LED Curve Tracer: First Light!

Measuring a handful of random LEDs from the heap produced a dataset that boiled down into a set of curves:

LED Curve Tracer - First Light

LED Curve Tracer – First Light

The Y axis (current) is logarithmic, so the traces should be straight lines. They’re loosely color-coded by LED color (black trace = white LED) and that blue trace looks mildly suspicious even to me. You’d want a better graphing program than OpenOffice Calc, but it’s OK for a quick look.

Note that the rated current for 5 mm LEDs is generally 20 mA, so 75 mA really puts the screws to them. That notwithstanding, the curve tracer machinery seems to work well enough.

The numeric values in the dataset have way more precision than the measurements have either accuracy or resolution. If we could put floats in those printf() format strings, then I’d be more inclined to prettify the results.

INOM is the nominal current in mA (and also the loop counter) and ILED is the measured LED current in μA. All the voltages are in mV, with a resolution of 5 V/1024 steps = 5 mV.

The dataset behind the curves, slightly massaged to weed out some, ah, bogosity that won’t appear with that firmware:

# LED Curve Tracer
# Ed Nisley - KE4ZNU - July 2012
# VCC at LED: 4867 mV
# Bandgap reference voltage: 1039 mV

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 1
0	0	4867	3745	1121	0	0	0	3745
5	4585	4867	3042	1824	1925	48	1877	2994
10	10087	4867	2980	1887	2070	105	1964	2874
15	14672	4867	2951	1916	2142	154	1988	2797
20	20174	4867	2917	1949	2243	211	2031	2705
25	25218	4867	2898	1969	2320	264	2055	2633
30	30262	4867	2879	1988	2392	317	2075	2561
35	34847	4867	2869	1997	2450	365	2084	2503
40	39891	4867	2854	2012	2527	418	2108	2436
45	45393	4867	2840	2026	2604	476	2127	2363
50	49978	4867	2835	2031	2667	524	2142	2310
55	54563	4867	2821	2046	2729	572	2156	2248
60	60066	4867	2816	2050	2806	630	2176	2185
65	65109	4867	2806	2060	2859	683	2176	2123
70	70153	4867	2797	2070	2912	736	2176	2060
75	75197	4867	2792	2075	2989	789	2200	2002

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 2
0	0	4867	3914	953	0	0	0	3914
5	5043	4867	1993	2874	1949	52	1896	1940
10	9628	4867	1863	3004	2070	101	1969	1762
15	14672	4867	1795	3071	2142	154	1988	1641
20	20174	4867	1713	3153	2243	211	2031	1502
25	25218	4867	1646	3220	2315	264	2050	1381
30	30262	4867	1583	3283	2397	317	2079	1266
35	34847	4867	1535	3331	2450	365	2084	1169
40	39891	4867	1482	3384	2532	418	2113	1063
45	44934	4867	1425	3442	2609	471	2137	953
50	50437	4867	1386	3480	2667	529	2137	856
55	54563	4867	1343	3524	2720	572	2147	770
60	60066	4867	1285	3581	2797	630	2166	654
65	64651	4867	1256	3610	2859	678	2180	577
70	69694	4867	1218	3649	2912	731	2180	486
75	74738	4867	1165	3702	2999	784	2214	380

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 3
0	0	4867	3577	1290	0	0	0	3577
5	4585	4867	1997	2869	1945	48	1896	1949
10	10087	4867	1877	2989	2070	105	1964	1771
15	15131	4867	1795	3071	2166	158	2007	1636
20	20174	4867	1738	3129	2238	211	2026	1526
25	25218	4867	1680	3187	2320	264	2055	1415
30	30262	4867	1617	3249	2397	317	2079	1299
35	34847	4867	1574	3293	2450	365	2084	1208
40	39891	4867	1521	3346	2527	418	2108	1102
45	45393	4867	1473	3394	2604	476	2127	996
50	49978	4867	1434	3432	2667	524	2142	909
55	54563	4867	1391	3476	2720	572	2147	818
60	60066	4867	1343	3524	2802	630	2171	712
65	64651	4867	1314	3553	2854	678	2176	635
70	69694	4867	1280	3586	2907	731	2176	548
75	74738	4867	1246	3620	2970	784	2185	462

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 4
0	0	4867	3736	1131	0	0	0	3736
5	5043	4867	1439	3427	1949	52	1896	1386
10	10087	4867	1323	3543	2070	105	1964	1218
15	15131	4867	1227	3639	2166	158	2007	1068
20	19716	4867	1150	3716	2243	207	2036	943
25	25218	4867	1073	3793	2315	264	2050	808
30	30262	4867	1006	3861	2402	317	2084	688
35	34847	4867	953	3914	2450	365	2084	587
40	39891	4867	881	3986	2527	418	2108	462
45	45393	4867	823	4044	2604	476	2127	346
50	50437	4867	760	4106	2676	529	2147	231
55	55022	4867	707	4159	2777	577	2200	129
60	59607	4867	659	4207	3134	625	2508	33

