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Laser power rating question

bakulaw

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Hi! Pls help me understand laser power rating. Im newb at electronics.

If the laser diode is rated for ex. 1.6W does it mean, if Im putting 4V to the laser, I shouldn't be pulling more than 400mA since 4V x .4mA = 1.6W even if my laser diode Current rating is 1.7A?

How about when Using Lm317 as constant current regulator, Should I also control voltage so that it doesnt exceed power rating of diode?
 
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Pi R Squared

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The power input to the driver has little to do with the power output of the laser diode and neither does the power output of the driver.

The power input to the driver will be higher than its output.

The power output from the driver will always be higher than the power output of the diode.

All laser diode drivers are constant current, never constant voltage, they will vary the voltage as needed to keep the current constant.

An LM317 driver if set to the correct current can't exeed the power rating of the diode, however that is a linear driver, if you give it a higher voltage than it needs the LM317 will simply get hot, with too high a voltage very hot.

Alan
 

bakulaw

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The power input to the driver has little to do with the power output of the laser diode and neither does the power output of the driver.

The power input to the driver will be higher than its output.

The power output from the driver will always be higher than the power output of the diode.

All laser diode drivers are constant current, never constant voltage, they will vary the voltage as needed to keep the current constant.

An LM317 driver if set to the correct current can't exeed the power rating of the diode, however that is a linear driver, if you give it a higher voltage than it needs the LM317 will simply get hot, with too high a voltage very hot.

Alan
Thanks for the reply! What im talking about is the voltage given to the diode not the driver.lets say im not using a constant current source, just a voltage regulator, If ex. 4V is going to the diode the max current should just be .4 so that it won't exceed 1.6w rating of the diode? Sorry for my Engrish :)
 

Pi R Squared

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No you must not give the diode a constant voltage, it will require some minimum voltage to begin lasing but after that the driver has to handle it. The diode would heat up and it's resistance drop and begin drawing more and more current until it kills the diode.

The diode will also emit some fraction of the power input to it anyway, so it will still heat up, that's why they have a duty cycle unless you have some form of active cooling beyond just a heat sink. The only way to tell a diodes output is with a Laser Power Meter (LPM), you can't calculate it's power because even 2 diodes of the same type will vary in how efficient they are.

Alan
 

bakulaw

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I thought if you know voltage and current you'll know the power. Is laser diodes power rating different than any other electronics?
 

Pi R Squared

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I thought if you know voltage and current you'll know the power. Is laser diodes power rating different than any other electronics?
No it isn't, I already explained it, I don't know how to make it clearer, yes P=IV and yes that's how much power the diode will consume but it has nothing to do with the output in the form of the beam. For example let's say you have a laser diode that is 30% efficient and it's drawing 2W of power, then the output will be 600mW and the other 1400mW will be dissipated in the form of heat.

Alan
 

WizardG

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I think what the OP is needing to understand is that the rated power of a laser diode is the output of the diode. If your diode is rated at 1.6 watts, per the OP, that means it can output 1.6 watts of energy in the form of light. If the diode is given 1.7 amps of current, and has a forward voltage of 4 volts that means it is using 1.7 * 4 = 6.8 watts of electrical energy to output 1.6 watts of light energy with the rest of the power lost as heat.
 

bakulaw

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Thanks I think I understand now. That 6.8W is what im talking about. So the power rating that im looking for is not written in datasheet. Thanks all!
 

WizardG

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You can that 6.8 number from the data sheet if you're willing to do a little calculation. Just multiply the rated current of the diode times the rated forward voltage for the electrical input in watts.

And please allow me to re-emphasize what PI R' said above; NEVER connect a laser diode you care about to a constant voltage source.
 

bakulaw

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Thanks again! I thought I can treat it like ordinary LED. Good thing it's not too late :)
 

WizardG

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Even regular LEDs will die rather abruptly if they're connected to a constant voltage source. You can get away with connecting an LED straight to a coin cell battery because the coin cell can't supply enough current to kill the LED. The best educational example I can think of is this: hook up a 1 watt LED, without a heatsink, to a well regulated power supply and turn up the voltage until the led is passing, say, 100mA. As the LED warms up it will pass more current, making it heat up more, which will cause it to pass more current, which will cause it to heat up even more. Lather rinse repeat. This is called thermal runaway. Laser diodes are just more sensitive to this than ordinary LEDs.
 




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