I am so confused. The watts aren't adding up.

Yes, I am very new to the forums, so please call me an idiot gently

I am mostly self-taught when it comes to electronics and I have been building circuits for about 5 years. So I'm not inexperienced as far as electronics and electricity goes, BUT I am getting more and more confused by these darned lasers each passing day. Ha. I read the "Complete Guide to Owning Lasers" sticky which was good information, but I still need help with one thing. Watts = Volts * Amps. We all know this. So how is it that the 445nm M140 diode is putting out 1.015 watts of power running on 4.2 volts @ 0.7 amps??? 4.2 * 0.7 = 2.94 watts... am I an idiot by thinking this? According to the image below, the M140 has 4.2V @ 700mA of power going into it, putting out 1.015 watts... is the output power of the laser measured totally differently than I understand? Are they even measuring watts in that picture?


I am going to build my second laser with the M140 diode and I am building a 1 watt driver for it as we speak. According to my calculations, I should only be putting about 240mA through it at 4.2V to make 1 watt. But someone else has told me that they built a 1 watt M140 laser driver and are putting 1250mA through it at 3.8V... but that is 4.75 watts total! Here is the full webpage with all the M140 test images https://sites.google.com/site/dtrlpf/home/diodes/445-m140-didoes (you'll need to scroll down a bit) Anyone who can help take away this headache will be loved very much I don't want to burn out my diode.

Laser diodes are not 100% efficient, the extra watts are dissipated as heat.
Hence so much heat sinking for the diodes.
So, in short, you don't get out the exact power you put in because of low efficiency.

Thank you very much for your prompt reply

Okay, so that makes a heck of a lot more sense to me, and I was wondering if that was the case and you have confirmed that for me. Would you say putting 1250mA at 3.8-4.2V through an M140 diode would be safe with active cooling?

There's a bit of difference between the watts. The one W=V*I measures the electric power, and the W of a laser diode measures its optical power, remember, a lot of energy is transformed into heat. All varies with the efficiency of the laser diode.

Got it. When I see a "2 watt laser diode for sale", I can safely assume they are talking about optical power, not actual electric power? So in reality with low efficiency a 1 watt laser could actually be sucking up nearly 3x the power needed to actually output 1 full watt of optical power? (obviously it's different across all diodes, I'm just trying to grasp the concept)

The watts always add up, just rarely in our favor. Other members have explained already how so.

As for thermal considerations; if you can keep the diode to under 40C it should live a long life, simple as that.


Edit: Yes, lasers are typically referred to by their optical power, not electrical power. For example; my 50mW 589nm laser draws a bit over 8Watts of electrical power. My 70mW Argon Ion Laser draws about 1.4 Kilowatts not counting cooling power.

Sigurthr said:

For example; my 50mW 589nm laser draws a bit over 8Watts of electrical power. My 70mW Argon Ion Laser draws about 1.4 Kilowatts not counting cooling power.

Click to expand...

HOLY CRAP! Your 50mW laser draws 8 freaking watts of electrical power??? Now I feel like I know nothing. LOL.

I have heard that there are major scams with laser diodes nowadays and like you said, the power ratings could be total BS. I will be extra careful. I am more than likely going to purchase an M140 diode with either the 3-element or G2 lens from the website I posted in my original post. I guess I will find out some way or another.

You were at the right place to get good info when you got the pic in the OP-
DTR is the best source of info and parts-& his pics are great-

good luck

hakzaw1 said:

You were at the right place to get good info when you got the pic in the OP-
DTR is the best source of info and parts-& his pics are great-

good luck

Click to expand...

Awesome! Thanks for the confidence booster! And thanks everyone for your help! You will all hear more from me again, I'll be sticking around.

I get 99% of my parts from DTR, and he is always a stand up guy and responds well to PM's. In an M140, I would drive the diode at 1.7A for a potential 2W of output using a G-lens. The M140 is also great for a first build due to the fact that it has a case pin...which keeps bugging me about our 9mm friends.

Just don't underestimate the brightness when you first go to turn it on. Go outside or someplace where the light isn't hitting something 4 feet from your face. The very first 445nm diode I made I turned it on pointing at a wall so it wouldn't bounce back. I had never had that high of power in a laser before though and didn't think about how painfully bright it would be. Had floaters in my eyes for hours. Now I always point it at a good distance away.

Thanks for the info, now I don't need to be scared of buying from him. Also good point about the brightness, I will keep that in mind. I went to your build/reviews page, awesome work!!! You say to drive the M140 at 1.7A (sounds good to me), but how about the voltage? I see that it can handle up to 4.7V, but I'm not sure if it's a great idea to drive it that high?

If you go with the module+driver option from DTR you will be getting an X-Drive. To run the M140 off of it you will need a total of two Lithium Ion batteries. LiIons are rated for 3.7V so a total of 7.4V is what you will use in a handheld. Unless you use a boost driver or are driving a red diode you will usually use two Lithium Ions.

I suppose I could do that. Now let's say I wanted to build an AC powered driver from 12V bench PSU and regulated the current and voltage using either an LM138 or LM338 regulator and a capacitor to catch voltage spikes (discharge the capacitor before putting the diode back on!!!). I'm just not a big battery guy, I like a constant power source that doesn't need recharging. And I have no problem building my own driver. So it sounds like to me that the voltage of 2 lithium ions is just the inital source voltage (which will definitely fluctuate based on the charge state of the batteries) but not the actual regulated voltage going into the M140. Based on what I have seen, it looks like an M140 can take between 3.8 and 4.7 volts, but I cannot find a datasheet. Sorry if I am annoying lol.