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 5
0	0	4867	3702	1165	0	0	0	3702
5	4585	4867	1713	3153	1959	48	1911	1665
10	10087	4867	1449	3418	2070	105	1964	1343
15	14672	4867	1304	3562	2147	154	1993	1150
20	20174	4867	1116	3750	2243	211	2031	905
25	25218	4867	982	3885	2320	264	2055	717
30	30262	4867	885	3981	2397	317	2079	568
35	35305	4867	770	4097	2469	370	2099	399
40	39891	4867	712	4154	2527	418	2108	293
45	45393	4867	621	4246	2647	476	2171	144
50	49520	4867	582	4284	2797	519	2277	62

# Insert LED, press button 1 to start...
# INOM	ILED	VccLED	VD	VLED	VG	VS	VGS	VDS	<--- LED 6
0	0	4867	3687	1179	0	0	0	3687
5	4585	4867	3004	1863	1954	48	1906	2956
10	9628	4867	2965	1901	2041	101	1940	2864
15	14672	4867	2932	1935	2137	154	1983	2777
20	20174	4867	2903	1964	2243	211	2031	2691
25	25218	4867	2888	1978	2315	264	2050	2623
30	30262	4867	2869	1997	2397	317	2079	2551
35	34847	4867	2854	2012	2450	365	2084	2489
40	39891	4867	2840	2026	2527	418	2108	2421
45	45393	4867	2826	2041	2604	476	2127	2349
50	49978	4867	2816	2050	2662	524	2137	2291
55	54563	4867	2806	2060	2720	572	2147	2233
60	60066	4867	2797	2070	2802	630	2171	2166
65	64651	4867	2787	2079	2859	678	2180	2108
70	69694	4867	2777	2089	2917	731	2185	2046
75	74738	4867	2773	2094	2975	784	2190	1988

  1. #1 by ewf on 2012-07-26 - 10:20

    In my toy-maker days, my department had to test tens of thousands of leds to select the approved vendors. We had an AIM-65 program to step the currents but used a complicated hardware fixture (flat-black and over a foot long, cone shaped) to manually take lumens values. The zif soket for the led under test did not precisely align the led so, we were never sure that the results were meaningful. Would have been nice to automate the whole process, but, never enough time. The goal was to find the best match to the drain current of the micro-controllers so that no limit resistor was necessary.
    Battery life was sometimes short and the limit resistors reappeared after an unfavorable review in Consumer Reports “Picks and Pans” on battery life of toys.

    • #2 by Ed on 2012-07-26 - 10:42

      so that no limit resistor was necessary

      Eliminating one cent from the recurring cost of a million toys pays for a lot of engineering time! [grin]

      Worked with a guy who designed TVs, back in the day, and he said they spent days in brutal meetings devoted to removing 25 cents from the chassis: if a resistor wasn’t doing two things at once, they merged it with a neighbor! Then, a month later, they’d do it all over again…

      • #3 by ewf on 2012-07-26 - 13:52

        The owners had “design” meeting without engineers present, just a technician, and would clip out every 2 lead part to see if they wasn’t really needed or could be replaced with a jumper. 100 million was a typical single component order.

        • #4 by Ed on 2012-07-26 - 16:01

          100 million was a typical single component order.

          Congratulations! You just got a Larval Engineer’s attention…

  2. #5 by Red County Pete on 2012-07-26 - 12:24

    I used to work for HP (later Agilent) in a semiconductor division (since shut down and sold off). One of our ICs was a resistor that we sold to a sister division for R-leds for 5 or 12V applications. More expensive than a simple LED, but they were just cost-effective enough to let us sell a bunch. No idea if these are still available; Lumileds got the LED side of the business in 2000 or so.

    • #6 by Ed on 2012-07-26 - 13:00

      just cost-effective enough to let us sell a bunch

      And it made just enough sense for the customers, too: both sides win!

      Hindsight says there might have been better ways to go about it, but (as with so many things) it made perfect sense at the time…

      • #7 by Red County Pete on 2012-07-27 - 12:03

        The approach was set by the state of the art (such as it was) for the mid ’70s when the products came out. Silicon wafers were 2″ in that fab at the time (we were late adopters of IC tech, and waay behind the curve) but the LED fabrication was even more primitive. More efficient tech (like National’s 1.5V LED blinker) was too expensive, and small processors were almost non-existent or hideously expensive. Not sure if you could integrate a control on an LED nowadays, or if it would be worth it

        • #8 by Ed on 2012-07-27 - 21:27

          National’s 1.5V LED blinker

          The LM3909 sits right up there with the LM741, NE602 and the NE555: as close to perfection as you’ll ever see!

          Well, apart from the original 555’s penchant for crowbarring the supply and the 741’s popcorn noise, that is…

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