In article <3vs96m$lj7@ixnews4.ix.netcom.com> roy_h@ix.netcom.com (Roy Hendricks on) writes: > I'm interested in a laser diode circuit using the internal pin diode > for feedback to limit the current so I dont blow them up. > Can anyone help? > the Visible 670nm laser diode that I want to use is the panasonic from > digikey or the NEC unit that mouser has.unless you have better > suggestions. > NEC specs: (mouser) > threshold current: 80ma Narrow range > operating current: 85ma > LD operating Voltage: ?? v > built in photodiode > $21.00 ea > Panasonic specs; (digikey) > threshold current: 25ma > operating current: 35ma > LD operating Voltage: 2.5v > built in photodiode > $37.50 ea > I cant afford to buy many of them so I want to be as safe as I can > Thanks in advance > Roy H You are correct to be cautious. They are easily damaged. Here are some comments relative to CD player diodes but they mostlly apply to visible laserdiodes as well. CD Laser Diodes: --------------- Note that these are infrared (IR) emitters, usually 780 nm. There is a spurious very low power line in the red part of the spectrum but the main beam is IR and invisible. Take care. A collimated 5 mW beam is potentially hazardous to your eyes. Typical CD laser optics put out about .3-1 mW at the objective lens though the diodes themselves may be capable of up to 4 or 5 mW depending on type. If you saved the optical components, these will be useful in generating a collimated or focused beam. Typical currents are in the 30-100 mA range at 1.7-2.5 V. However, the power curve is extremely non-linear. There is a lasing threshold below which there will be no output. For a diode rated at a typical current of 40 mA, the threshold current may be 30 mA. This is one reason why many applications of laser diodes include optical sensing (there is a built in photodiode in the same case as the laser emitter) to regulate beam power. You can easily destroy a laser diode by exceeding the safe current even for an instant. It is critical to the life of the laser diode that under no circumstances do you exceed the safe current limit even for a microsecond! Laser diodes are also supposed to be extremely static sensitive, so use appropriate precautions. Also, do not try to test them with a VOM which could on the low ohms scale supply too much current. It is possible to drive laser diodes with a DC supply and resistor, but unless you know the precise value needed, you can easily exceed the ratings. You can identify the laser diode and the photodiode because the photodiode's forward voltage drop will be in the approximately .7 V range rather than 1.7-2.5 V for the laserdiode. So, for the test below if you get a forward voltage drop of under a volt, you are on the photodiode leads. One approach that works for testing is to use a 0-10 VDC supply with a, say 100 ohm resistor in series with the diode, and slowly bring the current up until you get a beam. Use an IR detector for this! If you get the polarity backwards, the voltage across the diode will go above 3 volts. Then, turn power off and reverse the leads. This will not damage either diode. For an actual application, you should use the optical feedback to regulate beam power. You should also use a heatsink if you do not already have the laserdiode mounted on one. CD laserdiodes are designed for continuous operation. Here is an example of a CW laser light power supply. Obviously, you will need to make design changes to accomodate your diode polarities and current levels. Just consider a start. Uuencoded zipped postscript. --- sam begin 644 laser.zip M4$L#! 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