A linear regulator will produce the SAME amount of heat as a resistor
at the same current. W=VI If the volts and the amps are the same
the power is the same. However since the car mfg's are installing
LED's in the center brake light they have paid for the development of
simple constant current switching power supply IC's to drive the
LED's. The power dissipated in the LED is the same however the power
that was dissipated in the current limit ballast resistor has been
greatly reduced so that the TOTAL system power and heat generated is
about 50% of what it was to get the same light out of the LED using
resistors.
You have to add a small inductor and since it is a switcher you may
have radio interference if your layout and EMI filter is not good.
John

On Tue, 11 May 2004 09:01:23 +0100, anonymous coward
wrote:

On Wed, 05 May 2004 02:46:06 +0000, Ernest Christley wrote:

anonymous coward wrote:
Have you considered using a constant current regulator, instead of a
resistor? I believe there is an example circuit given in the LM337/317
datasheet showing how to build one using only the LM337 (normally used as
a voltage regulator) & one resistor.

It would need to be bolted to a heatsink, like the Luxeon Star LEDs, but
IIRC the LM337 and cousins also shut down if they overheat.

AC


Yes. I considered it. I opted for the simplicity of a single current
limiting resistor and the constant voltage regulator that is already
there. Number one rule of fault management. If it ain't there, there's
no way to break it. Regulators not only add an additional active
component with its list of failure modes, it also adds severl solder
connections and more heat,

A regulator will produce no more heat than a resistor passing the same
current with the same voltage drop. The ones I'm thinking of are 'in-line'
devices, so their (negligible) supply current does not have to be factored
in as an extra source of heat.

I buy what you're saying about complexity, but you would probably only
need a single current regulator + one resistor for each parallel bank of
series LEDs (eugh, but I can't think how better to put it).

BTW, do you need to use zener diodes / transorbs with aircraft power
systems as you do in cars, to avoid problems with voltage spikes?

AC

making it even more difficult to rig the system into a 1/4" piece of plexiglass.