Running ten random red LEDs (taken from the bag of 100 sent halfway around the planet) through the LED Curver Tracer produces this plot:
The two gray traces both come from LED 1 to verify that the process produces the same answer for the same LED. It does, pretty much.
Repeating that with the same LEDs in the same order, but stepping 10 mA up to 100 mA produces a similar plot:
The voltage quantization comes from the Arduino’s 5 mV ADC resolution (the readings are averaged, but there’s actually not much noise) and the current quantization comes from the step value in the measurement loop (5 mA in the first plot, 10 mA in the second). Seeing the LEDs line up mostly the same way at 80 mA in both graphs is comforting, as it suggests the measurement results aren’t completely random numbers.
Apply this bit of Bash-fu to the dataset file:
seq 1 11 > /tmp/seq.txt ; grep -E "^100" Red\ LEDs\ -\ 100\ mA.csv | cut -f 2,5 | paste /tmp/seq.txt - > "Red LED Vf at 100 mA.csv"
Produces a numbered listing of the LED current (in μA) and voltage (in mV) at a nominal 100 mA for each LED:
1 100415 2108 2 100415 2185 3 99957 2152 4 100415 2132 5 99957 2137 6 99957 2103 7 99957 2161 8 99957 2137 9 100415 2171 10 100415 2132 11 100415 2113
Putting three red LEDs in series could produce a total forward drop anywhere between 6.309 V (3*2.103) and 6.555 V (3*2.185), a difference of nigh onto a quarter volt, if you assume this group spans the entire range of voltages and the whole collection has many duplicate values and you’re remarkably unlucky while picking LEDs. For this particular set, however, summing three successive groups of three produces 6.445, 6.372, and 6.469 V, for a spread of just under 100 mV. That suggests it’s probably not worthwhile to select LEDs for forward voltage within each series group of three, although matching parallel LEDs makes a lot of sense. I have no confidence the values will remain stable over power-on hours / thermal cycling / current stress.
The capacity plot for the Wouxun KG-UV3D lithium battery packs shows that there’s not a lot of capacity left after 7.0 V, so shutting down or scaling back to lower current wouldn’t be a major loss. However, it’s not clear a fixed resistor will do a sufficient job of current limiting with 6.5 V forward voltage across the LED string:
- At 7.5 V, 100 mA calls for 10 Ω (drop 1 V at 100 mA)
- At 8.2 V, 10 Ω produces 170 mA (1.7 V across 10 Ω)
- At 7.0 V, 10 Ω produces 50 mA (0.5 V across 10 Ω)
Obviously, 170 mA is way too much, even by my lax standards.
A 100 mV variation in forward voltage between stacks, each with a 10 Ω resistor, translates into about 10 mA difference in current. This may actually call for current sensors and direct current control, although using a sensor per string, seems excessive. Low dropout regulators in current-source mode might suffice, but that still seems messy.
The test rig will run from a hard 7.5 V supply, which means I can use fixed resistors and be done with it.
