# How hot is too hot?



## mnm99 (Feb 2, 2015)

How hot should a heat sink get? I can test it with an IR gun when I get a chance. When I touch it I can hold it, but it seems pretty hot. Right on the edge of not being able to hold it too long.

Thanks


----------



## mercrazy (Feb 3, 2015)

if you have good thermal conductors between LED and heatsink, you are ok. sounds like your temperature is around 45C


----------



## mnm99 (Feb 3, 2015)

mercrazy said:


> if you have good thermal conductors between LED and heatsink, you are ok. sounds like your temperature is around 45C



They are secured with Arctic Silver Thermal Adhesive.


----------



## more_vampires (Feb 3, 2015)

When I touch a heat sink and it leaves shiny imprints on my fingers, I don't like touching heat sinks. That's too hot for me.


----------



## LedTed (Feb 3, 2015)

mnm99,

You may be looking at the wrong piece of the puzzle. The heat sink is one piece, the component is another.

If the heat sink gets “hot” it is doing its job; how well, that is the question. How “cold” can the heat sink keep the component?

Another related question, how “hot” does the manufacturer say the component can get? If you haven’t already, learn how to best use your IR gun, then measure how “hot” the component gets.

For example, I’ve seen “warm” heat sinks where their associated components got so “hot”, they effectively de-soldered themselves. 

And one more point, don’t forget about electrical isolation.

Hope this helps.


----------



## Steve K (Feb 3, 2015)

mnm99 said:


> How hot should a heat sink get? I can test it with an IR gun when I get a chance. When I touch it I can hold it, but it seems pretty hot. Right on the edge of not being able to hold it too long.
> 
> Thanks



Wikipedia tells me that aluminum melts at around 660C, so I'd advise keeping it cooler than that.

The better question is: what is the junction temperature of your LEDs? (just assuming that this is about LEDs)


----------



## mnm99 (Feb 3, 2015)

I picked up another IR gun today ( mine just broke ) and am going to check the top surface temp and backside when I get home. Link to lights

http://s270.photobucket.com/user/mnm1999/media/20150201_080230_resized_12.jpg.html?o=4


----------



## DIWdiver (Feb 3, 2015)

IR guns can be very deceiving. On low emissivity surfaces like bare aluminum, they read quite a lot low. A piece of black electrical tape can correct that.


----------



## mnm99 (Feb 4, 2015)

So far I ran the light for a total of 4.5-5 hrs with no problems to the Epoxy encapsulation or failing of lights.
I got the IR gun and did the piece of tape. Here is what I'm finding....Does this look OK?

With the water at 21.6*C / 71*F , the light is holding at 46*C / 115*F....Lights running for 1 hr at 12.5v

With the water at 32*C / 90*F , the light is holding at 56*C / 133*F....Lights running for 1.5hrs at 12.5v

The water near me will never get over 75*F, but if I want to sell these I need to make sure they will work in at least 90*F water too. Next test will be at 14.5v


----------



## LedTed (Feb 4, 2015)

DIWdiver said:


> A piece of black electrical tape can correct that.



Better yet, plain old colored masking tape.


----------



## mnm99 (Feb 4, 2015)

The electrical tape worked pretty good.


----------



## more_vampires (Feb 4, 2015)

A beautiful auburn-haired girl wearing an apron was teaching an apprentice young asian girl (maybe 13-15.) This smouldering fire girl was baking a turkey.

"Is it done yet?" asks the child.

"No dear," the redhead calmly replies as the timer rings. "When I open this door, we're going to shoot it with this laser gun.... *and stab it!*

A flurry of questions: 
"Do you really have a laser gun?" 
"Can I shoot your laser gun?"
"Is it really not dead yet?"
"Why are we going to stab it?"

"Tell you what, dear," says the redhead. "You're new at this, so when I open this door... you shoot it with the laser gun and check the indicator while I'll stab it."

The oven door opened. The turkey was cooked by non-bone contact of a penetration thermometer without touching the cook surface and towards center of material. The young girl was speechless.

Food isn't normally cooked that way!!!

I thought the whole exchange was hilarious. That redhead wasn't too hot, wasn't too cold... she was certainly "just right." Redheaded Goldilocks!

"Wow, this is really good!" later said the child. She was a fan of turkey with American style white gravy with black pepper and jalapeno pepper jelly.

See also: temperature convection by fluid dynamics. Temperature conduction is a separate topic. Temperature of a radiation is also of interest to many here.

