# How to calculate junction temp (lux 3) from slug ?

#### elfen1

##### Newly Enlightened
Any ideas how many degrees higher the junction temp is compared to a slug reading on a Lux III if so whats a good temp for the slug? I think the max junction is 120 C according to the pdf file http://www.lumileds.com/pdfs/DS46.PDF

Any help would be great.

The maximum is 135C

To calculate the junction temp by knowing the slug temp, use this formula:

Tj = ( (LED Vf in Volts) * (Current in amps) * 13) + (slug temp) (all in celsius)

To be exact, you should actually measure the LED Vf and Current instead of assuming values.

ok well i measured mine and got a about 88 and 110 measuring two setups. That is after your calculations. Do you think this is too high or should I only worry when near 135 max temp!

The rule is lower is always better. Even though you're not exceeding the maximum junction temp, a hotter LED will be less efficient as output degrades with increasing temperature, and running hotter accellerates the aging process, so it will dim sooner.

They are in the limits, but running hot, especially the 2nd one.

Whats your setup and temp ( slug Fv and amps)?? And emitter?

I have a Luxeon I in a 3D maglite. Maximum slug temp is about 30C, running at 580mA, Vf~3.5V, at 15C/W thermal resistance, that gives a Tj of about 60C.

I also have two Luxeon V in 2D maglites. Maximum slug temp is around 45C, running at 750mA, Vf=6.8V in one, 6.24V in another. With 8C/W j-s thermal resistance, that gives Tjs of 86C and 82C respectively. It's really hard to get a Luxeon V to run much cooler than that.

Note: These figures are after the light has been powered up for > 1/2 hour, free standing, and reaching thermal equilibrium - so that's all the higher the junction temperature will get.

I managed to get a set of 3 lux3 up to a heat-sink temp of 210F.. I don't even wanna calculate the Tj in that case!

You do want to keep the Tj as low as possible... on that light i mentioned i added in a fan.. and on my 'megasonic' i have a 4" circular finned heat sink.

-awr

Re: How to calculate junction temp (lux 3) from sl

[ QUOTE ]
evan9162 said:
The maximum is 135C

To calculate the junction temp by knowing the slug temp, use this formula:

Tj = ( (LED Vf in Volts) * (Current in amps) * 13) + (slug temp) (all in celsius)

To be exact, you should actually measure the LED Vf and Current instead of assuming values.

[/ QUOTE ]

where does the 13 come from?

Re: How to calculate junction temp (lux 3) from sl

The 13 is 13C/W thermal resistance from Junction to Slug, as specified in the Luxeon III emitter Datasheet.

Re: How to calculate junction temp (lux 3) from sl

So if using LuxIII stars would that be 17? But on ds45 & ds46 the thermal resistance headings are different.

Can you explain further.

Thanks!

Re: How to calculate junction temp (lux 3) from sl

It's 13C/W to the slug with emitters, and 17C/W to the star board with a Star. If you were to pop the emitter off the star board, it'd be 13C/W to the emitter slug.

DS45 and DS46 also have the new Red/Red-Orange/Amber Lux III parts in them, they have different thermal resistance values than the white/green/blue parts.