The raw data behind those graphs, with LED 1 and LED 11 being the same LED:
# LED Curve Tracer # Ed Nisley - KE4ZNU - December 2012 # VCC at LED: 4877 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 4877 3707 1169 0 0 0 3707 10 10087 4877 2970 1906 2084 105 1978 2864 20 20174 4872 2907 1964 2262 211 2050 2696 30 29803 4877 2869 2007 2412 312 2099 2556 40 39891 4877 2840 2036 2546 418 2127 2421 50 49978 4872 2821 2050 2681 524 2156 2296 60 60066 4877 2806 2070 2816 630 2185 2176 70 69694 4872 2792 2079 2927 731 2195 2060 80 80240 4877 2777 2099 3071 842 2229 1935 90 89869 4872 2768 2103 3196 943 2253 1824 100 100415 4872 2763 2108 3312 1054 2257 1709 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 2 0 0 4877 3803 1073 0 0 0 3803 10 9628 4872 2960 1911 2084 101 1983 2859 20 19716 4877 2898 1978 2257 207 2050 2691 30 30262 4877 2850 2026 2421 317 2103 2532 40 39891 4877 2816 2060 2551 418 2132 2397 50 49978 4872 2787 2084 2686 524 2161 2262 60 60066 4872 2763 2108 2816 630 2185 2132 70 69694 4872 2744 2127 2927 731 2195 2012 80 79782 4872 2729 2142 3052 837 2214 1892 90 90328 4872 2700 2171 3191 948 2243 1752 100 100415 4872 2686 2185 3331 1054 2277 1632 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 3 0 0 4877 3716 1160 0 0 0 3716 10 10087 4877 2960 1916 2094 105 1988 2854 20 19716 4877 2893 1983 2257 207 2050 2686 30 30262 4877 2850 2026 2416 317 2099 2532 40 39891 4872 2821 2050 2546 418 2127 2402 50 49520 4872 2797 2075 2681 519 2161 2277 60 59607 4872 2782 2089 2802 625 2176 2156 70 70153 4877 2763 2113 2932 736 2195 2026 80 79782 4872 2749 2123 3076 837 2238 1911 90 90328 4872 2734 2137 3182 948 2233 1786 100 99957 4872 2720 2152 3321 1049 2272 1670 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 4 0 0 4877 3716 1160 0 0 0 3716 10 10087 4877 2965 1911 2079 105 1973 2859 20 19716 4872 2903 1969 2253 207 2046 2696 30 30262 4877 2859 2017 2407 317 2089 2542 40 39891 4877 2830 2046 2546 418 2127 2412 50 49520 4877 2806 2070 2686 519 2166 2286 60 60066 4872 2787 2084 2821 630 2190 2156 70 69694 4872 2773 2099 2927 731 2195 2041 80 79782 4872 2763 2108 3052 837 2214 1925 90 90328 4872 2749 2123 3196 948 2248 1800 100 100415 4872 2739 2132 3331 1054 2277 1685 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 5 0 0 4877 3697 1179 0 0 0 3697 10 10087 4877 2965 1911 2079 105 1973 2859 20 20174 4877 2898 1978 2257 211 2046 2686 30 30262 4877 2854 2022 2412 317 2094 2537 40 39891 4872 2830 2041 2551 418 2132 2412 50 49520 4872 2802 2070 2681 519 2161 2282 60 60066 4877 2787 2089 2816 630 2185 2156 70 70153 4872 2768 2103 2932 736 2195 2031 80 79782 4872 2758 2113 3071 837 2233 1920 90 89869 4872 2744 2127 3177 943 2233 1800 100 99957 4872 2734 2137 3293 1049 2243 1685 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 6 0 0 4877 3764 1112 0 0 0 3764 10 9628 4877 2980 1896 2079 101 1978 2879 20 20174 4877 2922 1954 2262 211 2050 2710 30 30262 4877 2883 1993 2412 317 2094 2566 40 39891 4872 2859 2012 2551 418 2132 2440 50 50437 4872 2835 2036 2686 529 2156 2306 60 60066 4872 2821 2050 2816 630 2185 2190 70 69694 4872 2802 2070 2941 731 2209 2070 80 79782 4872 2787 2084 3081 837 2243 1949 90 90328 4872 2773 2099 3191 948 2243 1824 100 99957 4872 2768 2103 3307 1049 2257 1718 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 7 0 0 4877 3870 1006 0 0 0 3870 10 10087 4877 2970 1906 2089 105 1983 2864 20 20174 4877 2907 1969 2262 211 2050 2696 30 30262 4872 2859 2012 2412 317 2094 2542 40 39891 4872 2830 2041 2551 418 2132 2412 50 49978 4872 2802 2070 2686 524 2161 2277 60 60066 4872 2777 2094 2821 630 2190 2147 70 69694 4872 2758 2113 2927 731 2195 2026 80 79782 4872 2744 2127 3052 837 2214 1906 90 90328 4872 2724 2147 3196 948 2248 1776 100 99957 4872 2710 