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Digital Thermometer W/ Thermocouple to Determine Duty Cycle?

TaterMay

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So I've always been one to want to do everything as precisely and as scientifically correct(if that makes sense) as possible. Therefore, I have been less than satisfied with the "if it starts to get warm, turn it off" method of determining duty cycles. Upon searching, I haven't been able to find much more informant on the subject than that. Here's what I'm wondering.

First, I have a small digital infrared point-and-shoot style thermometer that has the perfect size sensor to aim directly inside the modules we use. Would this be an accurate way of determining duty cycles? By simply running it for predetermined amounts of time, then immediately monitoring the temp at the front of the diode can?

Second, would the following be better than the first option? Since these units come with small wires, I figure it would be possible to sandwich the thermocouple in the module right against the diode to get a pretty close reading on the actual diode temp. Thoughts?

http://www.amazon.com/gp/product/B00FFYEPVQ?ref_=cm_sw_r_awd_ht-MvbK5JRA2N
 
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TaterMay

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I have also wondered if ambient temperatures are taken into account when specifying duty cycles and should we adjust accordingly.

Oh and the link is a bit wonky so here is a correted one :)
Amazon.com: Signstek 3 1/2 6802 II Dual Channel Digital Thermometer with 2 K-Type Thermocouple Sensor Probe for BGA rework HVAC 1300°C 2372°F: Industrial & Scientific
Thanks! What did you have to change? I simply copied and pasted the URL into the link window that pops up when you press the add link button.
 

Cyparagon

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"duty cycle" isn't a precise value anyway. If you define your parameters beforehand (e.g. on-time duration until case reads 50C), that still depends on ambient temp, battery charge remaining, and whether it's the first on-time or the second. Even if you ignore those variables, choosing a specific case temp is arbitrary. In theory, cooler is better. Running them hotter decreases life slightly. But what reduction of life is acceptable to you? zero cannot be done in a handheld, and it's contradicted anyway by the "run the f:)cker 10 milliamps from death" mentality so many members hold.
 

TaterMay

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True, it definitely isn't a precise value without considering those variables. But I would like to be able to monitor the case temp so I can observe the effect that those variables have on it. I'm wondering if this is a fairly accurate method of doing so. In doing so, I believe it's very likely that I could make very useful determinations, such as "Laser A can be run for 45 seconds on/60 seconds off, up to an ambient temp of X degrees" or "Laser B can be run continuously at up to X degrees ambient without reaching a dangerous case temp" or "You better not even turn on Laser C if ambient temp is above X degrees" lol. I think that it could be a very useful tool, if accurate.
 

Cyparagon

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67 variables unaccounted for set aside for now, These thermometers are easy and cheap and have the added benefit of giving you a reading in less than a second. UN3F Pen Type Mini Infrared Thermometer IR Temperature Measuring LCD Display | eBay. The disadvantage is they will not read shiny metal properly. Anodized Al is fine. If shiny metal needs to be read, you can always put a piece of thin tape on the surface, and read the temp of the tape.
 

TaterMay

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I've already got a better IR thermometer than that one. That's what I was saying in the initial post. I'm thinking that something like this might be more accurate. I get that there are variables to account for here, but I don't see how that makes it any less beneficial to monitor one of the most critical aspects of diode life. You can't even know just how big a role those factors that you are referring to even play without monitoring diode temp. I suspect that there are only a few factors, maybe 4 or 5 at most, that actually have a significant interaction with case temp when it comes to a diode that is likely going to sit in one host and be used by one person until it dies. And if I had a reliable, accurate way of monitoring case temp, I could do some statistical analysis, probably throw the data into MiniTab, and determine which variables are actually worth considering while determining a healthy duty cycle.
 

Cyparagon

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The other variables make having a slightly better accuracy on your temp calculation pointless. The difference might be as high as 1C, which makes little difference in comparison with on-time. It might be 120 seconds to a temp rise of 30C for example, so a temp rise to 29C is going to be pretty much the same.
 

TaterMay

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Ah, are you saying the small amount of increased accuracy from the thermocouple isn't enough to justify purchasing it over what I already have? Essentially, what I want to be able to do is simply monitor the case temp vs on time and ambient temp(and other variables if I determine any are statistically significant) and be able to know that, if I have X degrees of ambient temp, I can safely run this laser for Y seconds. If I built in some tolerance for accuracy errors, I feel like I could come up with a fairly accurate calculation. I enjoy designing experiments, so I might end up getting a cheap thermocouple or thermistor and comparing it with the IR thermometer.
 

IanP2015

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Thanks! What did you have to change? I simply copied and pasted the URL into the link window that pops up when you press the add link button.
i just copied and pasted too but directly into the box you type your text message into :)
 

Pman

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Never seen an IR unit that cheap before Cyp. I have a couple inexpensive ones but also a Fluke 568 that can do bead (like type "k") and IR at the same time besides for (3) Fluke 52-2 and a couple other ones that can take lots of different probes that i have as I used them daily with commercial/industrial refrigeration. Now that i thnk about it i have at least 10 meters i can use just for temp as i sometimes worked on many units at the same time. I haven't bothered measuring any of my units for duty cycle as the build is the fun for me rather than the finished product besides for the cycle seeming a bit too random do to potential issues with conductivity between the module and heat-sink.
Besides the long list of other potential issues I've noticed that if the heat-sink isn't bored just right the module will slide too easily in and not make a tight contact plus tightening down the set screw will then open a tiny gap on the set screw side to part way around the module. This needs to be taken care of.
I am generally not too concerned for my own personal units as they are rarely used and if used it's always just a 10 second or less thing. I'm a builder/collector and only a burner when messing with cheapo builds I put together or modify.
 
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TaterMay

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Do you not use Arctic Silver or something equivalent in that case? I feel like it always helps, even if you do have a tight fit between the module and heatsink. I, too, enjoy the build more than anything. I also really enjoy problem solving and designing experiments, and I would love to be able to actually see the variable data to show exactly how much each factor affects duty cycle. If I can get a reliable measurement method down, I can also compare identical builds to each other to see if there's much variation from one build to the next. I expect it's not some highly variable, unpredictable process as some think. But you never really know until you have some actual data. I'm probably going to purchase a thermometer with thermistor or thermocouple today. I just need to find one that is small enough to fit as close to the diode as possible.
 

Pman

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Understand. I just posted a question in the "general" section about thermal conductivity for a specific heat-sink I've never used before.
 

TaterMay

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Well I just received the thermocouple/thermometer. It seems small enough to fit in there very close to the diode. Tone to set up some experiments. I might have to collect all my data at home and take it to work to put it into MiniTab, unless anyone knows of a MUCH cheaper(preferably free) alternative. I am pretty excited to see, with actual data, just how much things like ambient temp, driver current AND type(buck, boost, etc.), cool-down periods, etc. affect the case temp. Plus, I went through Design of Experiments training a few months ago, and this will be a great, fun way to keep my skills fresh. :)
 




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