DrewE wrote:
Let's see...20W at 9V is about 2.2A. The LM7809 is usually specified for up to 1A, and has built in current limiting.
I think the regulator would survive the attempt (as in it won't be destroyed), but by shutting itself down. The 780x regulators are pretty hard to kill.
Your heat sinking needs to be able to dump about 7W of power (3ish V * 2.2A). The junction-to-case thermal resistance of the 780x is listed as 5°C / W, so the junction would be about 35°C warmer than the heat sink assuming a very good thermal bond between it and the case. This means that the heat sink must not get warmer than 90°C to keep the device within its maximum rating of 125°C. (These details may vary from manufacturer to manufacturer, in fact; there isn't just one 7809 design.)
One 7809 per panel might work, though it's on the edge of the chips specifications, and sharing a heat sink may be feasible. You still have to remove the same total amount of heat, of course, as the power dissipated by the regulators remains the same.
Another option may be to use a PWM circuit to reduce the average power consumption of the LEDs to something reasonable. You could probably do this with a 555 and a power transistor and a few minor interface bits. That would be more efficient (less heat), though would probably need some sort of filtering to prevent excess RFI.
Wrong.
You need to understand just HOW these LEDs are wired..
I have one of these same LED modules that Mex has so I am very aware of the design and not to mention I have MEASURED current draw of said modules..
These modules are known as COB or chip on board. They are an array of 1W LEDs mounted on a plate.
There is three parallel ROWs of LEDs, each row has THREE WHITE LEDS in series..
Each white LED requires 3.4V to light so each ROW requires about 10.2 V..
Each ROW only consumes about 1W since LEDs in series share the same amount of wattage like a resistor in series)
Each row is 1/3 of the current draw (and wattage) but all the LUMENS are added together..
So technically all three parallel ROWs added together will draw about 3W at 10V if you really wish to be accurate..
The manufacturers kind of skew the actual wattage drawn since they add up the wattage of all the chips on the board.. Technically it is a 9W module but they round it up to 10W for marketing purposes.. But in reality it is drawing not much more than 3W at 10V..
