |     |
                          |           \     +---/\/\--+---+        +-----+
                          |           |     |         |                  |
                          |           |     |         +-------||---+     /
                        +_|_          |     |       __|__   C2 (1) |     \ R4
                     C1  ---          |     |     E /   \ C 100pF  |     / 3.9
                   10uF - |           +-----|------' Q1  '---------+     |
                          |           |R    |    BC328-25 (5)      |    C|
                          |       +---+     |       (PNP)          |   |/ Q2 (5)
                          |       |  _|_.   |                      +---|  BD139
                          |   VR1 +-'/_\    |                      |   |\ (NPN)
                          |   LM431   |     |                 C3 +_|_   E|
                          |   2.5V    |     |               10uF  ---    |
                          |   (3)     |     |X                   - |     |
                R1 3.9    |           |     |Y                     |     |
      Gnd o------/\/\-----+-----------+-----+----------------------+-----+
    Note the heavy capacitive filtering in this circuit. Changes would be needed to enable this circuit to be modulated at any reasonable rate.


    Derek Weston (Email: derekw@alphalink.com.au) has constructed an IrDA tranceiver based loosely on this design and a Crystal Semiconductor Corporation CS8130 IR transceiver IC. A complete description of this project may be found at his: UPN Laser Transceiver Web Site.

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Laser Diode Power Supply 2 (RE-LD2)

    This one, from the same manufacturer as the one described in the section: Laser Diode Power Supply 1 (RE-LD1), seems to be an improved design including a soft-start (ramp-up) circuit and an inductor in series with the laser diode. Otherwise, it is virtually identical and runs off of a 6 to 9 V DC source.

    Since both units were from the same company, I assume that these refinements were added as a result of reliability problems with the previous design - in fact, I have recently discovered that the unit from which I traced that schematic is not as bright as it should be!

    Interestingly, there does not appear to be any reverse polarity protection on the input - I don't know why that would have been removed! C1 and Q1, at least, would likely let their smoke out if the power supply was connected backwards.

               2SC517 (NPN) (6)
     Vcc o----+--.  Q1 .---+---------+---------------+--------+----+-----+
              |  _\___/_E  |         |               |        |    |     |
              |     |      |         |              _|_     __|__  \ R5 _|_ C4
           R1 \     |      |         |           PD /_\  LD _\_/_  / 1K --- .01uF
         3.3K /     |      |         /               |        |    \     |  (2)
              \     |      |     R2  \               |        |    |     |
              |     |      |    390  /         R3    |        +----+-----+
              |     |      |         \    +---/\/\---+--+                |
              +-----+      |         |    |   2.2K      |                +
                    |      |         |    |             +----||----+      )
                    |    +_|_ C2     |    |           __|__ C3 (1) |      ) L1
                    |     --- 33uF   |    |   R4    E /   \  47pF  |      ) (3)
                    |    - |         +----|--/\/\----' Q2  '-------+     +
                    |      |         |R   |  220   BC328-25 (6)    |     |
               C1 +_|_     |     +---+    \           (PNP)        |   |/ Q3 (6)
              1uF  ---     |     |  _|_.  /<-+ R6                  +---|  BD139
                  - |      | VR1 +-'/_\   \  | 10K                 |   |\ (NPN)
                    |      | LM431   |    |  | Power Adjust   C5 +_|_   E|
                    |      | 2.5 V   |    +--+ (4)          10uF  ---    |
                    |      | (5)     |    |X                     - |     |
                    |      |         |    |Y                       |     |
     Gnd o----------+------+---------+----+------------------------+-----+
    Note the heavy capacitive filtering in this circuit. Changes would be needed to enable this circuit to be modulated at any reasonable rate.


    This design is virtually identical to the circuitry found in typical laser pointers like the Radio Shack 63-1040 reverse engineered by Walter Gray (Email: soldersucker@geocities.com).

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Laser Diode Power Supply 3 (RE-LD3)

    This one runs off of a (wall adapter) power supply from about 10 to 15 V (12 V nominal).

    It was apparently designed by someone who was totally obsessed with protecting the laser diode from all outside influences - as one should be but there are limits. :-) This one goes to extremes as there are 5 levels of protection:

