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

The prototype was on standard FR4 and worked fine. We had to add an RC
damping circuit to get it stable on the Cobritherm board. I've got 2
led strings and controllers on each board and noticed that the two
channels syncronised to the same frequency. This definitely didn't
happen on FR4.

I'll measure the capacitance to see if it was the problem.

As to shared ground I'm sure that that wasn't the problem, which
probably means it is. The first rule of fault finding is that the
fault is where you are not looking for it.