# How much current does a 50mW laser need?



## eatkabab (Mar 18, 2014)

50mW lasers are typically listed as needing around 300ma (don't trust that number) of current at somewhere between 2.5-5v. I'm rather new to lasers, coming from LED's, but I just wanted to know if they operate similarly.

How much current does a 50mw laser require? I don't trust the 300ma number because 5mW lasers are also listed at ~300ma requirement. Doesn't make sense. I'm using some 350ma AMC7135's (because that's what my custom packs already have built into them) and am wondering if I'm going to have trouble when I pick up a 50mW laser. My 5mW laser seems to have no problems and outputs at max brightness from a fully charged lipo cell through the 350ma regulator.

Also, my 5mW laser doesn't seem to heat up at all. Would a 50mW laser require a heatsink beyond the little brass/metal housing it comes in?

Advice greatly appreciated.
I searched around and didn't find much. Sorry if this is a regularly answered question and I'm just asking it incorrectly.


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## Arilou (Mar 18, 2014)

300mA is about right for a 50 mW green DPSS laser. Most direct laser diodes are more efficient than that. At 300mA, my GaAlAs laser outputs 400 mW.


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## eatkabab (Mar 18, 2014)

Arilou said:


> 300mA is about right for a 50 mW green DPSS laser. Most direct laser diodes are more efficient than that. At 300mA, my GaAlAs laser outputs 400 mW.



Thanks for the quick reply!

I just rebuilt my little 5mW laser pen into something with a lithium and a recharging board. I noticed the circuit board of the laser gets very hot after about 4min of continuous use. Should I somehow try to get a tiny heatsink on there before encasing it in thinish plastic? Also, if I were to put a 50mW in the same configuration, would it get hotter? I'm assuming the 50mW draws more current than the 5mW. How much current does the 5mW probably draw? How much larger of a heatsink might I require?

Lastly, I feel like this 5mW thing is rather bright. Could it be significantly over-spec? In a moderate lit room I can easily see the beam from a 90deg arc behind the source.


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## Arilou (Mar 19, 2014)

Almost certainly over-spec. 5mW is not a lot of heat. The muscles in your finger put out more heat than that.


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## bshanahan14rulz (Mar 19, 2014)

I would bet that the linear driver is havign to drop all that extra voltage from the lithium cell. You might need a diode in series with the battery to lower the voltage that the laser driver sees. That way, on a fresh 4.2V lithium ion, the driver only sees ~3.5V, which is closer to the 3.2V that it is expecting from fresh alkalines.


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## eatkabab (Mar 19, 2014)

bshanahan14rulz said:


> I would bet that the linear driver is havign to drop all that extra voltage from the lithium cell. You might need a diode in series with the battery to lower the voltage that the laser driver sees. That way, on a fresh 4.2V lithium ion, the driver only sees ~3.5V, which is closer to the 3.2V that it is expecting from fresh alkalines.



So I actually have the lithium attached directly to the LED Driver board and it's been working fine. In tinkering today though I shorted the board and burnt the laser I think. Now when I turn it on it's just a really dim red/IR light that can only be seen if looked down the barrel.

Could it be possible that the circuitry on the driver was dumping the extra voltage and that's what was firing it up? If not, should it be heatsinked? (That's why I fried it, I tried putting a heatsink on and knocked it by accident during installation)

Also, any way to repair it? :-/

The bare laser modules from eBay state they'll take a 3-5v input which is a single lithium...makes sense. So I wouldn't see why a laser pen intended to take 2xAAA would be any different.


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## Arilou (Mar 19, 2014)

eatkabab said:


> So I actually have the lithium attached directly to the LED Driver board and it's been working fine. In tinkering today though I shorted the board and burnt the laser I think. Now when I turn it on it's just a really dim red/IR light that can only be seen if looked down the barrel.



If that was a green laser, then that dim red ight is the 808 nm pump diode. You do not want to be looking directly at that, it can burn your eyes.


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## bshanahan14rulz (Mar 20, 2014)

You don't have to heatsink anything if you aren't overvolting it. It doesn't matter if the driver only accepts up to 3V or up to 5V, the diode only drops ~2V. The rest of the voltage is dropped by the linear driver as heat. By adding a diode in series, the driver doesn't have to drop as much voltage, meaning it heats up less. 

Arilou's warning is very important. Green lasers are more like a classical ruby laser: there is a crystal that acts as the gain medium, and you pump it with light energy, except instead of using a flash lamp, they use an IR laser diode, because it is capable of high energy density compared to LED. Still, since the process is nowhere near 100% efficient, you need lots of pump power to get a little bit of green. Therefore, even though the final output may only contain ~5mW of 532nm radiation, there may be significantly more IR radiation leaked from the unit. Just think, your eyes are not designed to see 808nm or 1064nm, but these devices are pumping out so much of it that you are able to see it! 

A safer way to check for IR is to use your cellphone camera or other cheap digital camera. IR will show up as bright white or pink light to a digital camera. It is much cheaper to replace an iPhone than to replace an eyeball


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