    The first part of the circuit consists of the input filter, soft start circuit, voltage regulator, and DC-DC voltage converter. Its output should be s super clean, filtered, despiked, regulated, smoothed, massaged source of -10 V ;-).
                L1      D1     C       E   I +--------+ O                  -10V out
     +12 o--+--CCCC--+--|>|--+--. Q1  .---+--| LM7810 |--+-------+               o
            |        |1N4002 |  _\___/_   |  +--------+  |       |            C5 |
            |        |    R4 /     |      |      C|      |       |        +------+
            |        |   10K \     |      |       |      |      8| 7  6  5|  180 |
            |        |       /     |      |       |      |     +-+--+--+--+-+ uF |
          +_|_ C10 +_|_ C11  |     |    +_|_ C8   |  C7 _|_    |            |16V |
           --- 2.2  --- 2.2  +-----+     --- .22  |  .1 ---    |   LT1054   |  +_|_
          - |  uF  - |  uF   |     |    - |  uF   |  uF  |     |            |   ---
            |        |     +_|_   _|_     |       |      |     +-+--+--+--+-+  - |
            |        |   C9 ---   --- C6  |       |      |      1  2| 3| 4|  C3  |
            |        | 4.7 - |     | .047 |       +------+-------------+--|--||--+
            |   L2   |  uF   |     |  uF  |       |              C4 |+  - |.01uF |
     Gnd o--+--CCCC--+-------------+------+-------+       180uF,16V +-|(--+------+
    It was not possible to determine the values of L1 and L2 other than to measure their DC resistance - 4.3 ohms. The LT1054 (Linear Technology) is a 'Switched Capacitor Voltage Converter with Regulator' running at a 25 kHz switching frequency. A full datasheet is available at http://www.linear-tech.com/.

    The input to the LM7810 ramps up with a time constant of about 50 ms (R4 charging C9). This is regulated by the LM7810.

    The LT1054 takes the regulated 10 V input and creates a regulated -10 V output. There is no obvious reason for using this part except the desire to isolate the laser diode as completely as possible from outside influences. Like the use of an Uninterruptible Power Source (UPS) to protect computer equipment from power surges, a DC-DC converter will similarly isolate the laser diode circuit from any noise or spikes on its input.

    The second part of the circuit is virtually identical to that described in the section: Laser Diode Power Supply 1 (RE-LD1):

      Gnd o----------------+------------+------------------+-------+-----+
                           |            |                  |       |     |
                           |            |     Pwr Adj     _|_    __|__  _|_
                           |            /      R2 20K  PD /_\ LD _\_/_  --- C2 
                           |        R1  \     +----+       |       |     |
                           |       470  /     |    |       |       |     |
                           |            \     +---/\/\--+--+       +-----+
                           |            |     |         |                |
                         +_|_           |     |       __|__              /
                      C1  ---           |     |     E /   \ C            \ Rx
                    10uF - |            +-----|------' Q1  '-------+     /
                           |            |R    |       PN2907       |    C|
                           |            |     \       (PNP)        |   |/ Q2
                           |           _|_.   / R3                 +---|  PN2222
                           |      VR1 '/_\    \ 1K                 |   |\ (NPN)
                           |      LM385 |     /               C1 +_|_   E|
                           |      Z2.5  |     |             10uF  ---    |
                           |            |     |X             16V - |     |
                           |            |     |Y                   |     |
     -10 V o---------------+------------+-----+--------------------+-----+
    Note the heavy capacitive filtering in this circuit. Changes would be needed to enable this circuit to be modulated at any reasonable rate.

    I suspect that there are additional components inside the laser diode assembly itself (like the hypothetical Rx, probably a few ohms) but could not identify anything since it is totally potted.

  • Back to Diode Laser Power Supplies Sub-Table of Contents.

    Laser Diode Power Supply 4 (RE-LD4)

    This more sophisticated (or at least more complicated) driver board uses a dual op-amp (LM358) chip instead of discrete parts to control a transistor current source. Due to the relative complexity of this design, and the fact that it is entirely constructed of itty-bitty surface mount parts, errors or omissions with respect to both transcription and interpretation are quite possible!

    The schematic for the driver is available in both PDF and GIF format:

  • Get LDDRIVE-SCH: lddrive.pdf or lddrive.gif.

    The feedback loop consists of the photodiode (PD, part of D1), a non-inverting buffer (U2A), the inverting amp/low pass filter (U2B, R9, R11, C2, bandwidth of about 1 kHz), and emitter following current source (Q1, R13, R14, with a sensitivity of 36 mA/V) driving the laser diode (LD, part of D1).

    Separate DC inputs are shown for the laser diode/photodiode itself (Vcc1) and the other circuitry (Vcc2). Vcc1 must be a regulated supply as there is no on-board voltage reference. It appears as though Vcc1 and Vcc2 should be set equal to one-another though there may have been (external) power sequencing in the original application. If Vcc1 is less than Vcc2 by more than a volt or so, the laser diode will be turned off. The input voltage range can be from 5 to 12 VDC though I would recommend running on 5 VDC if possible since this will minimize power consumption and heat dissipation in the current driver transistor and other circuitry. This is adequate for laser diodes with an operating current of up to about 80 mA. For laser diodes with an operating current greater than this, a slightly higher voltage will be required.

    The set-point is at about 1/2 Vcc1 so that the laser diode optical output will be controlled to maintain photodiode current at: I(PD) = .5 Vcc1 / (R6||R7). Use this to determine the setting for R7 (SBT, Select By Test, Power Adjust) for the photodiode in your particular laser diode. Or replace R7 by a low noise variable resistor and