On Fri, 26 Jun 2009 13:04:23 +0100, Raveninghorde raveninghorde@invalid
wrote:

On Fri, 26 Jun 2009 09:52:54 +0100, Raveninghorde
raveninghorde@invalid wrote:

On Thu, 25 Jun 2009 17:20:18 -0700, Capt. Cave Man
wrote:

On Thu, 25 Jun 2009 11:40:39 +0100, Raveninghorde raveninghorde@invalid
wrote:


Here is my new light bulb, minus reflector.

168 112 lumen leds in 14 strings.

The main problem is the aluminium of the pcb is not earthed causing
problems with current control presumably due to much higher stray
capacitance than we had on FR4. Just got to find a neat way of
earthing it.


Is there not a current limit resistor at each LED?

Capacitance has an effect? Improper design then.

Current limit resistors ARE the cheapest, most efficient "current
control".


No. Resistors are not efficient they dissipate and take board space,
they are not even particularly cheap at these power levels. The leds
take about 180 Watts.

To follow up on that. The colour temperature is dependant on LED
current so the current needs to be correct. Dimming is done by PWM so
that the on current doesn't change.

How do you insure that the same voltage is found anywhere along the
physical rail?

The LED voltage is specified as 2.7V to 3.8V at room temperature with
0.4V change over the temperature range. The LED voltage variation is
bad because the LEDs are matched for brightness and colour
temperature.

OK, so you match and cull.

Based on the above the current has to be actively controlled. In this
case using a Supertex IC.

The series resistor is for max current limiting.