Easy & Efficient LED Convertion of 6V Recharable Flashlight

[zmint]

Hello everybody,

I am considering converting my conventional 6V Rechargeable Flashlight to a bright LED flashlight.

I came across this in respect to LED flashlights :

"LED flashlights are often electronically regulated to maintain constant light output as the batteries are depleted. By contrast, the output of a non-regulated flashlight changes proportionally to the voltage of the battery, resulting in a significant decline in output as battery capacity - and thus voltage - drops as the flashlight is used"

I was able to dig a nice and reliable DIY tutorial, to achieve the same result but it involved, programing a IC (ATtiny11L as charge pump), which is far beyond my availability.

Now, How can get the same job done with easily available electronic components.

The concluding facts for the project are :

1) Converting a conventional 6V rechargeable Flashlight to a bright LED flashlight.
2) The converted version should be equally bright and As quoted above it should be efficient in relation
to battery life (longer run-time), durability of leds.
3) We have to take in consideration a 6V 4AH maintenance Sealed Lead Acid battery which may give
around 6.6 V when charged afresh.
4) Three types of LED are Available :
'a' type) Good quality Simple white LEDS. Don't know their rating but bright enough and focused at one place.
'b' type) Super Bright Wide Angle 5mm White Helmet LED's, used extensively, in Torches, Lanterns etc
for their brightness, low power, compact size and a very long life of 11+ years.
The specifications :
Forward Voltage ... 3.2V(max 3.6V)
Forward Current ... 25mA
Type ... Straw Hat, Clear Lens
Colour ... White of-course
Colour Temp ... 6000K-10000K
Technology ... GaInN
Size ... 5mm
Expected Life ... 11+ years
Emission Angle ... 170 Degree

'c' type) The label states :
Part No : 10W 4S CPM
VF : 3.1 - 3.3
IV : 14000 - 16000
WL : W2


5) Number & type(a,b,c) of LED to be used are to be considered and also the amount of heat generated if any.
6) Housing the LEDS and assembling ... Leave it to me. (Just need to know the Driver Circuit)

Thanks for reading all this.

Zmint

 

[vne147]

There are a few ways you can achieve a regulated output to drive your LEDs and ensure constant brightness. You can use a linear voltage regulator (less efficient), you can use a buck converter (more efficient), or you can use a boost converter (more efficient). There might be more but those are the ones that immediately come to mind.

Linear regulator:

Linear regulator - Wikipedia, the free encyclopedia

Buck converter:

Buck converter - Wikipedia, the free encyclopedia

Boost converter:

Boost converter - Wikipedia, the free encyclopedia

If the foward voltage of your LED is 3.2V and the supply from the battery is 6V, then there is 6 - 3.2 = 2.8 V that you need to get rid of somehow. In the case of the linear regulator, the extra energy will be dissipated as heat both in the regulator and in a current limiting resistor that you'll need to place in series with the LED. The LM317 is a pretty standard and cheap adjustable linear voltage regulator that requires a minimum of external components.

There are also tons of buck and boost conveter ICs out there that you could use. Some of them will need to be "programmed" but this is usually done by selecting different sized external resistors. It doesn't mean you sit in front of a computer and actually write a program. The exact value and placement of the external components needed to achieve the desired output can be determined by looking at the data sheet for the specific IC chosen.

A buck converter can be used like a linear regulator in the sense that it will drop the battery voltage to a constant level throughout battery life. The boost converter will raise the battery voltage to a constant level (say 7V for example) throughout battery life. In that case you will place 2 LEDs in series instead of having a bunch of single LEDs in a parallel configuration.

If I were doing this project, I wouldn't start out by selecting a regulator. I would start out by deciding the type and number or LEDs I was going to use. Then you'll know how much current will need to be supplied and you can select an appropriate voltage source.

 

[Sceadwian]

Don't forget.
SEPIC converter - Wikipedia, the free encyclopedia
It does the same thing as both a buck and boost regulator at the same time.

I'm sure the tiny11's are beyond your availability, because they've been obsoleted by Atmel and are no longer made. I've been trying to find some for a few days now and no one has any left.

If you have a ready to go circuit and all you need is a programmed tiny11 I could do that for you, I have a small tube of them left and the only reason I'm looking for more is bulk experimentation. If you have a .hex file or AVR Studio compatible .asm file I'd be happy to burn one for you and mail it if you're inside the US. It'd cost me less than a dollar, no sweat off my back =)

Provide a link to the tiny11 charge pump you're talking about so I can look at it.

 

[zmint]

Thank you for the links. I can understand that but I cannot design the regulator myself, that's why I am here. I need some kind of semantic to start with.

