DIY Thermal LPM for under $50

[honeyx]

You can also use 10 x10 ohm SMD resistors to calibrate it. At 10V it will give you 1000mW. I calibrated mine this way and the readings are quite accurate. Check out my 400mW lazer review and compare to the power certificate that came with the laser.

Soldered the resistors to an "S" and added some thermal paste at the buttom of them to calibrate my LPM.


Here also some other voltages to compare with:

2,24 Volt - 50 mW
3,16 Volt - 100 mW
4,47 Volt - 200 mW
6,32 Volt - 400 mW

Just make sure your DMM is really very exact in showing the correct voltages.

 

[qumefox]

How do you compensate for the half of the power that's lost by radiating away from your sensor? Resistors don't just get hot on one side.

 

[honeyx]

How do you compensate for the half of the power that's lost by radiating away from your sensor? Resistors don't just get hot on one side.

I think you mean the half power to be lost by radiating away from the resistors not the sensor as the TEC is the sensor.

Well, if the resistors were all around surrounded by air, you would be right. But if you use thermal paste almost all of the heat is being so to say sucked to the colder TEC that is also cooled by the heatsink.

You can compare this with taking a big massive CPU heatsink and holding a small flame to one side of it. You won´t be able to get one side of the heatsink to be hot while the other side will be still cold. Just try this out and you will se what I mean. The whole heatsink will warm up slowly and both sites will have almost the same temperature.


It´s simply because by using thermal paste the heat transfer from the resistors to the TEC and from the TEC to the cooler is faster than to air. Air itself is indeed a heat isulator and thermal paste or aluminium are heat conductors. The heat takes the easy route and flows the road with the lowest resistance.

 

[lasersbee]

Without checking your LPM against a know calibrated
LPM... there is no sure way to know if your LPM is
accurate.... no matter how much thermal paste you
use. It's just the way it is...


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[honeyx]

Sure. There you are right. All LPMs must be checked against something. May it be the most expencive non plus ultra LPM, or a DIY one. You probably are checking your Laserbees also against a calibrated one that also was checked against a calibrated one and so on. Even the very first one in the world must have been calibrated somehow, so this all is relative depending of the point of view. No doubt your LPMs are more accurate as my self made one, but calibrating a LPM with resistors will be more accurate than putting a finger into water and guessing which temperature it has. Therefore I also didn´t say my LPM is as accurate as yours but quite accurate relatively to the readings of the power certificate that came with my laser.

 

[lasersbee]

Sure. There you are right. All LPMs must be checked against something. May it be the most expencive non plus ultra LPM, or a DIY one. You probably are checking your Laserbees also against a calibrated one that also was checked against a calibrated one and so on. Even the very first one in the world must have been calibrated somehow, so this all is relative depending of the point of view. No doubt your LPMs are more accurate as my self made one, but calibrating a LPM with resistors will be more accurate than putting a finger into water and guessing which temperature it has. Therefore I also didn´t say my LPM is as accurate as yours but quite accurate relatively to the readings of the power certificate that came with my laser.

Don't misunderstand me... I'm not comparing your DIY LPM
to ours.. I'm speaking in general hobbyist LPM calibration terms..

We spend money to have our Lab Quality Newport LPMs Calibrated
by the Manufacturer to NIST standards to assure that we can
calibrate our products as accurately as we can.

The average hobbyist can't afford to do that...

I agree that using a resistor to calibrate a DIY LPM is better
than sticking your finger in water to test for temperature... but
so is checking a stable laser on a known calibrated LPM and
using that to calibrate your LPM.

I see too many variables in using a resistor for the hobbyist.

1) the accuracy of the Power Supply used to heat the resistor
2) the accuracy of the DMM used to verify the resistor's value
3) the thermal properties of the adhesive holding the resistor
4) the thermal losses from the parts of the resistor that is not
attached by adhesive...

Like you said it is better than your finger...
but at what accuracy...

If you are satisfied with your results then you've done a good
job...:beer:


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[kiyoukan]

Well using heat to calibrate i thought was a very bad idea. as far as getting accurate results.
Due to it depends on the light absorption of the paint that determines power level.
That is why some paint does not work across all colors as other paints do.
BTW what paint is being used?
using paint that can stand up to 1200º F is good right?
RustOleum.com
This is what i have on my setup.
Is that the best stuff?

ps this stuff is fun
RustOleum.com

 

[honeyx]

Well using heat to calibrate i thought was a very bad idea. as far as getting accurate results.

But you know the principle of TEC and Thermophile LPMs turning light into heat and the heat being the basis to measure the amount of power?

There are a lot of variables as Jerry said like not all the heat of a rasistor being absorbed or not all the light being absorbed by the paint and turned into heat. Even soot will reflect a small amount of light that wont be turned into heat.

I for myself took this also into account. Also a small difference in the resistance of a resistor will have a big influence to the readings. That´s why I finally used 4 DMM´s to check the resistance and voltages and even calculated the drifts between all of them to compare with. So calibrating mine didn´t just take me a few minutes but all in all about two days.

If it comes to paint you don´t have to use this high temperature resistant paint. For a LPM up to 2W you can use normal paint with good absorbtion. The light won´t heat it up that much to destroy it. You can even cover the paint with a thin layer of soot before it drys to reduce the amount of reflections.

All in all a DIY LPM won´t be as accurate as a professional one, but in taking all variables into account and trying to get it as accurate as a hobbyist can, you can get a LPM that will give you a clue of what the power of a laser will be about allowing you to check it´s somwhere within the specs or way underspec.

 

[qumefox]

Don't forget thermal paste is a poor conductor of heat when used in large quantities. When just glooped on, a lot of times it can have worse thermal transfer than air, depending on the situation. Plus.. even if working as intended.. You will still be radiating heat to the atmosphere as well.

