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From: "R. Lewis"
Subject: Re: luxeon star leds
Date: Fri, 22 Nov 2002 22:19:31 -0000
Organization: Nextra UK
NNTP-Posting-Date: Fri, 22 Nov 2002 22:18:21 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"Uwe Zimmermann" wrote in message
> "R. Lewis" wrote:
> > I seem to have a problem with the luxeon star - or its data.
> > Luxeon quote a tempco of forward voltage of -2mV/degC and a theta
> > to board) of 17 degC/watt.
> > However, with a constant current source, if I power up a device (in free
> > air- no additional heatsinking) and measure the forward voltage (takes
> > 30mS for the first reading) and subsequently monitor the forward voltage
> > the temp on the aluminium pcb I get a far greater temp rise on the pcb
> > the temp rise of the junction.
> > How is this so -- or what am I doing wrong.
> > How can I otherwise determine junction temp?
> If it is the same datasheet, they state the temperature coefficient
> for a constant forward current of 350mA. It might be quite different
> for different currents!
> However, I find it hard to believe that they really achieve the same
> tempco for all those different semiconductor materials they must have
> in their different LEDs...
It was tested at a number of currents from 110mA to 343mA.
The tempco of 2mV/degC is quoted all over the place (data sheets, app
notes - even those specifically aimed at thermal management - so why do I
get nothing like this?
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