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Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
One of my flashlights uses a 2.4V 0.5A incadescent bulb and the batteries are notoriously empty. I thought I could do better and came up with this simple schmitt trigger boost converter. It draws 40mA at 3.8V and the six white LEDs are bright enough to be seen. I could buy a high power emitter with multiple chips and replace the low power LEDs - so I might as well just buy a suitable IC.
Should I a) just try to improve this circuit, then solder it together b) solder it together as is c) forget about it, use two LiIon Cells in series and build a MC34063 based buck converter instead?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I have made several flashlights running white LEDs directly off a single lithium cell and it works perfectly. It is more efficient that even the best boost regulators. At the cost of slight variations in brightness. If you add a PWM circuit like this you can get any brightness you want:
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I once built a beefed-up version of Big Clive's Joule Thief with regulated output current, that could run a 1W LED off a 1.5V battery. The current regulation worked by PWM: the duty cycle was varied by using a transistor to reset the core in the manner of a mag-amp.
But, it was only about 60% efficient. It was actually more efficient to use 4.5V of batteries and a dropping resistor.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
Ok maybe I just got caught by the fascination of simple transistor circuits but since it is this appeal that made circuits like the joule thief so popular, I don't want to feel bad about just having tried.
subtract the 0.02A offset
Next time I'll perhaps try a circuit with a LM393 comparator. Thank you all for the feedback.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
I really begin to like this circuit. In fact, I like it so much that I made a layout of it
the overall cost makes it an ideal giveaway / little something (and I think it is much more fun than the annoying astable multivibrator LED thing). Current draw is approx. 45mA (for maximum brightness)
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
Hey, I just built something useful
Ok, I know this is only a small LiIon cell (the 18650 cells require irreversible modifications and with smaller cells that fit in, I can still downgrade to alkaline batteries anytime) but the booster circuit only kicks in when the diode current drops below 60-70mA, then keeps it above 50mA until the cell goes below 2.3V (I shall better get a protected cell next time). I included a 1W LED that doesn't get too warm under these operating conditions. Otherwise, the cell runs directly from the battery with a small resistor.
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transistorized LED Booster (powers 3x2 white LEDs, with schematic and layout)