Specific heat? Heat capacity? Questions that plague us all...


----------



## LED Boatguy (Feb 4, 2015)

It's amazing how much heat these LEDs can take and still survive. One time I was reflow soldering a Cree XP-G to a MCPCB in an infrared oven controlled by a programmable PID. I used the PID so often for other things I do, some of the insulation on the K thermocouple had been worn off and shorted when inserted into the oven. Since the thermocouple was inop, the PID just kept raising the temp, WAY past the 215 C soldering temp. I knew something was wrong, as the temp on the PID display wasn't going up fast enough, so I repositioned the thermocouple. The PID suddenly displayed something like 350 C and shut down the oven.

After the oven cooled down, I removed the LED and the (previously) white MCPCB was now toasty brown. But damn if the LED didn't still work just fine! I never put it in one of my products, but it was cool that it worked. Good thing that screw up didn't happen when I was soldering 50 or so boards at a whack.


----------



## more_vampires (Feb 4, 2015)

Reminds me of pasteurizing wine to stop fermentation before clarification without sulfite compounds and without much alcohol loss with a thermocouple probe in an industrial microwave oven. Killed a few batches dead before I figured the flaky probe.

No I was not distilling! 

It was only 20 percent alcohol...


----------



## LED Boatguy (Feb 4, 2015)

more_vampires said:


> Reminds me of pasteurizing wine to stop fermentation before clarification without sulfite compounds and without much alcohol loss with a thermocouple probe in an industrial microwave oven. Killed a few batches dead before I figured the flaky probe.
> 
> No I was not distilling!
> 
> It was only 20 percent alcohol...



So, on the show "Moonshiners", one of them is you? Jim Tom, is that you?

I'm just glad I wasn't in production mode when that screw up occurred. That would have been expensive.


----------



## mnm99 (Feb 5, 2015)

??? So would you say its too hot. I didnt get too much of an opinion out of all the responses. Thanks. Im trying to get some specs from the manufacturer today.


----------



## more_vampires (Feb 5, 2015)

mnm99 said:


> ??? So would you say its too hot. I didnt get too much of an opinion out of all the responses. Thanks. Im trying to get some specs from the manufacturer today.



They won't give you a specific number, most likely. What they call "too hot" likely merely shortens component life. Heat path and rate of heat transfer are the critical things. Like how LED Boat had a cooked LED that still worked?

It's like how grain alcohol could evaporate at happen at 172 F... or about 180 F, depending. Heh, I'm no "Moonshiner."


----------



## SemiMan (Feb 6, 2015)

more_vampires said:


> They won't give you a specific number, most likely. What they call "too hot" likely merely shortens component life. Heat path and rate of heat transfer are the critical things. Like how LED Boat had a cooked LED that still worked?
> 
> It's like how grain alcohol could evaporate at happen at 172 F... or about 180 F, depending. Heh, I'm no "Moonshiner."



Heat sink temperature is almost meaningless without knowing how much power you are putting into the LEDs, how well they are mounted, etc. With the right LEDs, a good path between the LEDs and the heat sink, and lower drive current, the heat sink could be 90-100C and you could still get quite good life out of the LEDs ... 10's of thousands of hours if not 50K hours with reasonable lumen depreciation.

So ... details man, details!

How big is the heat sink, which LED, how much current are you driving into it, how is the LED mounted to the heat sink and everything between the heat sink and the LED.

MFR documents tell you how hot the LED junction can get, not the heatsink.


----------



## more_vampires (Feb 6, 2015)

Loving it, Semiman.

Average core temp of the heat sink is one thing, junction temperature is a different beast. Temperatures high enough to melt solder result in failures upon the shock of a drop (or similar.)

Rate of temperature flow. As Semiman said, size, led type, drive current, led heat sinking possibility versus reality.

...a whole lot of things to go wrong! Don't forget the "heat pump" hvac system. It ices up like mad as outside temps approach freezing. It works, but only for its temperature range... then auxiliary methods.


----------



## mnm99 (Feb 7, 2015)

So far I have 6hrs on these without a problem. I bought the LED off ebay (I know, I know). There are 3 in series, 8 in parallel, 24 total. Mounted to a 3.25" round, 5/8" thick 5086 Aluminum block using Artic Silver thermal adhesive. I'm using 25 feet of 22gauge twisted pair as a current limiter/Volt drop. The voltage at the led with 12.6 output on the battery is around 10.3. The Led's are encapsulated in optical epoxy baked at 150*F for 4 hrs. Here are the specs I got off the LED's. The led's were ran in 70*F water and 90*F water. Here is what I got and the specs of the LED's.

