Old 12-25-2009, 09:02 AM #1
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Default Question about LPC-815 LOC forward voltage

Does anyone know if there is a graph of the LPC-815 LOC diode forward voltage versus current draw? In particular, I am interested in the precise voltage needed to drive the LOC diode at 420, 475, and 500 ma.


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Old 12-25-2009, 10:46 AM #2
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Default Re: Question about LPC-815 LOC forward voltage

Unfortunately, there is no set value for any type, there is slight variation from diode to diode. Typically, I believe it is between 2.7 and 3v. The only way to find this value is to test each diode individually, and as it warms up or cools, the voltage will drift slightly, which is why we use constant-current drivers.
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Old 12-25-2009, 03:47 PM #3
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Default Re: Question about LPC-815 LOC forward voltage

Laser diodes are current driven devices. If you wish to drive at 420, 475, and 500mA, set your current regulator up for the required mA's. As far as voltage goes, for an LOC driven very hard like you wish to, the Vf will be around 3.1 to 3.3V. Ensure that your voltage source (battery or bench PS) can deliver this Vf plus whatever voltage overhead that your particular linear current regulator requires in order to remain in regulation.
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Old 12-27-2009, 08:44 AM #4
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Default Re: Question about LPC-815 LOC forward voltage

Thanks for the replies,

My problem is that I'm trying to power a LOC with a Li-ion battery and a 0.3v dropout regulator. If the forward voltage of the diode is too high, I wont be able to use this driver. If the diode Vf is a conservative 2.7v as charliebruce suggested, the total voltage I need will be 3v. This would be perfectly fine for powering the diode. If the Vf is higher, say the 3.5v, then I would need 3.8v total. While this would still work with a Li-ion, the battery life would be very short. In this instance I would probably have to find a different driver.

This is why the exact Vf of the diode is important to me. Does anyone know about how much the voltage varies between different LOCs at 420, 475, and 500ma?
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Old 12-27-2009, 05:10 PM #5
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Default Re: Question about LPC-815 LOC forward voltage

spectral532;

If your driver only has a 0.3 volt dropout,

a #18650 will drive the LOC easily.

I have done this many times with Red builds.

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Old 12-27-2009, 08:57 PM #6
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Default Re: Question about LPC-815 LOC forward voltage

The 18650 will be fine with a 0.3V dropout regulator. You are looking at needing 3.6V at the most. I just hope your diode does OK at 500mA. A little bench testing with your diode, regulator, and an 18650 should firm things up for you.
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Old 12-28-2009, 03:38 AM #7
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Default Re: Question about LPC-815 LOC forward voltage

Are you sure that it will only draw 3.6v, even when pulling 500ma? If its a bit higher, say the 3.8v I suggested, then the battery life would be very poor. The Li-ion could supply 3.8v while its fully charged, but the voltage would quickly drop with use.
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Old 12-30-2009, 05:37 PM #8
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Default Re: Question about LPC-815 LOC forward voltage

spectral532;

Dr. Lava's LPC chart over on the L. Enthusiast board shows only reaching 3.1 VDC at 500ma.

You are an experimenter, don't worry.

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Old 12-30-2009, 10:01 PM #9
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Default Re: Question about LPC-815 LOC forward voltage

Thanks for the info LarryDFW, that graph on Laser Enthusiast Forums was exactly what I was looking for. Additionally, I found a graph by Igort which also shows the Vf of a LOC versus current. Here are both graphs in case anyone else is interested.

Edit: Deleted Graph. You can still find it in the "Diode testing Graphs" thread at Laser Enthusiast.


Last edited by spectral532; 01-27-2010 at 10:10 PM. Reason: Deleted a graph
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Old 12-31-2009, 03:41 PM #10
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Default Re: Question about LPC-815 LOC forward voltage

spectral532;

I would delete the graph from Laser Surplus parts.

They provided the laser diode to Dr. Lava for the testing.

This forum and that forum do not normally share data without permission.

That IS the reason I didn't post it.

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