Electronics question regarding power supplies

[Trafiq]

Running a 70mW laser off a 4V 200mA power adapter?

Hello All,

I have a question regarding what voltage and current I need to supply for a load. I got a laser that's rated at 3-4V and 70mW and I have a power adapter that's providing 4V and 200mA.

My question is: How do I make sure I don't burn out the laser? For example, if my power supply were 4V and 800mA, I would have to place a resistor in series, wouldn't I?

To find out if I need a resistor with the 200mA adapter, do I need to use P=IV for the laser, then sub it in into V=IR for power adapter and find what resistance I need to balance out the equation?

Thank you.


Regards,
Trafiq

 

[davenn]

lasers are a little different to standard LEDs
they require a current driver ( so do LED's but for them it can be as easy as a resistor to limit current)
doing a google search for laser driver circuit should supply a few answers

cheers
Dave

 

[Trafiq]

I think the driver's already there (it's a laser module) operated by 2x1.5V AAA batteries in series, but I just wasn't sure what were to happen if I were to hook it up to a DC power supply. That's why I'm asking.

The batteries apparently provide 1500mAh (Energizer lithium) which comes up to over 0.41mA. And the diode has no trouble with running off the batteries, while my power adapter is max 200mA.

Thoughts?

 

[davenn]

as long as you are really sure it has an assoc driver
then it will only draw what current it needs

Dave

 

[Trafiq]

Great! I just connected it and it works fine. Gonna have a nice 70mW green scattered light source in my room now!

 

[davenn]

awesome ... have fun but be extremely careful 70mW is a quite powerful laser ... 65mW above that which is mormally available for public use.
you could easily totally zap your eyesight

Dave

 

[Trafiq]

Thanks for the heads-up Dave. I am aware it is a powerful laser and as such, I am quite careful where I point it.

One issue that I noticed is that the module seems to make the diode drop in intensity after about a minute or so. I think it has to do with heat dissipation and its inability to do so efficiently (the module's currently covered in plastic), so I'm wondering what would be the fix for this if I leave the module how it is.

Could I possibly lower the current to the module to reduce the power it has to dissipate?

 

[jim hardy]

In my day higher powered lasers usually had a third wire for temperature sensing. They expect a power supply that adjusts current to keep the laser within some temperature limit and that third wire provides temperature measurement of the laser element..

Modern ones might have that built in, I don't know.

Since its performance changes after a 'decent interval', I would investigate further. Maybe it has a part number someplace.

Does it get warm after that minute? Try the two finger test - we are more sensitive to temperature difference between two things than to one by itself.

And seconding Dave's advice, that thing should be considered eye-dangerous for hundreds of yards. Don't ever point it in direction of people.

 

[Trafiq]

Yes, it does get a bit warm, jim hardy. A rough, subjective, personal approximation puts the temperature at about 30+-5 deg Celsius after 2-3 minutes of turning it on. And that's through about 1mm of plastic. I can confirm that there is no third wire. There are the two positive-negative electrode wires and a spring - also negative electrode - at the end of the circuit board in case I were to decide to make it battery-operated, which is not my intention. I just want to point it at the ceiling so that the reflection off the matte surface can light up my room. Goes well with a black light!

Perhaps I should look into making some sort of heat sink for it? The diode is already housed in a copper tube to fulfill that purpose, but apparently it's not enough.

 

[davenn]

Perhaps I should look into making some sort of heat sink for it? The diode is already housed in a copper tube to fulfill that purpose, but apparently it's not enough

yes definately... it should be heatsunk
and heed what Jim said about temperature sensing -- current limiting
if you don't have a datasheet for the diode, I would suggest doing a google search


Dave

 

[jim hardy]

Hmmm

I suppose you verified with your ohm-meter that there is zero ohms between spring and 'negative' ?

