LED Forward Voltages vs. Color

Running a random set of colored LEDs from the Basement Laboratory Parts Warehouse Wing through the LED Curve Tracer produced this pleasant plot:

ROYGBUIW

The white LED doesn’t match up with either the blue or the UV LED. Perhaps the blue LED uses a completely different chemistry that shoves further to the right than seems proper? I suppose I should run a handful of white, blue, and UV LEDs through the thing just to see what’s going on…

The Bash / Gnuplot source code:

#!/bin/sh
numLEDs=8
#-- overhead
export GDFONTPATH="/usr/share/fonts/truetype/"
base="${1%.*}"
echo Base name: ${base}
ofile=${base}.png
echo Input file: $1
echo Output file: ${ofile}
#-- do it
gnuplot << EOF
#set term x11
set term png font "arialbd.ttf" 18 size 950,600
set output "${ofile}"
set title "${base}"
set key noautotitles
unset mouse
set bmargin 4
set grid xtics ytics
set xlabel "Forward Voltage - V"
set format x "%4.1f"
set xrange [0.5:4.5]
#set xtics 0,5
set mxtics 2
#set logscale y
#set ytics nomirror autofreq
set ylabel "Current - mA"
set format y "%3.0f"
set yrange [0:35]
set mytics 2
#set y2label "right side variable"
#set y2tics nomirror autofreq 2
#set format y2 "%3.0f"
#set y2range [0:200]
#set y2tics 32
#set rmargin 9
set datafile separator whitespace
set label 1 "IR" at 1.32,32 center
set label 2 "R"  at 1.79,32 center
set label 3 "O"  at 2.10,32 center
set label 4 "Y"  at 2.65,32 center
set label 5 "G"  at 2.42,32 center
set label 6 "B"  at 4.05,32 center
set label 7 "UV" at 3.90,32 center
set label 8 "W"  at 3.25,32 center
#set arrow from 2.100,32 to 2.125,31 lt 1 lw 2 lc 0
plot \
 "$1" index 0 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "red" ,\
 "$1" index 1 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "orange" ,\
 "$1" index 2 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "dark-yellow" ,\
 "$1" index 3 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "green" ,\
 "$1" index 4 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "blue" ,\
 "$1" index 5 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "purple" ,\
 "$1" index 6 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "magenta" ,\
 "$1" index 7 using (\$5/1000):(\$2/1000) with linespoints pt 1 lw 2 lc rgb "dark-gray"
EOF

And the raw data file:

# LED Curve Tracer
# Ed Nisley - KE4ZNU - March 2013
# VCC at LED: 4897 mV
# Bandgap reference voltage: 1041 mV

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 1
0	0	4892		3889		1002		0	0	0	3889
5	4613		4892		3264		1627		1990		48	1942		3216
10	10148	4892		3216		1675		2092		106 1985		3109
15	15223	4892		3182		1709		2199		159 2039		3022
20	19836	4892		3148		1743		2271		208 2063		2940
25	24910	4897		3129		1767		2354		261 2092		2867
30	30446	4897		3104		1792		2431		319 2111		2785

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 2
0	0	4892		3884		1007		0	0	0	3884
5	4613		4892		3124		1767		1985		48	1937		3075
10	9687		4897		3037		1860		2111		101 2010		2935
15	14761	4897		2964		1932		2189		155 2034		2809
20	19836	4897		2906		1990		2271		208 2063		2697
25	24910	4897		2848		2048		2349		261 2087		2586
30	30446	4892		2794		2097		2431		319 2111		2475

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 3
0	0	4892		3826		1065		0	0	0	3826
5	4613		4897		2862		2034		1990		48	1942		2814
10	10148	4897		2688		2208		2097		106 1990		2581
15	15223	4897		2552		2344		2194		159 2034		2392
20	19836	4892		2436		2455		2276		208 2068		2228
25	24910	4897		2349		2547		2354		261 2092		2087
30	29985	4897		2257		2639		2426		314 2111		1942

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 4
0	0	4892		3734		1157		0	0	0	3734
5	5074		4892		2935		1956		1976		53	1922		2882
10	10148	4897		2823		2073		2102		106 1995		2717
15	15223	4892		2722		2170		2199		159 2039		2562
20	20297	4897		2649		2247		2276		213 2063		2436
25	24910	4897		2567		2329		2349		261 2087		2305
30	29985	4897		2489		2407		2426		314 2111		2174

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 5
0	0	4892		4485		406 0	0	0	4485
5	4613		4897		1724		3172		1990		48	1942		1675
10	10148	4892		1443		3448		2097		106 1990		1336
15	15223	4897		1249		3647		2199		159 2039		1089
20	19836	4892		1099		3792		2276		208 2068		891
25	24910	4897		983 3913		2354		261 2092		721
30	29985	4892		862 4030		2426		314 2111		547

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 6
0	0	4892		4165		726 0	0	0	4165
5	5074		4892		1448		3443		1985		53	1932		1395
10	10148	4897		1322		3574		2102		106 1995		1215
15	15223	4892		1220		3671		2194		159 2034		1060
20	20297	4892		1147		3744		2276		213 2063		934
25	25372	4892		1075		3816		2354		266 2087		808
30	29985	4892		1002		3889		2426		314 2111		687

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 7
0	0	4892		4247		644 0	0	0	4247
5	5074		4892		3647		1244		1981		53	1927		3594
10	10148	4892		3618		1273		2107		106 2000		3511
15	14761	4892		3603		1288		2170		155 2015		3448
20	20297	4892		3584		1307		2271		213 2058		3371
25	25372	4892		3574		1317		2354		266 2087		3308
30	29523	4892		3565		1327		2412		310 2102		3255

# Insert LED, press button 1 to start...
# INOM	ILED		VccLED	VD	VLED		VG	VS	VGS VDS <--- LED 8
0	0	4892		4945		-53 0	0	0	4945
5	5074		4892		2160		2731		1985		53	1932		2107
10	10148	4892		2034		2857		2097		106 1990		1927
15	15223	4897		1927		2969		2194		159 2034		1767
20	19836	4892		1826		3066		2271		208 2063		1617
25	25372	4897		1734		3162		2349		266 2082		1467
30	29523	4892		1666		3225		2412		310 2102		1356

# Insert LED, press button 1 to start...
About these ads

  1. #1 by Jason Doege on 2-April-2013 - 09:44

    Is it odd that these appear roughly in spectrum order or is that just a coincidence?

    • #2 by Ed on 2-April-2013 - 10:24

      The forward drop roughly depends on the oomph required to toss photons out of the junction, so it’s inversely proportional to wavelength… plus a host of other factors that account for the variations & reversals in that plot.

      Sooo, although the white LED should be close to either the blue or UV LED, it ain’t!

  2. #3 by Red County Pete on 2-April-2013 - 22:42

    Assuming the Wikipedia light-emitting-diode article is accurate (looks right, but LEDs aren’t my forte) there’s a fairly wide range for Vf in blue LEDs. There’s a bit of wiggle-room in the chemistry, and the white example you have is within the range claimed for a blue one, and the blue example is also in the range. As I recall, it was hell for the early green LEDs, because small changes in process variables tweaked the color and Vf, thus making for problems in matching. Not sure how tricky blues are, but I doubt that white LEDs care much about the color of the forcing light.

    • #4 by Ed on 3-April-2013 - 08:29

      tweaked the color and Vf

      I vaguely recall some numeric (?) LEDs with a two-axis binning scheme sorting them by color and intensity-at-current. Fortunately, I didn’t really care, but if you were building a big multi-digit display, you’d care a lot.