Illuminating more than one very bright white LED requires a choice of configuration, either series or parallel. Of course, each configuration has design tradeoffs.
A parallel connection requires a lower voltage across each LED, yet ballast resistors or current sources are needed to accomplish matched light uniformity. Different levels of bias current, and thus light coming from each LED, create a disrupting light source. However, light matching with ballast resistors or current sources decreases battery life.
A series connection inherently has great current match, but it requires a higher voltage across the LED string. Common white LEDs are biased at 3.6 V at 20 mA (max) for useful illumination. The circuit in Figure 1 shows an inexpensive inductor boost circuit that regulates the brightness of a string of seven white LEDs.
The circuit is best described in two parts: one, the boost circuit function of Q1 and Q2, and two, the control circuit of Q3 and JFET1. Assume Q1 is off. With the battery voltage slightly above Q2's VVB, a positive Q2 base current [iB = (battery voltage VBE)/RJET1] would flow. Q2 turns on, which switches inductor L1 to ground.
Energy stored within L1's magnetic field builds as L1's current rises with a positive di/dt. As this current rises, it also flows through Q2's RSAT. (SD1 and the LED string are off.) Q2's collector voltage becomes sufficiently large to turn on Q1. Q1's base voltage is connected to Q2's collector by the feed-forward network of R1 and C1. R1 also serves as Q1's base current limit.
With Q1 turning on, the previous base drive to Q2 is now shunted to ground, and Q2 turns off. The switching off of Q2 enables L1's energy to be discharged into the LED string as the magnetic field collapses.
This flyback action of L1 forward-biases the LEDs at greater than 26 V, which emits photon illumination in the form of white light. The human eye integrates the LED's flashing frequency into a constant illumination. With L1 discharged, Q1 turns back off.
The self-oscillating action repeats under normal operation unless the battery voltage falls below Q2's VBE plus the IR drop of JFET1 (about 1 V). Then, Q2 no longer turns on. L1, Q2's RSAT, and the switching characteristics of Q1 and Q2 also contribute to the period and duty cycle of oscillation.
Figure 2 shows the flyback voltage at Q2's collector for battery voltages of 6 (color A curve) and 3 V (color B curve), using a Coilcraft DO1608-104 inductor and Red Line RL5-W10015 LEDs. This flyback voltage waveform is averaged by SD1 and C2 into a dc voltage of about 23 V.
Through R4, a small dc current (less than 20 A) biases Q3 as a VBE multiplier, which adjusts the channel resistance of JFET1 and in turn regulates the battery drain current for longer battery life. The voltage at the gate of JFET1 operates about 0.9 V above the battery-stack voltage. A p-JFET operates as a depletion-mode device. Its p-type channel conducts with zero VGS.
The source is connected to the battery terminal. Designers pull up on the gate (more positive than battery voltage) to turn off the channel. The more the gate is raised above the battery, the higher its channel resistance. This RJFET1 previously was discussed with the Q1 and Q2 oscillation.
So as the battery-stack voltage drops from 6 to 3 V, the boost frequency decreases (slight change in VGS of JFET1). With it, the LED brightness decreases slightly. Ideally, the control loop would keep the LED current constant. But human sensitivity to light is on a quasi-log scale. Therefore, the small linear decline in illumination isn't very noticeable until about a 2-V battery stack.
An alternate scheme would hold the battery power (voltage times current) constant. This would keep the LED brightness constant but shorten battery life due to the internal battery-resistance losses. Also, the circuit complexity would increase drastically. As a result, this simpler circuit's LED brightness varies a little over battery life and follows the battery current.
The LED string brightness can be slightly adjusted. Designers can adjust for manufacturing variations in the transistors and LEDs by making small changes in R2. Light output (lumens) then can be set to a fixed point from one circuit to the next.
At an end-of-life battery-stack voltage, a dim LED string could be shorted to connect just one LED, which would be very bright with as little as 1 V left on the battery stack. This one-LED connection would provide last-hope emergency light with the deadest of batteries.
Alkaline battery safety dictates the use of matched batteries. The safety problem arises when the weakest battery in the stack becomes completely discharged and the other batteries have enough energy to voltage-reverse the weakest, causing it to overheat and leak milky acid.
To accomplish matching, always change all four batteries with new batteries from the same package. Four AA alkaline batteries have a capacity rating of 4 1000 mAh, which would mean that the LEDs would be illuminated for about 61 hours. Test results of prototype units continuously illuminated for a little over two days (48 hours).
Nice..... If i use li ion (3.6v) battery from mobile phone, do i have some modification? if i can't find JFET1, can u have another type to replace it?
Anonymous -August 26, 2008
Nice..... If i use li ion (3.6v) battery from mobile phone, do i have some modification? if i can't find JFET1, can u have another type to replace it?
Anonymous -August 26, 2008
Best I'm seen so far is a flashlight kit over at Nuts and Volts, will drive 9 leds, low parts count (six), and no resistors, and really cool is low drain ! I'v built one and plan to built more....
Keith -May 06, 2008
how do i modify this circuit to use a 9 volt battery? also, how do i change this so i can use differently rated LEDS?
Anonymous -February 28, 2008
how do i modify this circuit to use a 9 volt battery? also, how do i change this so i can use differently rated LEDS?