If you read one diode drop.. well,

here's the link
Laser Diode Driver Basics

 

[davenn]

nice one Jim, Thanks ... have saved that page

Dave

 

[jim hardy]

Thanks !

here's a fun little kit ,
It wobbles mirrors to deflect a laser pointer beam onto a wall or ceiling.

Thought OP might find it interesting

LLS1 - Laser Light Show - Ramsey Electronics



If I recall it'll demonstrate lissajous patterns nicely.

 

[Trafiq]

Hmmm

I suppose you verified with your ohm-meter that there is zero ohms between spring and 'negative' ?

If you read one diode drop.. well,

here's the link
Laser Diode Driver Basics

I don't understand what you're trying to say, Jim. Please explain

Edit: Forgot to mention that I did check the resistance between spring and negative and it wasn't 0 ohms, it was more like 4 or 5 ohms.

 

[jim hardy]

I don't understand what you're trying to say, Jim. Please explain

Edit: Forgot to mention that I did check the resistance between spring and negative and it wasn't 0 ohms, it was more like 4 or 5 ohms.

I'm suggesting that there's something in that package besides just a laser diode. I'd have guessed the spring is a power connection and the third wire is for monitoring temperature or power output..

That link describes typical driving schemes .

Since you have a meter, measure how much current it's drawing.
Not knowing anything about the device I'd limit current to ~100 ma by adding resistors.

If you're interested in lasers, here is a hobbyist page about tinkering with them.
This is lifted from just one of his many pages.
http://repairfaq.ece.drexel.edu/sam/laserdio.htm#diodlm4

How to Determine if You Have a Diode Laser Module
So you found a bag of cute little brass devices marked 'barcode lasers' at a garage sale. They have wires coming out of one end and a lens at the other. Are they bare laser diodes or do they have a built in driver circuit? Size alone is no real indication as the driver circuits can be quite tiny. •If there is any sort of model number, try to check that out first since as we have seen, laser diodes are not very forgiving of even minor abuse.


•If you really have a *bag* of the thingies, the surest way to know what is inside is to sacrifice one and reverse engineer it - unless, of course, they are totally potted in Epoxy or something even more impervious to 20th century technology. Perhaps your friendly dentist would be willing to X-ray one for you (with or without root canal). :)
Assuming that analyzing the circuit isn't possible or appealing and they are not clearly labeled (in which case you wouldn't be reading this anyhow), closely examine the wire leads: •If there are three solid gold plated wires and they connect directly to the bottom of what looks like a metal can transistor, you have a bare laser diode. This sort of diode laser module without driver circuitry is common in equipment like laser printers and barcode scanners. Some laser diodes only have two wires but not the kind you are likely to run across in a grab bag unless one broke off. :-) If you cannot tell where they go, also assume you have a bare laser diode. See the chapter: Laser Diode Power Supplies to determine their electrical characteristics and power requirements


•If there are only two wires, probably stranded and color coded red and black, there is almost certainly an internal driver circuit. Red will be positive. A typical power requirement is 3 to 12 VDC at 100 mA. Start low while monitoring output (using an IR detector if it may be an IR laser diode). Most diode laser modules operate at a fixed maximum output power which will be where the intensity stops increasing as you raise input voltage. You can probably assume the circuit will handle, say, 25 percent more input voltage beyond this point but there is no way to how much more without reverse engineering or testing the unit to destruction. The power curve will also probably be quite non-linear - starting out as an LED until the threshold current is reached and then increasing rapidly.
CAUTION: Some diode laser modules are current controlled using optical feedback but expect a regulated DC power supply input. With these, the output will continue to increase more or less linearly as the input is cranked up until the point at which the smoke comes out. :-(



•If there are three (or more) stranded color coded wires, the additional ones may be for on-off control, modulation, or a shield or ground. Have fun determining exactly what they do (but be prepared for frustration).

Or you could just go with what you have and see how long it lasts.

