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Old 08-10-2010, 10:53 PM #1
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Default Designing power supply - tech questions

Ok, so many of the laser diode supplies are constant current, but why not a standard constant-voltage one?

Do LD's drastically change their junction voltage as they age?
If they do, then which way does it go (higher, lower)?
What tends to happen when they are failing? Open? Short?

Thanks!
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Old 08-10-2010, 11:09 PM #2
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Default Re: Designing power supply - tech questions

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Old 08-10-2010, 11:22 PM #3
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Default Re: Designing power supply - tech questions

Constant voltage can work, BUT, as the LD warms up it also begins to draw more current. It becomes more of an issue with high mW LD's.

An example of this would be to compare a PS3 405nm LD vs a PHR. The PS3 diode runs under 30mA and will barely produce any heat. They can be run somewhat safely without constant current regulation, but will still wander and draw more than is set in the first place..

With a PHR, it will come to temp quicker at 100+mA and begin to draw more current faster.

Thermal runaway is the term most commonly used to describe this.
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Old 08-10-2010, 11:39 PM #4
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Default Re: Designing power supply - tech questions

One of the problems is that diodes are very sensitive to fluctuations in voltage.

If you connect one to a variable power supply and start increasing from 0V, you will at some point reach operating/forward voltage. That is when the diode opens and start conducting, it then takes very little voltage increase to get a huge increase in current.

Of course, if you are using batteries the increased current will drop the voltage due to internal resistance in the batteries. (See Kipkay Driver. )
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Old 08-11-2010, 02:55 AM #5
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Default Re: Designing power supply - tech questions

That's what I was afraid of - negative temp coefficient.

I was working on a 2A and 5A micro power supplies and constant current adds parts.

Meh.
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Wicked Arctic G1
TriAngle DPSS Green 150mw (200mw)
Tmart DPSS Green 1000mw (800mw)
Many 30mW 405s
Many 405nm modules from Hightechdealz flopping around.


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