Anonymous -January 28, 2008
Any suggestions on how to modify this circuit for the use of 1A LEDs, such as Z-LED or Luxeon? What burntimes could be expected?
Anonymous -July 21, 2007
So, Elio, what is the simplest circuit to safely power a couple of white LED's (3.5V/20mA) from, say three AA (1.5V) bateries?
CageMan -April 11, 2007
This is not a good circuit !!! The circuit as is will generate large current pulses into the LEDs generating large temperature peaks consequently greatly reducing the life of the LEDs. A levelling capacitor (1 uF) should be used across the output as suggested by Stan to reduce trapezoidal cuurents typical for this circuit. Secondly, the eye does not integrate light outputs but rather reacts to average light intensity. Thirdly, SD101 is a Schottky diode, not a zener as kl seems to think. A suitable replacement is a 1N5819. Fourthly, this circuit does not have a simple way of changing the output voltage/current, for example will the circuit work without modification with only 2 LEDs ?. Fifthly, hard to get/expensive P-channel FETs should be avoided.
Elio -March 14, 2007 (Article Rating: )
any recommendation on the type of Zener diode to be used? ie SD1...
kl -February 23, 2007
My design is for 6 volts maximum, or a fully charged dry lantern battery which might produce 7 V when new. Respectfully suggest connecting to 12 volt supply is extremely risky, despite the negative feedback control loop. Try it if you must, but use a current limited supply, say maximum of 100 mA to limit any catastrophic failure and possibility of a fire.
Stan -February 21, 2007
This is an interesting circuit... but what is the maximum input voltage it will support? If I apply 12 volts, will it then power a string of 13 or 14 LEDs? Or will it just burst into flames?
Roy -October 10, 2006 (Article Rating: )
I find the appetite for LED related application articles to be insatiable. This is one of the better ones.
Anonymous -September 13, 2006 (Article Rating: )
could someone please explain this to the average American? We aren't all electrical engineers. How long do batteries last in the products that are sold to the American public? For example, I bought a lantern with 24 LED bulbs in it. The lantern is powered by four D cell batteries, how long will it stay illuminated? They are little tiny bulbs like little twinkly christmas lights. How long can I expect them to illuminate the great treatisies you all have written? I can't wait to read them during the next hurricane.
Anonymous -May 26, 2006
See www.6voltled.9ax.net
Stan -April 02, 2006
Interesting article and circuit! I shall test it out.
Anonymous -March 23, 2006 (Article Rating: )
Forgot to mention that the inductor I wound must possess considerably greater inductance than the original 100 uH because the frequency has dropped from nominally 250 kHz to 33 kHz. Maybe this accounts for improved efficiency and easier switching characteristics for the transistors?
Also the device gives much greater light output by running two parallel strings of LEDs, in my case five in each string and NOT interconnected, purely parallel. In this way small manufacturing variations are averaged out within each string (same current through each series string) eliminating the need for current equalizing resistors, though with such healthy drive voltage available, resistors of up to 1 k are of no consequence. All LEDs are therefore of equal brightness regardless.
Have also tried a half-wave voltage doubler on the output to drive the LEDs. There is a marginal increase in brightness, but it also increases the component count by 2. Good if you want just one series string of 12 or 15 LEDs though.
Stan -March 09, 2006 (Article Rating: )
This is a seriously good circuit! I wanted to replace those energy-hogging 750 mA krypton bulbs in 6 V square-battery lanterns, which produce only dim yellowish light below about 4.5 V. 130,000 mcd x 10 mm white LEDs are great for this application but the usual constant current circuits are rather inefficient, being very lucky to achieve 70%. This circuit is an epoch-making step forward. I tried the following changes and found them very highly beneficial with respect to brightness and battery life:
1. Reduced C2 from 100 nF to 1 nF disk ceramic; it still suppresses ripple to the gate and keeps loop control via the JFET while allowing Q2 to switch on faster.
2. Added a 1N914 Si diode and 100 nF reservoir capacitor between collector Q2 and the white LED string so the LEDs run primarily on DC.
3. Wound an inductor on a little old ferrite toroid from a TV monitor to enclose the magnetic field and reduce DC resistance; this was the biggest single improvement to the efficiency, greatly reducing battery current from peak of 59 mA @ 6 V, down to 22 mA - right across the range from 6 V to 1.5 V! Plus it improved brightness at the low end, i.e. from 3.5 V down to 2.0 V.
4. Reduction from seven to four LEDs gives a just-perceptible increase in brightness, with a major reduction in parts cost.
Notes: (a) Works fine with 1N914 (or 1N4148, very cheap diode) instead of more expensive Schottky barrier diode. (b) The secret to this project is definitely the inductor efficiency! Almost anything works, but to get really low battery drain use a closed toroid of some sort rather than an axial or bobbin wound type; wind it for at least 100 uH but keep resistance as low as practical.
What I still don't understand is how the LEDs can be so bright with a measured current of only 4 mA. At first I thought the peaks and eye-integration must be doing it, but there are no peaks, as seen on the CRO, just 21 V DC plus 900 mV of ripple. Can anyone explain it?
Stan -March 09, 2006 (Article Rating: )
one of the better ideas !
MyMonster -March 05, 2006 (Article Rating: )
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