How strict is the voltage rating on laser diodes?

[Ravenous Travis]

Hey everyone, this is semi-related to electrical engineering and a quick search didn't appear to turn anything up regarding this question specifically so I figured this is an appropriate place to ask this. I've bought this laser diode rated for 1.9 - 2.2 volts for a project I'm working on. I've built a driver circuit that's ended up being a few percent over the maximum voltage. It clocks in at about 2.22 volts according to my free harbor freight multimeter, so I know I'm close to the recommended voltage... probably. How set in stone are those ratings? Am I going to fry this diode if I'm just a few fractions of a volt over the top end? I'd wire it up and find out myself but I spent 20 U.S. dollars on this diode so I would rather have a little patience and get some advice.

 

[scottdave]

Typically manufacturers try to build in some margin of error with the specs, so it might be OK. But a good designer will design the circuit such that normal operating conditions falls well within the specs of the components. So what I'm saying, it may not fry today, but it most likely will fry much faster than you want it to.

Have you considered that the diode will draw some current? How will that affect the voltage at the output of your circuit? Multimeters are typically designed to draw almost no current, unless your multimeter has a "battery tester" mode. If it is still above range, can you add or swap out a resistor, which will bring the voltage down some?

 

[Ravenous Travis]

Typically manufacturers try to build in some margin of error with the specs, so it might be OK. But a good designer will design the circuit such that normal operating conditions falls well within the specs of the components. So what I'm saying, it may not fry today, but it most likely will fry much faster than you want it to.

Can you add or swap out a resistor, which will bring the voltage down some?

Hmm alright, that's what I was afraid of. I can probably find something to replace one of my resistors with if I dig around enough, maybe it's about time I just order an assortment of some shiny new resistors. Thanks for the advice.

 

[jim hardy]

Have you a datasheet for your laser diode?
Some of them have an extra wire to sense temperature. It's used to regulate current so you won't fry it.

 

[Rive]

The problem with diodes/LEDs is that once they are open even a tiny increase in voltage can mean a far greater increase in current => not good for the dissipation.
That is why it is better to give them constant current instead of constant voltage.

My opinion is, that with good cooling this 0.02V won't mean much harm.
My suggestion is to check and/or modify the circuit if possible, because if that supply unit is sensitive to anything, then...

 

[Ravenous Travis]

Have you a datasheet for your laser diode?
Some of them have an extra wire to sense temperature. It's used to regulate current so you won't fry it.

I don't, the best I could find is the pinout posted in a review on the site I purchased it from which claims that the third pin isn't used. Lack of information seems to be the trend when buying cheap laser diodes.

 

[Ravenous Travis]

The problem with diodes/LEDs is that once they are open even a tiny increase in voltage can mean a far greater increase in current => not good for the dissipation.
That is why it is better to give them constant current instead of constant voltage.

My opinion is, that with good cooling this 0.02V won't mean much harm.
My suggestion is to check and/or modify the circuit if possible.

I wasn't entirely sure how to go about making the driving circuit, and I saw a lot of people using voltage regulating circuits to drive their diodes so I just went with that. It looks like I could convert my circuit relatively easily to regulate current instead of voltage.

 

[jim hardy]

It looks like I could convert my circuit relatively easily to regulate current instead of voltage.

My suggestion is to check and/or modify the circuit if possible, because if that supply unit is sensitive to anything, then...

I'll Second that .

 

[Ravenous Travis]

I'll do that then. Thanks for all the help guys!

 

[berkeman]

Why are you using a voltage source to drive a laser diode? The voltage that is listed is provably Vf (forward voltage drop) versus current. You don't put a voltage across an LED or laser diode -- you drive a current through it. That can either be from a current source, or a voltage source with a series resistor to set the diode current based on the diode's Vf and desired current.

The exception would be if the laser diode package includes its own current-setting resistor. But then the drive voltage listed would be more like 5V, not the 2.2V that is a typical Vf for a red laser diode.

Look at datasheets for other laser diodes...

 

[berkeman]

Laser diode with built-in current-limiting resistor: http://www.laser66.com/html/DL-405-150-SF.pdf

Good source of basic info on laser diodes: https://www.repairfaq.org/sam/laserdio.htm

However, unlike LEDs, laser diodes require much greater care in their drive electronics or else they *will* die - instantly.

 

[Ravenous Travis]

Why are you using a voltage source to drive a laser diode? The voltage that is listed is provably Vf (forward voltage drop) versus current. You don't put a voltage across an LED or laser diode -- you drive a current through it. That can either be from a current source, or a voltage source with a series resistor to set the diode current based on the diode's Vf and desired current.

The exception would be if the laser diode package includes its own current-setting resistor. But then the drive voltage listed would be more like 5V, not the 2.2V that is a typical Vf for a red laser diode.

Look at datasheets for other laser diodes...

My knowledge of electronics is fairly limited, so I've been approaching this project with google in one hand and community help in the other. The information given about the diode stated it had a "working voltage" of 1.9-2.2 so I had assumed applying that potential across the pins would be sufficient so long as I used a voltage regulator. That's my excuse for my initial approach. I've since converted my circuit to limit current to the listed operating current.

 

[berkeman]

I've since converted my circuit to limit current to the listed operating current.

Good. Have a read through that reference link that I posted. It looks to be a pretty good introductory reference.

 

[Ravenous Travis]

Good. Have a read through that reference link that I posted. It looks to be a pretty good introductory reference.

Will do!

 

[cabraham]

Under no circumstances should any diode, laser or otherwise, be directly connected to a voltage source. Drive it with a constant current driver. Search for a laser diode driver. I have one I thought about publishing. I used it years ago with great results.
Again, do not even attempt to drive a diode with voltage, never can that work. Look up "thermal runaway".