<font size="2" face="Verdana, Arial">As long as you seal the light as you suggest [to prevent any heat transfer by air exchange], there is no need to make any provision to thermally couple the heat source to the flashlight. This is the very subject that we flogged in another thread recently. As long as the heat is generated inside, it will get out.Originally posted by McGizmo:
Wayne and Doug,
Thanks for these useful pieces of information! this is one of those posts that it's a bummer that it will slide off the chart and this info will be hard to find later on.
Doug, how do you suggest imparting the resistor heat to the battery tube? Will thermal grease suffice? Wont some of the heat dissipate into the air in this case as opposed to a closed flashlight where the heat will one way or another need to pass through the body of the light? Probably want to seal off the light in as close to similar fashion as when the light will be in functional form?
- Don
<font size="2" face="Verdana, Arial">As long as you seal the light as you suggest [to prevent any heat transfer by air exchange], there is no need to make any provision to thermally couple the heat source to the flashlight. This is the very subject that we flogged in another thread recently. As long as the heat is generated inside, it will get out.</font><hr /></blockquote><font size="2" face="Verdana, Arial">My original answer is assuming that heat conduction within the metal is so good that temperature gradients within the length of the flashlight body can be ignored. This isn't a very good assumption. Those of you that are playing with these metal flashlights know this as the head of the flashlight feels warmer than the opposite end. If you perform the type of testing that Wayne and I are suggesting, it makes sense to measure the flashlight temperature in the immediate area of the heat source and to thermally bond the heat source to the inside of the flashlight body in a way that at least somewhat approximates the proposed ultimate arrangement for transferring the heat to the flashlight body. Your suggestion of using a thermal compound is a good one.Originally posted by Doug S:
</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by McGizmo:
Wayne and Doug,
Thanks for these useful pieces of information! this is one of those posts that it's a bummer that it will slide off the chart and this info will be hard to find later on.
Doug, how do you suggest imparting the resistor heat to the battery tube? Will thermal grease suffice? Wont some of the heat dissipate into the air in this case as opposed to a closed flashlight where the heat will one way or another need to pass through the body of the light? Probably want to seal off the light in as close to similar fashion as when the light will be in functional form?
- Don
<font size="2" face="Verdana, Arial">I strongly agree with this [ not the math part, Peter, only you know that for sureOriginally posted by Gransee:
Personally, I flunked math and even so, the variables to account for the interface between the die and each progressive layer of material to get to the outside surface, is a bit complex. The possibility of misrepresenting one of the interfaces is good and it would make the results deceptive.
Peter Gransee
<font size="2" face="Verdana, Arial">Strongly disagree. Two problems here. The viewing aspect ratio of most inexpensive IR thermometers [the type the typical CPFer member would likely have] is not narrow enough to look at only the LED. More importantly, even if the IR thermometer was looking at the top of the LED only, what it is looking at is the surface temperature of the plastic housing of the LED. The materials of the housing are likely poor thermal conductors and thus, at the power levels that we are typically concerned with, the measured temperature of the LED housing will be much lower than the die temperature.Originally posted by Gransee:
To get a good picture of how the die is doing in your maglight, remove the lexan lens and shoot the top of the LED with your IR thermometer. Move the sensor around to find the maximum reading.
This is the best method I recommend.
Peter Gransee
<font size="2" face="Verdana, Arial">I interpret the IR data in two ways. Relative and absolute. Absolute to make sure I am not too close to the 135C limit. In the LS3, I measured 85C max pointing the sensor directly at the top of the LED dome.Originally posted by McGizmo:
Peter G,
I have a IR thermometer as well as a thermacouple accessory for my DMM. I have found that invariably i get higher temp readings with the thermacouple if I'm measuring the temp on the aluminium housings. If you shoot the LED itself with the IR, do you think there is any significant error from reflected IR on the lens or even IR eminating from the LED?
dummy in the back of the class
<font size="2" face="Verdana, Arial">Right. Adequate sealing should not be hard. A couple of cotton balls or a dirty sock should do it.Originally posted by McGizmo:
Doug S.,
I intentionally set myself up for that one. ;-) Of course it may not be easy to totally seal the light with two wire leads coming out so I suppose that a good thermal path from resistor to housing would encourage the transfer of heat to housing direct.
