Switch mode LED torch

I modify a cheap chinese rechargeable torch with a switch mode LED drive circuit.

 The advantage over the original circuit is that it has constant brightness even when the battery voltage drops as it discharges, down up to the very end when its voltage goes below 2.7 volts.

The torch had seven LEDs in an aluminized plastic reflector, and a three position switch lighting zero, three or seven LEDs on successive positions. The LEDs were all connected to the 4 volt sealed lead acid rechargeable battery with independent resistors.

When the battery died, I replaced it with a 3.6 volt nicad pack from a defunct mobile phone I had lying around, and it worked, but the brightness had dropped. Replacing all resistors to raise the current through the LEDs (all seven of them) was thought to be too much of a chore, so I went ahead and wired them all in series. Now I had seven LEDs in series, requiring about twentyfour volts to light them, at around twenty milliamps.

Then I used an integrated circuit driver to raise my battery voltage to the value required.
 

Step 1 The schematic

This is the circuit I intend to use: The texas instruments TPS61041 LED driver, wired up according to the circuit in the datasheet, reproduced below.

TI make the circuit, so they are the people to tell me how to use their chip. It looks simple: Two capacitors to store energy and decouple the supply voltage and output voltage, a rectifier, an inductor, and finally a resistor to set the LED current.

According to the datasheet, as long as the other components are correctly chosen and wired up right, the LED current depends only upon the value of  the resistor Rs in the figure below. It doesn't matter how many LEDs there are, and what voltage the battery has.

 This particular integrated circuit can drive LEDs in series up to 28 volts, and the input voltage has to be in the range 2.7 volts to 6 volts. Since a white LED requires around 3.5 volts, upto eight of them in series may be used. Since the minimum voltage has to be 2.7 volts, at least two NiCd cells in series would be required to drive it - or one Li-ion.

Comments

[qwerty156]

wow, amazing job, im going to build this myself now :)

[Thereyouhaveit]

You seem to like TI alot ! Tell me more .

[bhvm]

Was there any Improvement in the brightness mate? How much lumens does this give? This may be of your interest- http://www.instructables.com/id/Make-yourself-Powerful-LED-Off-road-lights-headli/

[neelandan]

This mod was to use a different type of batteries, and to have an acceptable performance while doing so. Three ni-cads, while fully charged, measure about 4.2 volts. The voltage quickly falls to around 3.6 volts on load, where it stays for most of the discharge. With the original circuit, individual resistors set the LED current. They will have to be dimensioned to be within the safe current for the LEDs at the maximum battery voltage. Therefore, during most of the discharge of the battery, the LEDs will be running at less than maximum current and therefore the light output will be less than the maximum possible. This circuit allows me to set the LED current at near maximum and this current is maintained until the battery is almost completely discharged.

[bhvm]

Excellent thinking, Being an LED freak myself, Its well known that drving LEDs at slightly below their maximum rated power will work wonders for long life, at negligible drop in performance. .. Have you worked with Seoul p4 or Luxeon Stars neel?

[newzy]

hello neel, thanx 4 the ckt.. what is the value of zener diode used and how to calculate the output voltage and current of this ckt theoretically, ???thanx

[neelandan]

If you go to the website of Texas Instruments, ti.com they have the datasheet for this chip as well as useful info on driving LEDs and more theory than you would ever wish to wade through. Happy hunting!

[.Unknown.]

lol....replacing all the resistors is a chore, so you put them in series.

[neelandan]

Three nicads in series will have voltage in between 3.6 volts and 4 volts or so depending on the state of their charge, so calculating the new values to put in was something of a riddle. Much easier to connect them in series, so that their currents will all be equal, and then use active electronics to hold that current near the maximum rated current for the LEDs.

[yoyology]

I'm always impressed by your builds.  They seem so straightforward, and then we see them in context and realize how tiny and intricate they are.  Great instructable. Thank you!