If I were doing this project, I wouldn't start out by selecting a regulator. I would start out by deciding the type and number or LEDs I was going to use. Then you'll know how much current will need to be supplied and you can select an appropriate voltage source.

Good advice regarding type and number of LEDS. I decided to go for 20 LEDS of type 'B' (refer original post).

I'd be happy to burn one for you and mail it if you're inside the US. It'd cost me less than a dollar, no sweat off my back =)
Provide a link to the tiny11 charge pump you're talking about so I can look at it.

So nice of you but I am far away from US (but if you still interested let me know).
Regarding tiny11 charge pump, you mush be well aware of that. It was published in one of issues of Nuts & Bolts magazine, dealing with flashlights. I can send the PDF if you like to give it a look.

Thanks once again
Zmint

(Waiting eagerly for more Updates)

 

[vne147]

Thank you for the links. I can understand that but I cannot design the regulator myself, that's why I am here. I need some kind of semantic to start with.

Give me a few days and I'll draw up a schematic for you. Question, what's the minumun voltage your battery will supply when it's near death?

 

[Mr RB]

He said a 4Ah low maintenance sealed lead acid battery. So probably as low as 5v if the torch is used in a life or death situation.

25 LEDs at 25mA, obviously in parallel at 3.3v so 0.6 amps.

I would use a 0.1v current sense resistor into a comparator driving a high efficiency buck constant output current converter (FET + schottky diode).

 

[vne147]

He said a 4Ah low maintenance sealed lead acid battery. So probably as low as 5v if the torch is used in a life or death situation.

25 LEDs at 25mA, obviously in parallel at 3.3v so 0.6 amps.

I would use a 0.1v current sense resistor into a comparator driving a high efficiency buck constant output current converter (FET + schottky diode).

He said he only wants 20 LEDs.

I was going to recommend that he use a simple switcher from National in a boost configuration. Pump up the voltage to 7V and place 10 sets of 2 LEDs in series with one current limiting resistor per pair. The boost converter was advertised to be 90% efficient so (.025A x 10)/.9 ≈ .28 A out of the battery.

So the battery should last 4/.28 ≈ 14.4 hrs.

Is there an error in my thought process or is there a reason why that would be a bad idea?

 

[vne147]

Here is the schematic. I used the National online software for component selection and value calculations. Schmitt trigger introduced me to it. It's pretty awesome.

For zmint,

Go here:

**broken link removed**

This is a link to the National Semiconductor online software for desinging switching regulator circuits. It's very useful. Click the button next to LM3224-ADJ that says "Start Design". It will bring you to a page complete with a schemtic and BOM.

 

[zmint]

He said he only wants 20 LEDs.

The number of LED can vary if you think It could increase the efficiency.

Here is the schematic.

Thanks for that.

Before proceeding further, Let me start over with a different approach. Can you give me the list of IC that can be used for this sort of work ( designing switching regulator circuits). So that I can check my options, about their availability. If think it will be better to start with something I have access to.
In the schematic why are Cap. in ohm denomination, I usually see them x uF y V.

Thanks
Zmint

 

[zmint]

In the mean time, (before the Big Boys are ready) can I have a very down to earth circuit that we may find in most common LED flashlights in which Leds are connected in Parallel or Series with a resistor or so. How many LED and in what arrangement, would be most efficient.

 

[Ubergeek63]

Grrrrrrr

Hello everybody,

I am considering converting my conventional 6V Rechargeable Flashlight to a bright LED flashlight.

I came across this in respect to LED flashlights :

"LED flashlights are often electronically regulated to maintain constant light output as the batteries are depleted. By contrast, the output of a non-regulated flashlight changes proportionally to the voltage of the battery, resulting in a significant decline in output as battery capacity - and thus voltage - drops as the flashlight is used"

it always pisses me off when fanboys suggest complicated solutions to basic problems.

series/parallel combinations are LESS efficient than power LEDs if you can handle the heat and don't mind a little more money for the convenience of simplicity for maximum light output.

Lead acid cells are unreliable for long term use (typically emergency lights need them replaced every few years) though, like the crappy LEDs, they are inexpensive initially. also the "AHr" rating is around double what you can actually expect if you are running them near their rating.

the best combination for general long life short term use is one of the latest generation power LEDs, like the Luxeon Rebel, a lens suited to your needs (tight focus for a flashlight beam out to a "bubble" for a desklamp), and Lithium Iron Phosphate batteries (LiFePo4)

As to regulators, there are a number of switching regulators that are both common and easy to use. if you opt for series current limiting resistors, remember that the power loss is directly proportional to the ratio of the drop across the resistor to the voltage supplied to the resistor/LED string.

Dan

 

[Mr RB]

I don't see a reason for a "Grrrrr".