The only way to get accurate results would be to try it on something calibrated to see what your losses were.. which still means needing a calibrated LPM.. And heh.. Getting thermal grease all over the sensor of one would in itself ruin it's calibration.

 

[honeyx]

There you are right. Thats why I suggested SMD resistors as they are flat and you just need a relly thin layer of thermal paste to make sure the paste is working as intended. Sure there will be some sort of lost, but even all the expensive termophile LPMs do have to deal with some sort of lost as not all the light is being absorbed. The only way to get really accurate results would be to count all the photons. So like said a DIY LPM will of course be not as accurate as a professional one, but if taking all the variables into account, you can get something that will tell you your lasers are about up to specs or way below. A laser of known power will be of course a big help in calibrating such a LPM.

 

[qumefox]

A laser of known power will be of course a big help in calibrating such a LPM.

That's generally what most people who make DIY meters do. Though in actuality you need lasers of different wavelengths with known powers. Preferably with different powers of each, so you can check both linearity of the readings, as well as the response of the coating that's used to different wavelengths.

 

[honeyx]

Dude. Nothing is perfect, nor the coatings are. Just a black coating that also appears totally black to your eyes will absorb all the visible wavelengths about the same. Else it would appear reddish, blueish or greenish to your eyes like the AR coatings do. However some black paints are absorbing visible light but reflecting IR light. That´s why I suggested soot. It has almost a lineary absorbtion also in the IR spectrum. So like Jerry already said there are a lot of viariables to be taken into account.

 

[arcaneinsomniac]

Hi guys. I, like many others here, got curious after finding out that only a peltier module is needed (essentially) to build a DIY LPM. So I ordered a $15 15x15mm unit from digikey. It's got 8 elements along a side. Pretty much run of the mill. Have been knifing the silicone out of it. Don't think I've broken it yet.

Right now I seem to have misplaced my single rail op-amps but I hooked up the TEC to my voltmeter (which measures 0.1 mV increments).

Sure enough it works. I put my finger on one end, I get a few positive milivolts, put my finger on other end, negative milivolts.

I have no rustoleum hi-temp paint so I decided to color in the ceramic with pencil for the preliminary laser test. Graphite's dark enough so at least it doesn't reflect most of the light. Now I haven't got a heatsink attached to it yet either.

Here's what's got me a bit confused. If I shine my 405 on one side I get -28mV but if I shine it on the other side it only reaches 21mV. It does revert to an equilibrium value of around 0mV after a while. But it seems to be more sensitive to heat applied to the side with lettering on it (producing negative volts). Is this normal?

Once I get an opamp hooked up to this thing I might be able to get more solid numbers out of the DMM. But if not, I reckon I won't even need to build a circuit at all. Just a TEC thermal glued to a heatsink, and solder some banana plugs onto leads. Then, with the aid of a simple lookup chart (whose information content can be encoded in a single number in W/V) you've got the cheapest LPM ever. Like about $20. Because it doesn't even have its own voltmeter.

 

[qumefox]

Well all the circuit does is amplify the TEC output to a 1v=1W scale (typically) To eliminate the need for a conversion chart. But yes, if you have a sensitive enough voltmeter, there's no reason why you can't just measure straight off the TEC..

However, Unless you have a REALLY sensitive voltmeter, don't expect much precision on your readings. You won't be able to say, get down to the exact mW unless you have a voltmeter that will read down to the picovolt range or so. Which is the whole point of the op-amp amplifier. To expand the signal to a range that's easier to deal with using the types of voltmeters most have available.

 

[arcaneinsomniac]

Help me calibrate my new DIY meter!

I made some measurements with it today. This is a $15 peltier module arctic alumina'd to a 1.25"x2.5"x7/16" heatsink and front painted with rust-o-leum high temp matte black grill paint.

Recorded highest value reached. Usually they peak and drop off slightly within 30 seconds.

Ordered chronologically, reading is direct off of peltier leads.

445nm @ 1500mA, aixiz glass 405 lens
635nm @ 775mA, aixiz glass 650 lens
405nm @ 475mA, aixiz glass 405 lens
650nm @ 417mA, aixiz glass 650 lens

1 445nm -69.5mV
2 635nm -32.2mV
3 405nm -40.5mV
4 650nm -19.2mV
5 445nm -73.1mV
6 635nm -31.9mV
7 650nm -19.3mV
8 405nm -40.9mV
9 445nm -71.2mV
10 445nm -59.2mV
10.5 I put new battery in 445 unit at this time
11 445nm -79.0mV
12 445nm -80.2mV
13 635nm -32.3mV
14 445nm -83.4mV
15 405nm -41.6mV
16 635nm -31.1mV
17 445nm -82.3mV
18 405nm -41.0mV

when FlaminPyro metered my 635 for me before he sent it out he reported 449mW peak. Based on that I arrive at about 1150mW for my 445. It does sound very reasonable. But when the 635 was metered it was not in the same host it is in now. Also have put at least 5 hours of runtime on it, so its output characteristics may have changed.

I suppose i will need to use a stable laser that has been measured with a calibrated meter in order to calibrate mine. Ideally as many lasers as possible to ensure good calibration. But how are those numbers? They look pretty stable to me.

I figure there's no point in me building an op-amp linear amplifier at this point since I'd want it to be the calibrator so I end up with mV = mW. Need the all important ratio at this point. I guess I could just leave out the resistors... Hmm. I should order potentiometers. I may need them for the correction amp for my soundcard DAC. Can anyone recommend good potentiometers?

 

[qumefox]

i'd say 13.9mW/mV but that's just a guess. To know for sure you either need a calibrated LPM to check your lasers against, or have a positively stable known power laser.