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super simple and cheap pwm driver for led ?

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danjel

Member
I have a pair of high power 1W leds (in series 350ma) that are being driven with a LM317 to supply constant current. (the ouput of the lm317 is connected to the ADJ pin via resistor that sets the LED current. The leds are connected in series from ADJ to the - terminal of the battery).

I want to drive these with a 9v battery but I need to get more life out of it.

What is the cheapest way to make a fixed rate pwm driver to reduce power consumption? (i.e. I will not be dimming the leds, they will be constantly at highest brightness but I want to pwm them at an appropriate duty cycle to reduce power used).

555 chip based circuit? dedicated ic?

Anything with few components and cheap is ideal! (isn't it always? :p)
 

Boncuk

New Member
It doesn't make any difference between PWM and reduced forward current.

Using PWM with less than 100% duty cycle will dim the LEDs as well as reducing the current.
 

danjel

Member
what about persistance of vision? From my experience pwn was an effective way of maintaining the illusion of brightness while reducing current but maybe I am wrong?


I only need these to last 30 minutes max. By my calculations of a typical 9v battery mAh I would only get about 20 minutes max.
 
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danjel

Member
Why not use a battery with a bigger capacity?
Like a larger voltage or just a 9v with higher mAh rating?

The reason I was planning to use regular 9v batteries is because they are super cheap and relatively small.

I am making several hundred of these things for a performance event so I want each one to be as cheap as possible.

However, maybe I should explore battery technology a bit more. Maybe there a larger capacity and maybe even higher voltage that is not that much money if you buy in bulk.
 

crutschow

Well-Known Member
Most Helpful Member
Using a few AA or AAA alkaline batteries in series will give you a lot longer life than a 9V, which won't last long at a 350mA draw.

A Joule-Thief circuit would give you the longest battery life from any battery.
 

audioguru

Well-Known Member
Most Helpful Member
Light dimmers use PWM. An LED is at max brightness when its current is continuously at max (no PWM).

The amount of power in a little 9V battery is so low that you will see the LED dim as the voltage of the battery runs down.
Here is a 400mA load on a typical 9V alkaline battery:
 

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Boncuk

New Member
what about persistance of vision? From my experience pwn was an effective way of maintaining the illusion of brightness while reducing current but maybe I am wrong?
The persistance of vision averages since the human eye is much too slow to termine min and max brightness.
 
I am making several hundred of these things for a performance event so I want each one to be as cheap as possible.
I always find this link entertaining DealExtreme: Flashlight DIY Parts & Tools (Page 1). Lots of cheap parts. Stock on hand, delivery dates, and communcations are always buyer beware. It took almost two months on some led drivers (that were on back order) to arrive in the US from HK from a similar web site like this.

If the form factor is right?, it would seem that those small, high power flashlights might be the cheapest way to go.
 

ronsimpson

Well-Known Member
Most Helpful Member
I think you have 6 volts across the LEDs and 3 volts across the LM317 and resistor. Two watts into the LEDs and 1 watt into the regulator. Using a buck down PWM you should not loose the 1 watt in the regulator. This is not a great saving but it will help.
 

marcbarker

New Member
If the supply voltage is constant enough (i.e. lithium or NiMH) then just a dropping resistor is enough, instead of needing precise regulation. The internal resistance of the battery can be counted as part of it.
 
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audioguru

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If the supply voltage is constant enough (i.e. lithium or NiMH) then just a dropping resistor is enough, instead of needing precise regulation. The internal resistance of the battery can be counted as part of it.
But a battery voltage drops as it discharges which will dim the LEDs.
 

danjel

Member
thanks guys! lot's of great tips.

I had indeed looked at the deal xtreme stuff. Only problem with them is slow delivery so I may use them eventually but for demo purposes I need to build something asap.

That Joule thief circuit looks interesting! The description and example seems like it is just to drain batteries. Need to look further at this if I can use it for an efficient led driver with low voltage source.

I had looked at the Luxeon Buck Down module before but was turned away by the price. Is there a schematic for an equivalent circuit?

I definitely need to use AA batteries instead of 9V. The question is do I use 6 or more of them in series? or can I use a circuit like the Joule Thief or something else that can step up the voltage high enough to drive the leds?
 

audioguru

Well-Known Member
Most Helpful Member
Simple arithmatic shows that six AA alkaline cells in series make 9V when new. The circuit of two LEDs driven from an LM317 current regulator needs a minimum of 9V but the six cells will drop to only 6V and the regulator will not work.

A Joule Thief has a very low output current. Then the LED is not bright.
 

danjel

Member
yeah the lm317 drops 3v and each led is about 3-3.4V .

How much current can an AA battery safely supply?

I could put the two leds in parallel instead of series and use 5-6 AA batteries in series as the supply.
 

marcbarker

New Member
What's a 317 minimum drop-out voltage, with Iout= 350 mA?

Plus, how much would the battery terminal voltage drop to because of its internal resistance ?

Why I ask, is would there actually be any regulation overhead for a 317 to work with? If not, it's likely the only thing useful the 317 'd do is an overvoltage/overcurrent protection, rather than regulate.
 

audioguru

Well-Known Member
Most Helpful Member
How much current can an AA battery safely supply?
Why not look at the datasheet for an AA alkaline battery on the website of a battery manufacturer like Energizer?
The max amount of output current is about 10A into a dead short. The datasheet shows 1A for 1 hour when the voltage has dropped to 0.8V.

I could put the two leds in parallel instead of series and use 5-6 AA batteries in series as the supply.
LEDs do not work in parallel unless their voltages are matched. Their voltages are very different which will cause one LED to be bright and the other LED to be dim or not lighted. Then the bright one uses the current for two LEDs and burns out.
 
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