2161 3302 1049 2253 1660 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 8 0 0 4877 3702 1174 0 0 0 3702 10 10087 4877 2970 1906 2084 105 1978 2864 20 20174 4872 2903 1969 2262 211 2050 2691 30 30262 4877 2859 2017 2412 317 2094 2542 40 39891 4877 2830 2046 2546 418 2127 2412 50 49978 4872 2806 2065 2676 524 2152 2282 60 59607 4872 2792 2079 2802 625 2176 2166 70 70153 4872 2777 2094 2932 736 2195 2041 80 79782 4872 2763 2108 3076 837 2238 1925 90 90328 4872 2749 2123 3196 948 2248 1800 100 99957 4872 2734 2137 3302 1049 2253 1685 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 9 0 0 4872 3721 1150 0 0 0 3721 10 9628 4877 2975 1901 2084 101 1983 2874 20 19716 4877 2898 1978 2257 207 2050 2691 30 30262 4877 2854 2022 2407 317 2089 2537 40 39891 4877 2821 2055 2546 418 2127 2402 50 49978 4872 2787 2084 2686 524 2161 2262 60 60066 4872 2763 2108 2821 630 2190 2132 70 69694 4872 2744 2127 2927 731 2195 2012 80 79782 4872 2724 2147 3052 837 2214 1887 90 90328 4872 2705 2166 3196 948 2248 1757 100 100415 4872 2700 2171 3297 1054 2243 1646 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 10 0 0 4872 3702 1169 0 0 0 3702 10 9628 4872 2980 1892 2070 101 1969 2879 20 20174 4872 2912 1959 2253 211 2041 2700 30 30262 4872 2874 1997 2412 317 2094 2556 40 39891 4877 2840 2036 2546 418 2127 2421 50 50437 4877 2821 2055 2691 529 2161 2291 60 60066 4877 2802 2075 2816 630 2185 2171 70 69694 4872 2782 2089 2927 731 2195 2050 80 79782 4872 2773 2099 3052 837 2214 1935 90 90328 4872 2753 2118 3182 948 2233 1805 100 100415 4872 2739 2132 3331 1054 2277 1685 # Insert LED, press button 1 to start... # INOM ILED VccLED VD VLED VG VS VGS VDS <--- LED 11 0 0 4877 3707 1169 0 0 0 3707 10 10087 4877 2970 1906 2084 105 1978 2864 20 20174 4877 2907 1969 2257 211 2046 2696 30 30262 4872 2869 2002 2412 317 2094 2551 40 39891 4872 2845 2026 2546 418 2127 2426 50 50437 4872 2821 2050 2686 529 2156 2291 60 60066 4872 2806 2065 2821 630 2190 2176 70 70153 4872 2792 2079 2941 736 2205 2055 80 80240 4872 2777 2094 3061 842 2219 1935 90 90328 4872 2773 2099 3187 948 2238 1824 100 100415 4872 2758 2113 3317 1054 2262 1704 # Insert LED, press button 1 to start...
The Bash / Gnuplot script that produces them:
#!/bin/sh
#-- 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 "%6.3f"
set xrange [1.8:2.2]
#set xtics 0,5
set mxtics 2
#set logscale y
#set ytics nomirror autofreq
set ylabel "Current - mA"
set format y "%4.0f"
set yrange [0:120]
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 "\t"
set label 1 "LED 1 = LED 11" at 2.100,110 right font "arialbd,18"
set arrow from 2.100,110 to 2.110,103 lt 1 lw 2 lc 0
plot \
"$1" index 0:9 using (\$5/1000):(\$2/1000):(column(-2)) with linespoints lw 2 lc variable,\
"$1" index 10 using (\$5/1000):(\$2/1000) with linespoints lw 2 lc 0
EOF
And the Arduino source code, which bears a remarkable resemblance to the original firmware:
// LED Curve Tracer
// Ed Nisley - KE4ANU - December 2012
#include <stdio.h>
//----------
// Pin assignments
const byte PIN_READ_LEDSUPPLY = 0; // AI - LED supply voltage blue
const byte PIN_READ_VDRAIN = 1; // AI - drain voltage red
const byte PIN_READ_VSOURCE = 2; // AI - source voltage orange
const byte PIN_READ_VGATE = 3; // AI - VGS after filtering violet
const byte PIN_SET_VGATE = 11; // PWM - gate voltage brown
const byte PIN_BUTTON1 = 8; // DI - button to start tests green
const byte PIN_BUTTON2 = 7; // DI - button for options yellow
const byte PIN_HEARTBEAT = 13; // DO - Arduino LED
const byte PIN_SYNC = 2; // DO - scope sync output
//----------
// Constants
const int MaxCurrent = 100; // maximum LED current - mA
const int ISTEP = 10; // LED current increment
const float Vcc = 4.930; // Arduino supply -- must be measured!
