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LED Current Limmiter

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Frosty_47

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Hello Folks,

I want to use a 2.8 Amp 10 Watt LED (Digikey part number: 897-1021-1-ND) to make a bright flashlight that retails for over $200. I need your help in designing an efficient and adjustable current limiter. Perhaps some form of PWM control to limit the current ?

Thanks
 
How are you going to heat sink it?

Does it matter if you use a proprietary IC or do you have to use common components?

This site contains a few ideas on the discrete component front.
2-transistor Black Regulator

What sort of batteries are you using?

I'd recommend four D cells and if you're going with rechargeable then you need real D cells which will have a capacity of at least 8500mAh, not AA cells with extra padding.
 
You'll need a constant current driver. Most manufacturers sell constant current ICs, which could make a 2.8A limiter with only a handful of support components. Have a look at what Maxim and Allegro have to offer.

A
 
can I assume that the P channel FET can be replaced by any FET of similar characteristics ? I have plenty of IRF9540 parts laying around, how is the current regulated ? is that diagram for a fixed current or can it be varied to suit different leds ?
 
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Yes, the resistor controls the current setting but it's not very practical to vary a resistor which is carrying several amps because it would require an old fashioned rheostat.
 
well I was thinking for permanent seting of current but making it usable for various currents, so if i do not require 2.8 amps (as my leds do not) I can make that one time change and run my leds off it too. which value is it ? I guess the 0.33 ohm one ?
 
The voltage across the resistor is equal to Vbe, roughly 0.7V. The current can be calculated using Ohm's law.

You could use a rotary switch with different resistor values.

There are other similar methods using a comparator described in the thread I linked to. These are better because a pot. can be used to adjust a reference voltage which can provide a greater range of adjustment.
 
yea I'd probably go with the comparator method, microchip did an ap note on using a pic to make a small current regulated switch mode supply for leds and had the decency to provide the initial diagram using a comparator rather than a pic, the advantage of the pic was known frequency and various programmed light levels etc.
 
yea I'd probably go with the comparator method,
That's probably the best solution.

microchip did an ap note on using a pic to make a small current regulated switch mode supply for leds and had the decency to provide the initial diagram using a comparator rather than a pic, the advantage of the pic was known frequency and various programmed light levels etc.

Except they made a mistake with the inverting and on-inverting inputs, the thread explains all.
 
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