On Fri, 03 Jul 2009 15:11:10 -0700, John Larkin
wrote:

On Fri, 26 Jun 2009 09:46:01 +0100, Raveninghorde
raveninghorde@invalid wrote:

On Thu, 25 Jun 2009 17:22:21 -0700, Capt. Cave Man
wrote:

On Thu, 25 Jun 2009 07:11:19 -0400, Spehro Pefhany
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.

That is aluminum clad? There are holes through it? Are they isolated?


I think he is nuts. Looks like copper to me. What PCB houses use Al
clad media? What etching process works with AL cladding?

What solder sticks to Al cladding.

Current control should be easy and precise, even with ALL of them
floating without "earth" grounding.


The pcb is an aluminium plate with a very thin insulator and a copper
layer.

http://www.aismalibar.com/archivos/8...pdf?download=1

Current control was stable on FR4 prototype but the higher capacitance
is causing interaction between current controllers.

It's hard to imagine that the pcb adds much capacitance. You could
measure it.

Maybe the instability comes from a shared-ground problem.

John

Apparently the dielectric between the copper cladding layer and the
aluminium core is only 12um in this product. I think from previous
work that this can raise capacitance into the 400pF/cm^2 range. I had
thought from the previous description that he was using an al-cored
stack-up (fr4 layer insulation).

I don't know what control or modulation method is being used, so I
can't comment on sources of chaotic behaviour. I hope he's substituted
a known-good control section at some point, before beginning to
suspect the new layout.

Previous comments re thermals on copper-clad Al are amplified
three-fold with this particular material. It takes a lot of energy to
make an Al plate follow the recommended thermal profile of the opto
expected standard reflow. Dwell times are particularly difficult to
terminate.

RL