<font size="2" face="Verdana, Arial">N0! Please don't go.Originally posted by McGizmo:
The better the thermal path, the quicker steady state housing temp is reached from a cold start, correct? If I'm wrong on this as well, I think it's time for me to find other pursuits......
<font size="2" face="Verdana, Arial">It would be narrow enough if you held it at point blank.Originally posted by Doug S:
Strongly disagree. Two problems here. The viewing aspect ratio of most inexpensive IR thermometers [the type the typical CPFer member would likely have] is not narrow enough to look at only the LED. More importantly, even if the IR thermometer was looking at the top of the LED only, what it is looking at is the surface temperature of the plastic housing of the LED. The materials of the housing are likely poor thermal conductors and thus, at the power levels that we are typically concerned with, the measured temperature of the LED housing will be much lower than the die temperature.
<font size="2" face="Verdana, Arial">It would be narrow enough if you held it at point blank.Originally posted by Gransee:
</font><blockquote><font size="1" face="Verdana, Arial">quote:</font><hr /><font size="2" face="Verdana, Arial">Originally posted by Doug S:
Strongly disagree. Two problems here. The viewing aspect ratio of most inexpensive IR thermometers [the type the typical CPFer member would likely have] is not narrow enough to look at only the LED. More importantly, even if the IR thermometer was looking at the top of the LED only, what it is looking at is the surface temperature of the plastic housing of the LED. The materials of the housing are likely poor thermal conductors and thus, at the power levels that we are typically concerned with, the measured temperature of the LED housing will be much lower than the die temperature.
Originally posted by Gransee:
<font size="2" face="Verdana, Arial">You are going to look at the die only and not the slug its mounted on? Dimensions of the die are on the order of 1mm. I would be surprised if your equipment can do this. If your equipment is good enough to see only the die plus slug, you could use the approximation that the measured temperature is the slug temperature [this approximation works since the slug area is large relative to the die area in the view of the instrument] and then use the die to slug thermal resistance to calculate the die temperature [15C/W in the case of the 1W emitters]To be sure, and as I alluded earlier, removing the plastic dome from the top of the LED would provide a more accurate reading So, although not ideal (in a perfect world), I still feel that measuring the dome with a pyrometer is the "best method". At least until something "better" comes along.
What I am going to do to increase my confidence in this method is to sacrifice a luxeon. I will let you know what the difference was between dome on and dome off.
Peter Gransee
If you really must use IR measurement methods, a better way to "calibrate" your technique would be to take a "Star" configuration device and measure the backside under the emitter, calculate the die temp using the Luxeon datasheet thermal resistance, and then compare to your IR measurement of the front of housing. Measurements should be made after thermal steady state has been reached.
For other discussion of thermal measurement methods, see the link below:
http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=14;t=000376# 000019
<font size="2" face="Verdana, Arial">This would be an excellent test. To get actual die temperature you would still need to add in the effect of the thermal resistance from your point of measurement to the die. In the case of the 1W devices this would be 17 C/W and for the 5W devices 11 C/W.Originally posted by McGizmo:
Doug,
For a one off, it probably isn't worth the effort but if one really wanted to know what was happening, would a good, real world test be the following?
Mill or drill a cavity for thermocouple in the Aluminum heat sink pad of the host and glue the emitter to sink pad with thermocouple contained in this "laminar" junction. If arctic silver epoxy were used for instance, it seems to me that you would be getting as close to the source as possible. Comments?
<font size="2" face="Verdana, Arial">Peter, Peter, I hardly know where to start. I will be brief. I fear that you are giving yourself a sense of security that is not justified by your experiment. Even if we accept your data as accurate [and there are good reasons not to] you are showing a 50% greater temperature rise over ambient between your two measurements. Assuming an ambient of 25C, your 85C measurement in your LS3 now becomes 115C. The problem with your data, is that you are trying to draw conclusions about a 2C difference in temperature when your instrument only has a resolution of 1C. As you know, IR measurements are also affected by the emissivity of the surfaces being measured. In your experiment you are measuring different surfaces not known to have similar emissivities. A further complication is with the IR instrument pictured in your photos. As I recall, it has a specified aspect ratio of 6:1. This however breaks down at very close measurements. It is hard to know exactly what your instrument is looking at in your measurements.Originally posted by Gransee:
Without claiming my experiment is the last word in die measurment, I can say that I feel the accuracy of the method is usefull in ensuring that the LEDs in Arc flashlights are not being driven at dangerous levels.
Peter Gransee