The OP has already talked about availability of items being an issue so if he is happy to go with his easily available 25mA white LEDs then there's no harm. I've got 20000 mCd 5mm white LEDs here, and 20 of them EASILY kicks a 1W Luxeon LEDs butt.

The suggested 4Ah 6v lead gel battery is a good choice, at the 300mA into a buck regulator it will give many hours constant use and have a long life and be easy to charge. And they are cheap, probably under $15 for that battery.

Zmint- have a look at this webpage there are a few high efficiency circuits for driving white LEDs;
2-transistor Black Regulator
and more in this thread;
https://www.electro-tech-online.com/threads/high-power-led-controller.95832/

 

[zmint]

I don't see a reason for a "Grrrrr".

The OP has already talked about availability of items being an issue

No second opinion on that one.

Keeping aside all the fancy stuff and in spite of poor efficiency, if, I have to choose from, what are my most favorable options with Parallel/serial, resistor combination. How many LED and in what arrangement, would be most efficient.
Or I am asking the Sun how to produce a spark. (bothering a well learned man, to answer a stupid question)

Thanks.

 

[Ubergeek63]

I don't see a reason for a "Grrrrr".

The OP has already talked about availability of items being an issue so if he is happy to go with his easily available 25mA white LEDs then there's no harm. I've got 20000 mCd 5mm white LEDs here, and 20 of them EASILY kicks a 1W Luxeon LEDs butt.

The suggested 4Ah 6v lead gel battery is a good choice, at the 300mA into a buck regulator it will give many hours constant use and have a long life and be easy to charge. And they are cheap, probably under $15 for that battery.

Actually, no they don't. it depends on the application. the 20Cd is a tightly focused beam that is still only 1Lm@20mA or 14Lm/W.

20 of them would be 20Lm@1.4W compared to a Rebel (not a "luxeon" which is a BRAND and not an LED, ie a Luxeon Rebel) that puts out 100Lm at 1W ... how does 25% of the light for 40% more power qualify as butt kicking in your world? whatever you are smoking could you send some my way?

hours of use? hmmm.... at what you consider to be a fair amount of light the luxeon takes 100mA or less combined with proper electronics and a $4 LiFePo4 gets 13 hrs on a charge and overall a longer service life than your $15 lead acid.

**broken link removed** $4.19 1350mAHr LiFePo4
**broken link removed** $5.50 80Lm/W rebel star
**broken link removed** $6.46/3 350 mA LED drivers find the current sense resistor and up it 4x

 

[Ubergeek63]

No second opinion on that one.

Keeping aside all the fancy stuff and in spite of poor efficiency, if, I have to choose from, what are my most favorable options with Parallel/serial, resistor combination. How many LED and in what arrangement, would be most efficient.
Or I am asking the Sun how to produce a spark. (bothering a well learned man, to answer a stupid question)

Thanks.

efficiency in a series parallel configuration means NOT using resistors. the best option in that case is separate current regulators for each string of 3 LEDs. use a 12V instead of a 6V or else you can not put any in series... 13-14V at full charge to 10V at full discharge means you can not get resistors to maintain full brightness at the low end without blowing the LEDs at the high end.

2 diode drops to the base of NPNs with 25 ohms to gnd to force 20mA on each string.

Dan

 

[Ubergeek63]

I don't see a reason for a "Grrrrr".

sorry actually the reason for the grrr was the explained in my post:

it always pisses me off when fanboys suggest complicated solutions to basic problems.

after which i answered the actual question. and of course have since recommended using a 12V gel instead of 6V to get around double the life out of his mod. it gives him 120-140mA on a 2AHr battery (16-20hrs) instead of 400mA on a 4AHr battery (8-10Hrs) and requires no switchers to do it.

 

[Hero999]

I probably wouldn't worry about regulation and go with an unregulated SMPS consisting of an oscillator (555 timer, logic gates, transistors etc.) an inductor and a diode.

A simply comparator SMPS as posted by RB is also a good idea.

 

[zmint]

I probably wouldn't worry about regulation and go with an unregulated SMPS consisting of an oscillator (555 timer, logic gates, transistors etc.) an inductor and a diode.
A simply comparator SMPS as posted by RB is also a good idea.

A schematic to start with, is highly appreciable.

 

[Hero999]

For a roughly regulated circuit, see Mr RB's post.

Here's a simple unregulated circuit.

 

[vne147]

For a roughly regulated circuit, see Mr RB's post.

Here's a simple unregulated circuit.

Nice circuit Hero. It will be interesting to see what the OP does with it. I posted a very similar circuit a while ago that used a dedicated IC instead of a 555 but it wasn't "easy" or "efficient" enough apparently.