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Old 10-31-2008, 11:15 AM #1
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Default Maximum voltage?

Do laser diodes have a maximum voltage, or is it all about the current?

I was looking at some datasheets and some don't have a max voltage, but some have one ~0.3V above the typical voltage.

For example I'm currently testing a red laser diode with my new current regulator at 40mA and pretty much bang on 2.7V.
If I changed this to 40mA at 5V is this going to fry the diode, or does it just use what it needs?

Thanks


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Old 10-31-2008, 02:37 PM #2
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Default Re: Maximum voltage?

Quote:
Originally Posted by 12dstring
Do laser diodes have a maximum voltage, or is it all about the current?
I was looking at some datasheets and some don't have a max voltage, but some have one ~0.3V above the typical voltage.
For example I'm currently testing a red laser diode with my new current regulator at 40mA and pretty much bang on 2.7V.
If I changed this to 40mA at 5V is this going to fry the diode, or does it just use what it needs?
Thanks

When you use a current control driver, the diode uses whatever voltage it needs. The driver won't "send" more voltage than that. Maximum voltage is usually a restriction of the drivers components. On a linear driver the excess voltage is converted to heat. Ideally, you want to supply just above the requirement of the diode and the driver combined. For example, if you are using a Rkcstr the minimum voltage is 2.2V and the maximum is 12V. If your diode needs 3V and the driver needs 2.2V then ideally you would provide 5.4V. If you give it 9V (which we do for more power), the driver converts the excess voltage to heat. If you go over 12V, you will damage the driver.

Excess voltage will damage the driver. Excess current will damage the diode.

Peace,
dave
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Old 10-31-2008, 06:25 PM #3
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Default Re: Maximum voltage?

Thanks for that Dave

I only ask because after the 2.7V (flat CR2) I connected a fully charged CR2 (measuring 3.1V) and the diode suddenly became an LED. I'm not using a capacitor though so I guess there was a spike which surged its way through the driver and killed the diode. The driver's a bit different to a Rkcstr, having a max voltage of 6V but only using 0.12V itself.

I sort of answered my own question later when I used the 2.7V on the IR diode (they need 1.8V?) and it worked fine.

I've borrowed a capacitor out of something, so will try it again with that in to see if that was the cause...
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Old 10-31-2008, 07:11 PM #4
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Default Re: Maximum voltage?

I tried it again with a capacitor and 3.1V input and this time it started off nice and bright (I say bright, I mean about 5mW) but then seem to slowly decrease in brightness by about a third to a half - and there it stays.
The voltage measured across the diode was 2.35V when using both 2.7 and 3.1V input. The diode gets warm in it's little heatsink but the driver is cool unheatsinked.

Not sure :-/

Edit: Think I've got it figured now. With 2.7V, even though the voltage across the diode was the same as 3.1V, and the brightness looked the same, I've just hooked up an ammeter and it was actually only putting out about 32mA. I assume once it had enough power and could put out the full 40mA - it was a bit too much for these diodes, and they started to fizzle out.
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