December 12, 2010
Okay cool. In the meantime, I posted a separate comment explaining how to set the max current so that people don’t have to bother thinking about it too much unless they feel like it.
Just to save people some work, here’s how to set the max motor current:
Vref = 8 x Rs x Imax = .88 x Imax
Rref = 8 x Rs x Imax x Rtot / Vdd = 2.1982 x Imax
For your convenience, here’s a table of values based on the current board design and Vdd = 5 V, in 0.25 A increments:
Imax Vref Rref (k ohm)
0 0 0
0.25 0.22 0.55
0.5 0.44 1.1
0.75 0.66 1.65
1 0.88 2.2
1.25 1.1 2.75
1.5 1.32 3.3
1.75 1.54 3.85
2 1.76 4.4
Do we know why the Vref resistors weren’t updated to match the new Vsense resistors? What’s the point of having over half of the pot’s range have no effect on Imax? And why does the schematic say you can adjust the current limit from 0-2.4A when the chip only claims to be able to provide 2A?
Since Sparkfun has provided very little information about this light strip, I did some investigating of my own. I believe the circuit is identical to this one shown on Adafruit except that it has a 150-ohm resistor instead of a 130-ohm one for the blue and green LEDs: http://learn.adafruit.com/rgb-led-strips/schematic
On paper, giving the strip 12V and grounding the other three pins should result in a current of 20mA through each red LED and 18mA through each blue and green LED, totaling 56mA per group of 3 RGB LEDs. Since there are 20 groups of 3 RGB LEDs per meter, the current should be 1.12A per meter and 5.6A per 5-meter strip.
I tested a small strip to see if my numbers were close. I put 12V on a strip that was 6 groups long, and it drew 300mA, which comes out to 50mA per group (11% less than expected).
expected current @12V = 56 mA/group, 1.12 A/m, 5.6 A/strip
measured current @12V = 50 mA/group, 1.00 A/m, 5.0 A/strip
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