02GF74
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 posted on 22/1/08 at 10:33 AM |
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LED current driver - circuits?
I'm gonna make up some LED bike lamps and plan to use a pb acid that I have with my current lamps.
This gives out 5.9 V under load (2 A) and the high Power LEDS have a volt drop of 3.2 V at high current so means I cann connect to in series.
I am trying to avoid using a drop resistor as halg my battery power will go up to heating it up i.e. waste + global warming.
Is it possible to have a driver that supplies pulses to the LED to thus avoiding a drop resistor so ineffect connecting the LED to 6 V but for a very
short period of time, short enough to not burn it out?
Otherwise I'll be looking at a > 4 or > 7 V battery - RC battery?
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bigandy
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| posted on 22/1/08 at 10:52 AM |
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Does the maximum forward current across the LEDs need to be around 2amps?
If so, then you would need something like a 1.5Ohm current limiting resistor, which would be dissipating a shade under 5watts (for around the same
light output).
You can get higher efficiency drivers that work using a number of methods. Most of them are basically switching power supplies to supply the correct
forward voltage to the LEDs to be driven. Some work by stepping up a low voltage, some work by stepping down voltages. All of them will be more
efficient than using a basic current limiting resistor.
You might struggle in getting a single driver that can handle 2 Amps for a reasonable cost though. If you are using multiple LEDs, then it might be
an idea to use a driver for each LED. having said that, it has been around a year or so since I last did any design that needed something like this,
so technology has probably progressed since then.
Here is one part I have used in the past with a 200mA LED:
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1094,P10995
Cheers
Andy
Dammit! Too many decisions....
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02GF74
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| posted on 22/1/08 at 10:55 AM |
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thanks - I am googling drivers as you write ..... I would be swtiching up to 1 A per LED.
so far found LM 3551/2 that can supply up to 1 A BUT max input volts is 5.5 V. close but not quite there yet....
----- WOO HOO!!!!
The LT3474/LT3474-1
* Wide Input Range: 4V to 36V
* Up to 1A LED Current
[Edited on 22/1/08 by 02GF74]
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bigandy
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| posted on 22/1/08 at 11:16 AM |
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Have you consider switching to a slightly better battery chemistry that is more closely matched to your max forward voltage?
with a forward voltage of 3.2v maximum, then a single cell Lithium polymer/ion battery would be ideal. They have a maximum voltage of 4.2v (fully
charged) and an "empty" voltage of around 3volts. That would work with a wider range of driver ICs, and also have the benefit of having a
higher energy density (i.e. lighter weight for the same duration!). The only downside is that you would need a slightly more advanced charger, and
probably some method of encasing the cells for their protection.
Cheers
Andy
Dammit! Too many decisions....
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02GF74
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| posted on 22/1/08 at 11:34 AM |
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ouch!! just seen price of li-ion batterys ..... seeing as I have 6 V battery, 4.5 Ah that works fine I'll stick to that ... until it wears out.
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