NEWBIE: LED Running Temp and connecting wire

[AMazeOfTwistyPassages]

I have a cutter triple PCB which with the heatsink I provided got very hot and caused the insulation on the wire connecting to the PCB to melt and short.

How hot should a light assembly run?

Should I use wire with some form of heat resistant covering? The wire I used probably had a low melting point. Which/where?

Thanks in advance
AMOTP

PS

Cree R2 emitters

 

[Th232]

Was the PCB attached to a heatsink? If not, that's the problem.

 

[LEDninja]

LEDs self-destruct at around 140°C (284°F).
The LED light bulbs I buy claim max temperatures of 50°C (122°F) to 70°C (158°F).

The PCB board is just a method of transferring the heat to a bigger heat sink/dissipation device such as a flashlight body or a C channel. What is your board connected to thermally?

 

[Mr Happy]

If the insulation on the wire got hot enough to melt with an LED something went terribly, terribly wrong.

The most likely cause is that something shorted first, and the short circuit current in the wire then caused the insulation to melt.

 

[VidPro]

only time i have melted the insulation on wire when assembling LEDs was from the soldering at the time of assembly, if its getting that hot, wonder what the poor led is going through.

on the other hand, there is Wire in existance that is Very poor quality, high resistance cheap alloy too small for the job, add to that not solidly connected. but it would have to be pretty poor wire.
i have never NEEDED primo high heat wire, like teflon or something for leds, even overdriving 3amp ones, but i always pick copper over the loads of cruddy wire that came from other broken items that i have avilable.

 

[AMazeOfTwistyPassages]

The Triple LED board was attached using arctic silver thermal adhesive to a khatod heatsink. The heatsink was 30mm diameter, as is the PCB containing the 3 XPE R2 emitters. The heatsink depth is 15mm including fins. I was supplying the emitters at 750mA.

Does it sound like I was trying to use too small a heatsink and that's why I got thermal runaway?

What temperate should the PCB be operating at?

Thanks in advance
MOTP

 

[Mr Happy]

I don't think you should get temperatures higher than, say, a hot cup of coffee. Temperatures hot enough to melt plastic are way out of range.

How does the heat leave the heat sink? Is there ventilation or is it enclosed inside a light?

 

[AMazeOfTwistyPassages]

I had it enclosed at the front and rear with say 60% of the fins open to the air at the sides.

I was pushing 1000mA into the XPE R2s... maybe I was overdoing it?

It was operating very hot... > 100 degrees C ? finger burning hot. Given that this was my first LED assembly I did not know what to expect.

The setup was in an old headlamp; by the end all I had left was a sticky plastic mess! So well up in the plastic melting range!

Thanks for your patience!

Regards
AMOTP

PS

Revised 750mA to 1000mA.... As this is probably what I was trying for my last experiments.

 

[Mr Happy]

For a rough calculation, assume 1 A though an emitter with a Vf of about 3.5 V, that's 3.5 W dissipated. Times three, that's about 11 W. That's quite a lot of heat confined to a small space. Substantial cooling is going to be needed.

 

[VidPro]

you can "get Away" with a heat sinc of small sizes if its sinced TO something, like a aluminum body or even steel body of a flashlight.
meaning the inital heat sinc is just a "spreader".
fins should be usefull, if they can get a flow of air across them, but often they can just get in the way of transfer of the heat to the flashlight case.

 

[AMazeOfTwistyPassages]

What is the typical current used to drive XPE R2s? (My application is a headlamp for Night Orienteering).

... or do people just have lots of cooling? If so... just a big heatsink or fans?

I feel like I was running before I could walk with this project.

Thanks for the V. useful info.

Regards
AMOTP

 

[VidPro]

9 watt headlight .
headlights rarely have enough heat sincking as it is, that is why many of them have dribble for output.

lightweight fins should be very usefull, then running at lower powers MOST of the time, then "boosting" up to high power for short bursts. that is one way of having your cake and lighting it with 300 candles too.

 

[AMazeOfTwistyPassages]

My normal headlight is 20W Halogen, LED is a new departure for me.

For longer runs I have a 10W bulb. Both are 6v driven from a voltage regulator from a 12v NiCd source.

I was hoping to replace the Halogen setup with an LED equivalent.

Regards
AMOTP

 

[Mr Happy]

What is the typical current used to drive XPE R2s? (My application is a headlamp for Night Orienteering).

... or do people just have lots of cooling? If so... just a big heatsink or fans?

I feel like I was running before I could walk with this project.

Thanks for the V. useful info.

I think the maximum current for a white XP-E R2 according to the data sheet is 700 mA. However, this would be considered "overdrive". The recommended current would be about 350 mA. As VidPro said you probably want to work with 350 mA as normal and 700 mA as a temporary boost mode, unless you can come up with some really efficient cooling.

There's a significant difference between LEDs and incandescent bulbs. With bulbs much of the heat is radiated out of the reflector as infrared, so less is soaked up in the housing. With LEDs there is much less infrared so much more of the heat has to be handled by the heat sink. This generally leads to some significantly different design and mounting decisions for LED lights compared to incans.

 

[AMazeOfTwistyPassages]

Thanks for your help.

I will go and do some experiments at lower currents.

Regards
AMOTP