const float RSense = 10.500; // current sense resistor
const float ITolerance = 0.0005; // current setpoint tolerance
const float VGStep = 0.019; // increment/decrement VGate = 5 V / 256
const byte PWM_Settle = 5; // PWM settling time ms
#define TCCRxB 0x01 // Timer prescaler = 1:1 for 32 kHz PWM
#define MK_UL(fl,sc) ((unsigned long)((fl)*(sc)))
#define MK_U(fl,sc) ((unsigned int)((fl)*(sc)))
//----------
// Globals
float AVRef1V1; // 1.1 V bandgap reference - calculated from Vcc
float VccLED; // LED high-side supply
float VDrain; // MOSFET terminal voltages
float VSource;
float VGate;
unsigned int TestNum = 1;
long unsigned long MillisNow;
//-- Read AI channel
// averages several readings to improve noise performance
// returns value in mV assuming VCC ref voltage
#define NUM_T_SAMPLES 10
float ReadAI(byte PinNum) {
word RawAverage;
digitalWrite(PIN_SYNC,HIGH); // scope sync
RawAverage = analogRead(PinNum); // prime the averaging pump
for (int i=2; i <= NUM_T_SAMPLES; i++) {
RawAverage += (word)analogRead(PinNum);
}
digitalWrite(PIN_SYNC,LOW);
RawAverage /= NUM_T_SAMPLES;
return Vcc * (float)RawAverage / 1024.0;
}
//-- Set PWM output
void SetPWMVoltage(byte PinNum,float PWMVolt) {
byte PWM;
PWM = (byte)(PWMVolt / Vcc * 255.0);
analogWrite(PinNum,PWM);
delay(PWM_Settle);
}
//-- Set VGS to produce desired LED current
// bails out if VDS drops below a sensible value
void SetLEDCurrent(float ITarget) {
float ISense; // measured current
float VGateSet; // output voltage setpoint
float IError; // (actual - desired) current
VGate = ReadAI(PIN_READ_VGATE); // get gate voltage
VGateSet = VGate; // because input may not match output
do {
VSource = ReadAI(PIN_READ_VSOURCE);
ISense = VSource / RSense; // get LED current
// printf("\r\nITarget: %lu mA",MK_UL(ITarget,1000.0));
IError = ISense - ITarget;
// printf("\r\nISense: %d mA VGateSet: %d mV VGate %d IError %d mA",
// MK_U(ISense,1000.0),
// MK_U(VGateSet,1000.0),
// MK_U(VGate,1000.0),
// MK_U(IError,1000.0));
if (IError < -ITolerance) {
VGateSet += VGStep;
// Serial.print('+');
}
else if (IError > ITolerance) {
VGateSet -= VGStep;
// Serial.print('-');
}
VGateSet = constrain(VGateSet,0.0,Vcc);
SetPWMVoltage(PIN_SET_VGATE,VGateSet);
VDrain = ReadAI(PIN_READ_VDRAIN); // sample these for the main loop
VGate = ReadAI(PIN_READ_VGATE);
VccLED = ReadAI(PIN_READ_LEDSUPPLY);
if ((VDrain - VSource) < 0.020) { // bail if VDS gets too low
printf("# VDS=%d too low, bailing\r\n",MK_U(VDrain - VSource,1000.0));
break;
}
} while (abs(IError) > ITolerance);
// Serial.println(" Done");
}
//-- compute actual 1.1 V bandgap reference based on known VCC = AVcc (more or less)
// adapted from http://code.google.com/p/tinkerit/wiki/SecretVoltmeter
float ReadBandGap(void) {
word ADCBits;
float VBandGap;
ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1); // select 1.1 V input
delay(2); // Wait for Vref to settle
ADCSRA |= _BV(ADSC); // Convert
while (bit_is_set(ADCSRA,ADSC));
ADCBits = ADCL;
ADCBits |= ADCH<<8;
VBandGap = Vcc * (float)ADCBits / 1024.0;
return VBandGap;
}
