Although the boost circuit in Figure 1 comes off as rather simple (only two npn transistors), its benefits include low startup voltage and long battery life. White LEDs have gained immediate popularity because of their high brightness and multiwavelength spectrum. These characteristics can be extended to single-alkaline battery portability if the drive circuit can start from a low input voltage (<1.5 V).
White LEDs typically require 3.6 V at 20 mA (max) for proper operation. So an inductive dc-dc boost circuit solves the requirement of outputting a higher voltage than its input. Previous LED boost drivers required a VBE plus a VSAT to start up (approximately 1 V)—and many more components. Figure 1's circuit has only a few components and starts at a VBE around 0.7 V. This lower startup voltage means that more of the battery energy is used (longer battery life).
The circuit description is cyclical, meaning there's a series of events that loops back on itself. The cycle starts with the battery voltage slightly above Q2's VBE. This creates a positive Q2 base current:
iB = (battery voltage − VBE)/R2
and Q2 turns on, thus switching inductor L1 to ground. Q1 is off. 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 (D1 is 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.
As Q1 turns on, the previous base drive to Q2 is then shunted to ground and Q2 turns off. The switching off of Q2 discharges L1's energy into the LED (D1) as the magnetic field collapses. This flyback action of L1 forward-biases D1, which gives up photon illumination in the form of white light. With L1 discharged, Q1 turns back off. The self-oscillating action repeats until the battery voltage falls below Q2's VBE.
L1, Q2's RSAT, and the switching characteristics of Q1 and Q2 dominate the period and duty cycle of this oscillation. LED brightness depends directly on the average current flow through D1. D1 is on while Q2 is off and off while Q2 is on.
The oscilloscope waveforms in Figure 2 show the battery current, D1 current, and D1 voltage during the switching period of 10.8 µs (93 kHz). With a 1.0-V battery voltage, the average battery current is 41 mA, and the average LED current is 14 mA. Using a Coilcraft DO1608-104 inductor and a Nichia BSPW500BS LED, the nominal input/output power efficiency is 23%, 34%, and 72% with a battery voltage of 0.8 V, 1.0 V, and 1.5 V, respectively.
There's a limitation on the largest battery voltage that can drive this boost circuit. Because the base of Q2 connects directly to the battery by R2, a battery voltage greater than 1.5 V will cause excessive Q2 base current. A single alkaline-cell battery (<1.5 V) is best. Yet, almost any discarded alkaline battery that comes out of portable electronics (toys, PDAs, and so on) will be able to start up this boost circuit and provide some illumination. Therefore, the usable lives of so-called dead batteries are extended—albeit in another application.
I've just built that follow circuit but Q1 & Q2 using BC 548 (common part in our country), Inductor (L1) I got from my fail electornic SL LAMP Philips at garage. The Inductor have 4 lead, actually only 2 lead connected. It's works !
Anybody can help me how to modify the circuit to work with 2 AA batteries (3V input) or 4 AA batteries (6V Input).
regard
zon -November 23, 2007
Rating Only
Rating Only -June 06, 2007 (Article Rating: )
I tried this circuit and am having some problems - I want to step a 1.2 V NiCd battery up to power a white LED, but when i read the voltage where the LED will be, it is only 1.2 V. I checked and double-checked my wiring...any suggestions would be most appreciated.
jim c -June 05, 2007
@Sanjiv If you are searching till now here´s the solution. Take the circuit from http://joulethief.com/kit.php This will work with 3V input voltage. Put your LEDs in SERIES. The device will create itself the voltage needed for all the LEDs in series. You should choose a bigger inductor. Maybe 1 - 2mH. Get a ferrit bead, isolated copper wire and experiment a little. ;-)
Anonymous -April 06, 2007
dear friend if you have any information about white LED than please send me ok thanks
xing_bist -February 28, 2007
I saw lot of variations of this circuit on NET for long. But none are flecible. Can you help me how to modify the circuit to work with 2 AA batteries? I.r., 3V input, and the output current should be about 150 mA at 3.2 V to drive 6 white LEDs @ 25 mA. This will help most readers to built a portable video light!
Sanjiv -June 07, 2006
Can any one send me instructions on how to design the transformer
Rahul -January 30, 2006 (Article Rating: )
i dont know how to design the transformer can any one send me instructions on how to design the transformer--whoever helps me i am going to provide a lot of amasing circuits to that person
shubham -August 07, 2005
This circuit is simply amazing. I can't wait to try it. I've been experimenting with LEDs lately as night lights to save on electricity bills, now adays, using my rundown batteries in series, to run the LEDs at nominal voltage, helps but only lasts for a week. Now that I ran out of used batteries, I'm using new ones. This circuit will help eliminate using multiple batteries and extend the life of just using 1 battery, especially a C or D cell. I can't wait to see how long it will actually last and at what level brightness. I've been researching for the best circuit and this one seems to be the easiest and cheapest to make. Now I just have to go buy the parts. I'm not sure if these are typical Radioshack parts, but I'll have to check out some online electronic part suppliers for the cheapest prices.
John Shagena -September 29, 2004
Replace Q2 with a 2N4401, or better yet, a BC338 or better, a NTE11. It puts out much more light. The 2N3904 is a wimp, can't handle enough current to light the LED fully. Also, it's a good idea to put a 100-uF cap across the battery.
Acme Fixer -January 04, 2004
just, i have built it, works nicely
d.prabakaran -December 24, 2003
I've just built something like that with just one transistor, one resistor and one little transformer on ferite bead and one battery. It works with "dead" cells and gives bright light with one cell until cell voltage is below 0.3V.
andi -December 12, 2003
For an improved version from two years ago, visit:
It has compensation for voltage droop as the cell ages and can use almost any size inductor -- all at a small increase in the number of components.
Al Dutcher -November 12, 2003
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