Scroll through Pictures (The 24 LED light) .. http://s270.photobucket.com/user/mnm1999/media/20150201_080230_resized_12.jpg.html?o=4

With the water at 21.6*C / 70*F , the light is holding at 46*C / 115*F....Lights running for 1 hr at 12.5v

With the water at 32*C / 90*F , the light is holding at 56*C / 133*F....Lights running for 1.5hrs at 12.5v


 
 *Parameter* *Symbol* Test Condition​ Min.​ Typ.​ Max.​ *Unit* Luminous Flux Iv​ IF=700mA​ 80​ 90​ Lｍ​ Chromaticity Diagram CIE​ IF=700mA​ Dominant Wavelength λd​ IF=700mA​ 445​ nm​ Emission Angle 2θ1/2​ IF=700mA​ 120​ Deg​ Forward Voltage VF​ IF=700mA​ 3.50​ 4.00​ V​ Reverse Current IR​ VR=5V​ 100​ uA​
 *※**Absolute Maximum Ratings at Ta=25**℃**:* 
 Parameter​ Symbol​ Rating​ Unit​ DC Forward Current IF​ 700​ mA​ Peak pulse current(1/10 duty，pulse width 1msec) Ipulse​ 1000​ mA​ Reverse Voltage VR​ 5​ V​ Operating Temperature Topr​ -30～110​ ℃​ Storage Temperature Tstg​ -40～120​ ℃​ Manual Soldering Tine at 260℃(Max.) Tsol​ 5​ Seconds​


----------



## SemiMan (Feb 8, 2015)

That is a good start, but without knowing what the LEDs are mechanically so I can see the mount, it is hard to tell how good the thermal contact is. The junction/ambient thermal resistance of the LED is also missing from the data.

So you know, by todays standards, these are really crummy LEDs.


----------



## Anders Hoveland (Feb 27, 2015)

It can be just hot enough that you can't keep your finger on it, but it should not be much hotter than that. (about 85°C).



LED Boatguy said:


> It's amazing how much heat these LEDs can take and still survive. One time I was reflow soldering a Cree XP-G to a MCPCB in an infrared oven...
> 
> After the oven cooled down, I removed the LED and the (previously) white MCPCB was now toasty brown. But damn if the LED didn't still work just fine! I never put it in one of my products, but it was cool that it worked.


It is a different thing when we are talking about how much outside heat the LED can take. 

But when the LED is in operation, and current is being supplied, the outside temperature needs to be lower so the heat generated in the diode can dissipate. So the maximum operating temperature is substantially lower than the mounting temperature in the oven.


----------



## SemiMan (Feb 28, 2015)

Anders stop making up stuff you have no clue about. For the most part its all about duration. It was only in the oven a short time. You could run the LED really hot for a short time with little ill effect if the current is in tolerance.


----------



## lucca brassi (Feb 28, 2015)

Led temp should be measured at source , that mean under emitter core and heatsink all other is inaccurate .

How hot ... depends what you want long life ? , best efficiency ? , stable color temp ?


----------



## htsystems (Mar 10, 2015)

I hooked up some 100w led grow lights. I was told to try and keep them under 40C I think, this was last year. I know they can run hotter but the will kill their life.
I really don't think they should ever get above 60C. Also driving at a lower power level, they put out more lm/w and should last longer and much less heat.
I think many grow lights are running at 50 to 75% to make the leds last longer.
Arctic Silver seems to work nice too and make sure its on very thin!!!!!!
Something to think about.


----------



## DIWdiver (Mar 10, 2015)

Yes, some LEDs need to be run pretty cool for very long life. But those from most name brand makers can be run much hotter and still maintain long life. All modern CREE parts are specified at 85C for the life of the part.

Keep in mind it's the DIE temperature that determines life. Depending on how well the heat is transferred from the die to the heatsink, the heatsink may need to be somewhat cooler, or may need to be lots cooler.


----------



## SemiMan (Mar 10, 2015)

Not just die as phosphor temp may play in and that can be ambient related.


----------



## SemiMan (Mar 10, 2015)

Some LEDs can hit useful 50k+ hours lumen maint at well over 100C.


----------

