I have built a heat sink from of a 6 mm x 100mm aluminium disc with a sst-90 on it and I have run it for a short burst but I am concerned about the heat. How can I test the LED temperature with only basic equipment? What should the maximum temp I should aim for? I can increase the heat sink easy by adding an aluminium battery holder fastened to the heat sink but this would warm the 3 D cells would this be a bad idea?
heat test
- Thread starter joohn
- Start date
What do you call a short burst? I take it youre using a star screwed down to the aluminum disc? Though you cant really measure LED temperature, with a good star, and good method of mounting, if you can keep your hand on the disc, then chances are the LED isnt too hot, provided you are driving it within normal specified currents. Just a SWAG(scientific wild *** guess) , you should be able to run for at least a minute or two on your heatsinnk, probably more.
If you want to get a worst case scenario idea of how long you can run a burst, assume no heat is removed from the disc, calculate or measure the weight of it, multiply by the specific heat of the aluminum and temperature difference to get your heat capacity, and see how many joules/calories/watt-hours it would take to heat the plate from your ambient temperature to 60C or so, and then assume 95% of the energy into the LED comes out as heat, (or 100% for real worst case scenario) and divide the heat capacity by the led's heat output to get a time. In practice it will take longer because heat will be dissipated from the heatsink via convection any any other air flow. (Radiation doesnt dissipate that much heat when your heatsink is only a few dozen degrees hotter then ambient! people get hung up on increasing radiant dissipation when they really shouldnt even worry about it)
If you want to get a worst case scenario idea of how long you can run a burst, assume no heat is removed from the disc, calculate or measure the weight of it, multiply by the specific heat of the aluminum and temperature difference to get your heat capacity, and see how many joules/calories/watt-hours it would take to heat the plate from your ambient temperature to 60C or so, and then assume 95% of the energy into the LED comes out as heat, (or 100% for real worst case scenario) and divide the heat capacity by the led's heat output to get a time. In practice it will take longer because heat will be dissipated from the heatsink via convection any any other air flow. (Radiation doesnt dissipate that much heat when your heatsink is only a few dozen degrees hotter then ambient! people get hung up on increasing radiant dissipation when they really shouldnt even worry about it)
I have screwed it on to the ally disk and it is on a star, “short burst” was on to see it works then off till I was sure about the temp. It will have to run for 20 to 30 mins constant, Its for diving, so the outer of the body of the torch is cold, I could fit a short aluminium tube around the inside of the body welded to the disc to aid cooling? Or just a larger disc behind the one I have?
Thanks again for your help.
Thanks again for your help.
Check out the dive light section on here, lots of nice lights and examples on there. Whatever way you can connect the LED to the body of the light with a good thermal path would be best, since all that water makes for a very nice sink for all the heat. A weld or a braze/soldered joint (metal to metal to metal) would be best, but if you have to have something bolted or pressed etc... make sure to use some sort of thermal grease to aid in thermal conductivity.
I have looked at that section on diving and found it helpful and thanks for that, But what I need to know is I think the LED will be running at a higher temp than the heat sink and what I was looking for was what temp would the ally disc run at neat to the LED if the led was getting too hot. I will set up a test soon using a normal thermometer on the disc to check the heat, but not sure what is hot?
bshanahan14rulz
Flashlight Enthusiast
joohn, it all depends on how well you bolted down the star, what you used between it, what the star is made of, how well the LED is soldered to the star, etc. Even if you knew all that information and more, the error allowed in calculation would make it more worthwhile to just estimate.
Your best bet is to just test-run it. If it sizzles your finger, you know it's too hot ;-)
If you can hold your finger on the star without being hurt, it's pretty safe.
You want the thermal sensor as close to the LED thermal slug as possible. Often, MCPCB desined for high-power LEDs specifically will have a thermocouple trace that you can solder a thermistor to, but also provides a good spot for temperature measurement. The only time I've seen the manufacturer provide a generic equation for junction temperature relative to thermocouple sensed temperature was when the LED was provided from manufacturer on the metal substrate.
Your best bet is to just test-run it. If it sizzles your finger, you know it's too hot ;-)
If you can hold your finger on the star without being hurt, it's pretty safe.
You want the thermal sensor as close to the LED thermal slug as possible. Often, MCPCB desined for high-power LEDs specifically will have a thermocouple trace that you can solder a thermistor to, but also provides a good spot for temperature measurement. The only time I've seen the manufacturer provide a generic equation for junction temperature relative to thermocouple sensed temperature was when the LED was provided from manufacturer on the metal substrate.
MikeAusC
Enlightened
An SST-90 running at full power will put out at least 25 watts of heat.
You will need a heatsink with lots of fins and a fan.
Your cheapest solution is to look around for a CPU heatsink with fan for an outdated model of CPU. I even found one with a heatpipe for $25.
You will need a heatsink with lots of fins and a fan.
Your cheapest solution is to look around for a CPU heatsink with fan for an outdated model of CPU. I even found one with a heatpipe for $25.
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hellokitty[hk]
Enlightened
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Read posts first.
Max junction temperature is rated at 150°C; though In general, if you can touch the star, then it's ok.
MikeAusC
Enlightened
What ?????
It's for a dive light, so a fan is probably not needed or desired.
MikeAusC
Enlightened
If the Divelight has a plastic case, you still have an Insulator between the water and the air inside the light.
I built a divelight that used a 50 watt Halogen bulb inside a plastic case and used a fan to stop local heating - the same applies to keeping an LED heatsink cool in a small volume where there will be minimal airflow and you can't conduct the heat away via metal.
I built a divelight that used a 50 watt Halogen bulb inside a plastic case and used a fan to stop local heating - the same applies to keeping an LED heatsink cool in a small volume where there will be minimal airflow and you can't conduct the heat away via metal.
Halogen is totally different than LED. Halogen does not mind getting warm, in fact, the hotter the bulb, the more efficient it is, and the more light it makes. So it was perfectly fine to isolate a halogen bulb from conduction losses and put it inside a silvered reflector that insulated it from the case.
LED efficiency and lifespan go down with heat, and acceptable temps are far, far lower than the surface of a halogen bulb.
Your dive light will make such good use of the fantastic cooling available in water that you will typically design it with a power/heatsink size ratio far too high to operate OUT OF the water. You may want to include a safety shutoff in case it is accidentally turned on outside the water, as it will quickly overheat and burn up the LED.
LED efficiency and lifespan go down with heat, and acceptable temps are far, far lower than the surface of a halogen bulb.
Your dive light will make such good use of the fantastic cooling available in water that you will typically design it with a power/heatsink size ratio far too high to operate OUT OF the water. You may want to include a safety shutoff in case it is accidentally turned on outside the water, as it will quickly overheat and burn up the LED.
onetrickpony
Enlightened
- Joined
- Mar 10, 2011
- Messages
- 262
I just outlined my method for mounting stars here, it may be of use: http://www.candlepowerforums.com/vb/showthread.php?313317-screwed-on-star
Remounted the star, I take your point about lapping in the star and heat sink with a fine paper, the star is only stamped out and far from flat I will flatten it when I have soldered on the LED. See below
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