//-- Print message, wait for a given button press
void WaitButton(int Button,char *pMsg) {
printf("# %s",pMsg);
while(HIGH == digitalRead(Button)) {
delay(100);
digitalWrite(PIN_HEARTBEAT,!digitalRead(PIN_HEARTBEAT));
}
delay(50); // wait for bounce to settle
digitalWrite(PIN_HEARTBEAT,LOW);
}
//-- Helper routine for printf()
int s_putc(char c, FILE *t) {
Serial.write(c);
}
//------------------
// Set things up
void setup() {
pinMode(PIN_HEARTBEAT,OUTPUT);
digitalWrite(PIN_HEARTBEAT,LOW); // show we arrived
pinMode(PIN_SYNC,OUTPUT);
digitalWrite(PIN_SYNC,LOW); // show we arrived
TCCR1B = TCCRxB; // set frequency for PWM 9 & 10
TCCR2B = TCCRxB; // set frequency for PWM 3 & 11
pinMode(PIN_SET_VGATE,OUTPUT);
analogWrite(PIN_SET_VGATE,0); // force gate voltage = 0
pinMode(PIN_BUTTON1,INPUT_PULLUP); // use internal pullup for buttons
pinMode(PIN_BUTTON2,INPUT_PULLUP);
Serial.begin(9600);
fdevopen(&s_putc,0); // set up serial output for printf()
printf("# LED Curve Tracer\r\n# Ed Nisley - KE4ZNU - December 2012\r\n");
VccLED = ReadAI(PIN_READ_LEDSUPPLY);
printf("# VCC at LED: %d mV\r\n",MK_U(VccLED,1000.0));
AVRef1V1 = ReadBandGap(); // compute actual bandgap reference voltage
printf("# Bandgap reference voltage: %lu mV\r\n",MK_UL(AVRef1V1,1000.0));
}
//------------------
// Run the test loop
void loop() {
Serial.println('\n'); // blank line for Gnuplot indexing
WaitButton(PIN_BUTTON1,"Insert LED, press button 1 to start...\r\n");
printf("# INOM\tILED\tVccLED\tVD\tVLED\tVG\tVS\tVGS\tVDS\t<--- LED %d\r\n",TestNum++);
digitalWrite(PIN_HEARTBEAT,LOW);
for (int ILED=0; ILED <= MaxCurrent; ILED+=ISTEP) {
SetLEDCurrent(((float)ILED)/1000.0);
printf("%d\t%lu\t%d\t%d\t%d\t%d\t%d\t%d\t%d\r\n",
ILED,
MK_UL(VSource / RSense,1.0e6),
MK_U(VccLED,1000.0),
MK_U(VDrain,1000.0),
MK_U(VccLED - VDrain,1000.0),
MK_U(VGate,1000.0),
MK_U(VSource,1000.0),
MK_U(VGate - VSource,1000),
MK_U(VDrain - VSource,1000.0)
);
}
SetPWMVoltage(PIN_SET_VGATE,0.0);
}
The Smell of Molten Projects in the Morning 
Why not use a constant current switcher made for LEDs? Or boost the battery voltage up to where all the LEDs can be in a single series string, combined with a current limiting resistor?
It may come down to that… [grin]
I want several strings, which would require a current-controlled switcher for each string. Because I only need two or three of these lights, I’m thinking I can mix-and-match some LEDs so the strings share the current equitably. Might not work, but it’ll be interesting to find out why or why not.
The great thing about a current-control driver is that you don’t need a ballast resistor. It generally does, however, require a current-sense resistor, which I’d like to avoid. Eliminating the sense resistor would allow operation with less margin between the battery voltage and the LED forward drop; ideally, the MOSFET doing the switching could also serve as the current limiter, right down to the point where the MOSFET on-resistance becomes the limiting issue.
That’s why I doodled up those notes on a Hall effect current sensor… it might come in handy some day!