1
\$\begingroup\$

I am trying to run a 0.5W white LED on a 2.4 V supply from a rechargeable battery. The LED needs a voltage drop of 3V to light up. My only constraint is to use commonly available components only.
Tried using a 555 timer followed by a Boost convertor,
but the 555 itself needs Voltage > 3V to run.

I also tried a joule thief but it is not easily available, and it reduces life of the battery and cannot be used for a long time.

Any suggestions are welcome.


Battery: 2.4V rechargable, 1000mAh.

Target run time is 3 to 4 hours (or more) per charge.

\$\endgroup\$
  • 1
    \$\begingroup\$ What are the other characteristics of your 2.4V supply? Amperage? Is it constant/well-regulated? Is it a battery? Wh, Ah, etc. And what is "a long time"? \$\endgroup\$ – Fizz Oct 17 '15 at 19:52
  • \$\begingroup\$ Its a 2.4V rechargable battery, 1000mAh. By long time, I mean for say 3-4 hours atleast, till the next recharge \$\endgroup\$ – Siddd Oct 17 '15 at 20:05
  • \$\begingroup\$ The 555 timer is not a low voltage part .It is featured in the www.badbeetles.com website .You could use discrete components or use chips that will run down to less than 1V like maxim. \$\endgroup\$ – Autistic Oct 17 '15 at 20:09
  • 1
    \$\begingroup\$ You battery can only output 2.4Wh. So even assuming a very good boost converter, of say 90% efficiency, you're looking at 2hrs running time at the most. \$\endgroup\$ – Fizz Oct 17 '15 at 20:14
  • 3
    \$\begingroup\$ Please list all the components you can use. Nobody here is a mind reader and most don't know what's available in Bombay/Mumbai. \$\endgroup\$ – Andy aka Oct 17 '15 at 21:40
1
\$\begingroup\$

You MUST specify your battery "chemistry" as "2.4V" could mean several things and results will vary. That sounds like 2 x AAA NimH cells. Are you limited to that capacity or the size chosen or ...?

I'll assume 2.2V = 2 x 1.1V = 2 x average output voltage of a NimH cell at moderate load.

If using NimH then voltage will fall to 2V at end of battery life so system must run off 2V.

At say 2.2V x 1000 mAh = 2200 mWh battery energy and say 80% converter efficiency, for 4 hours operation you can get 2200 mWh x 80% /4 hours = 450 mW.

This is so close to 1/2 Watt as to not matter. You can get 80% "easily enough" - 90% may be possible but is harder across the whole voltage range.

A very common and low cost IC is the LM319 / LM339 dual/quad comparator. These are almost certainly easily obtained where you are.

These will operate with Vcc from 2V to 36V and can be used to make oscillators and to act as switching elements in switching reqgulators. They do not have enough current drive to drive the LED directly but will drive a suitable MOSFET or bipolar transistor plus inductor in a boost converter circuit.

LM319 dual comparator data sheet

LM339 quad comparator data sheet


The circuit below, which is Fig 3 on this page could have the 555 oscillator replaced with an LM339 oscillator and a current sense resistor between LED cathode and ground would provide LED-current feedback control. Components could be adapted to suit.

enter image description here

\$\endgroup\$
  • \$\begingroup\$ I did manage to get the square wave using LM 339 in the square wave oscillator circuit here But when I try to drive a transistor (BC 547) followed by a boost convertor using this circuit, it gives me a flat output. Any suggestions? \$\endgroup\$ – Siddd Oct 19 '15 at 15:51
  • \$\begingroup\$ @Siddd Suggestion: Answer all the questions, provide a circuit of what you are doing. Doing otherwise wastes your time and ours. When you say "to drive a transistor" we have no certainty that you are doing it well, and a boost converter may have needs we cannot know of until we see your circuit. \$\endgroup\$ – Russell McMahon Oct 20 '15 at 10:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.