 

[Trafiq]

The diode's got a circuit board attached to it and I can see an op amp-like component, as well as a few resistors or capacitors, so it does have a driver. I guess I'll try to limit the current to 100mA and see how I go (the laser still drops in intensity after a few minutes if I have it connected to two AAA lithium batteries, so I'm pretty sure it's the driver that does this. Which sucks :< )

 

[jim hardy]

With a magnifying glass you should be able to see the part numbers and figure out what the board was designed to do and whether it is still doing it.
One starts by looking up the part numbers and getting the datasheets.
Then try to trace out the circuit from studying the board and draw it neatly on a clean sheet of paper.. Usually it'll be close to one of the suggestions in manufacturer's "Application Hints" section.

When you search for datasheets, always choose the offerings from the manufacturer's website.
There's quite a few really annoying 'middleman' sites that barge in and promise you a datasheet but you have to put up with a lot of advertisements to find even a second rate copy. You'll figure that out quickly.

You just might become the neighborhood "Laser Driver Expert" !

good luck and have fun !

 

[Trafiq]

Thanks for the info, Jim. I'll have to keep it in mind for next time however, because it looks like the diode's borked. I took the focusing lens off to see what's going on. When I first turn the laser on, everything is coherent and there is one large blurry green circle under the laser. But about 30sec-1min in, as it warms up, it looks like the circle gets separated in a number of dimmer circles with the same diameter as the center one. It looks like the whole beam becomes out of focus as the cicles are not centered anymore which means the lens can't focus all the circles that are not incident on it, hence the drop in intensity.

Here are some pictures to show you what I mean.

The module:

Before:

30s-1min after laser on

Thoughts?

 

[jim hardy]

Are you saying it does okay until it gets warm?

That sounds like it needs its controller board, or something similar, to limit input power.

Picture of the module doesn't show (windows, ie10)

 

[Trafiq]

Are you saying it does okay until it gets warm?

Yes. It seems that it doesn't matter if I run it off batteries or adapter, it still does it after a while.

That sounds like it needs its controller board, or something similar, to limit input power.

I tried with an 100 ohm resistor in series, but then it wouldn't light up. Would you say that's too much? I stuck an ammeter in series and it was telling me about 125mA was going through, but then the laser wouldn't light up either.

Picture of the module doesn't show (windows, ie10)

[PLAIN]http://s7.postimg.org/oc2wkg8dm/layz_hurr3.jpg[/PLAIN] 
http://s23.postimg.org/m31nj8ah7/layz_hurr1.jpg
http://s23.postimg.org/hg4s8y1vv/layz_hurr2.jpg

 

[jim hardy]

Those links work, thanks.


Here's another article on laser diodes
I found it by searching on "Laser diode typical current"
it explains quite a bit about the basics.
http://www.newport.com/Tutorial-Laser-Diode-Technology/852182/1033/content.aspx


Do you know how to make a controlled current source?
I'd think making one that'd go from about 5 or 10 ma to ~100 ma would be handy for you to experiment with.
Then you could power it from a "wall wart" supply from an old video game or something.
That would let you find a current at which it 'lases' yet doesn't overheat, if it's not already toast.
And you could test your next one with it.

Here's a thread with discussion of a few different current source circuits.

https://www.physicsforums.com/showthread.php?t=580295&highlight=controlled+current+source

 

[Trafiq]

Do you know how to make a controlled current source?

Not specifically, no. I looked at the link you provided and I'm just trying to remember basics about op amps from back in 2nd year; it's been a while lol. Correct me if I'm wrong, but I understand that the reason why the diode doesn't light up when I stick a 100Ω resistor in series is because the voltage drops as well. Is that correct?

Also, can lasers run both off of AC and DC power? I was told that the reason why this misfocus of the small circles may happen is because the laser is expecting AC current to regulate temperature and because I'm supplying it with DC, all the heat dissipation occurs on the diode substrate, or something along the lines of, impeding the proper operation of the diode.