Hello all. Am very glad to have found this forum! Been reading and studying, trying to relearn some of my electronics skills from the 2 year electronics engineering program I took at a community college in the mid 1980s. Never used it much, but was very glad to have done it.
My project is replacing the 12V, 20W (2x10W) halogen bulbs in light fixtures (in my old sailboat project) with dimmable LED modules I hope to build.
Have been reading here a dozen hours or so total and find many helpful articles on PWM, LEDs and so on, but still have a few questions.
The driver I selected uses this chip: http://www.msc-ge.com/download/pcn/macroblock/mbl_application_note_mbl6651.pdf
To use the dimming function it wants a PWM signal on the DIM pin. If I'm reading the spec correctly (bottom of pg 2, "DIMMING"), the wave should be about 1 kHz, at least 3.5V high.
Have looked at two simple 555 PWM circuits:
http://forum.allaboutcircuits.com/showthread.php?t=24028
and
http://www.dprg.org/tutorials/2005-11a/index.html
Any ideas on which may be more suited to my application, and why?
In the first circuit, I see no spec for CR1 and CR2? The second circuit uses 1N5818 - would these also work in the first?
The driver chip datasheet says, "The high voltage of DIM pin is 3.5V." Does this mean it needs to be a minimum of 3.5V to trigger reliably, or what might be the implied min/max voltage spec?
I guess the output of a 555 PWM running on ~12VDC will be ~10.3V (read somewhere that 555 output is Vcc - 1.74...), no? Is this too much for the DIM pin, and if so, what is the best way to reduce it to an acceptable range?
My order with Allied is mostly made up with all the stuff to get going to breadboard these (or other) circuits, depending on what advice I get here. Already have a soldering iron and VOM, but other than that I'm starting from scratch in this hobby. Pretty exciting for me!
Any advice or comments will be much appreciated.