ULN2003 - PWM led driver & 3 watt LED's

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i made nano board attached uln2003, runs @ 12v transistor array.  if calculate correct, 12v @ 250 ma = 3 watts [3 watts = (3) led modules, 1 watt each wired in parallel].  if read data-sheet correctly notes uln2003 500 ma each signal output puts me 1/2 way mark comfortable safety purposes.

please correct me if wrong.  testing current purpose: hook cheap hand-held dmm, inline +12v, out dmm led+, , led- goes transistor lead.  trigger pwm arduino mimum pulse width.  extend width pulse until dmm can read 250 ma.  whatever pulse width when reading "250"  maximum limit should consider safe use.  if unpredicted event happens, arduino locks - , transistor locked high, expect 3-watt led should burn out, , cancel circuit.

by limiting width of pulse how limit current - correct? 


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by limiting width of pulse how limit current - correct?
no.  control-limit average  current affects perceived brightness.    and, dmms won't read pulsed voltage or current accurately.   dmms work dc or ac sine waves.  if have old electro-mechanical meter (with needle indicator), inertia pretty-good @ averaging.

if "led modules" rated 12v @ 250ma, should draw 250ma when apply 12v.    ...that's led strip current limiting resistors built-in. 

if have "raw" high-power leds, require constant-current source, switching-type of circuit inductor , feedback-control.  (simple pww voltage switching not work.)


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