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Old 01-31-2012, 04:27 PM #1
edy edy is offline
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Default Voltage regulation

Hi,

i've been thinking since I ever saw that LM317 based driver about why it is just not used as voltage regulator. It is used as current regulator and so are all I've ever seen.

My question is why are there not any voltage regulators? Are there any drawbacks? I can think of resistivity of LD is dependent on temperature, but it should not matter that much, no?

Check the image, so you know what I mean.



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Old 01-31-2012, 04:43 PM #2
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Default Re: Voltage regulation

Of course there are voltage regulators...
A Laser Diode is a current device... to run a LD in a stable
mode you will need to supply a constant current.
If the LM317 is set up as a current source any variation in
normal input voltage will not change the output current.

You can use a Voltage regulated drive but the possible
variations in the Diode heating up and changing internal
resistance will change the current going through the LD...

That makes for a much less desirable Driver....


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Old 01-31-2012, 07:15 PM #3
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Default Re: Voltage regulation

It is also very difficult to set up. Small changes in the output voltage will lead to large changes in current.
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Old 02-01-2012, 07:22 AM #4
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Default Re: Voltage regulation

A laser diode has a Negative Temperature Co-efficient (NTC), meaning as it is heated, its resistance decreases, im not sure which diodes are affected the most but it does happen, i thinks its dependent on the semi condutor medium's make up.

When driven with Constant Voltage (CV) it will alter the current to achieve its set voltage, which would be fine if its resistance didn't decrease when heated.

When the diode is at its defined voltage and the diode heats up, the resistance decreases causing the current to increases. If this continues to happen i believe its called thermal run away.

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