LED Circuit Help

I have looked at this forum many times and have finally registered!!! I DIY hunting lights for personal use and I am really 'green' w/ LEDs, but I am learning.

I want to build a light with a high/low option and need help w/ several things (forgive my crude drawing, I am not an electrician or engineer by trade, experience, or education...LOL)

LiON 11.1v 2.6AHà on/offà10v2.1AH constant current Driverà SPDT(high 2.1AH) à3.2v LED
Same SPDT (low) àΩ??? (1.0AH)àSame 3.2v LED


I want the 'low' side of the SPDT switch to power only 1.0AH to the LED. Will this work? What resistor do need? Is there a better or simpler way to do it?


Thanks in advance,

Matt

Matt there's no drawing showing.
Also do you really mean AH this means a number of amps pullied over an hour, it's a measure of capacity rather than flow. Straight amps will give you an idea of the current flow to the Led,. Have a look at the numbers and get the drawing up and we can help.
P

LiON 11.1v 2.6AHà on/offà10v 2100ma constant current Driverà SPDT(high 2100ma) à3.2v LED
..........................................................................................................Same SPDT (low) àΩ??? (1000ma)àSame 3.2v LED

Sorry - this was my crude drawing. I want to use a SPDT toggle switch where the common pole is 2100ma coming from the driver and one of the out going poles is 2100ma to the LED and the other out going pole goes through a resistor to provide 1000ma to the LED. Basically make an 'on/on' switch (high/low).

Will this work or is there a better way? What resistor do I need to reduce 2100 to 1000? Will the resistor still allow for 3.2v to power the LED?

Again, I apologize for my 'ignorance' and I am grateful for any help.

Thanks,

Matt

This'll help, still can't see the damn drawing....
http://www.ohmslawcalculator.com/led-resistor-calculator
P

Peter, the text IS the drawing.

Matt, you don't want to use a resistor. If you do, you'll just be wasting a phenomenal amount of power.

What you need is an LED driver that can handle the max battery voltage at full charge, which is 12.6V. You want a buck-type switching driver, so you can drop the voltage without wasting 2/3 of your battery power.

Unfortunately, I have never run across one of those that can be controlled the way you want, at least straight out of the box. If you are handy with a soldering iron, I could show you how to modify one to do what you want.

If you could run it on a 3.7V battery, I've got a few suggestions that would work better and be easier to do. If you took the 3 cells in your 11.1V battery and connected them in parallel instead of in series, you'd have a 3.7V, 7.8AH battery.

Peter - sorry I dont think my 'line diagram' is showing up right if you are looking on a mobile device - thanks for the link.

DIW - here is the components I was considering: LED Cree high density (10watt, 3.2v, max current 3000) and a buck driver (10-30v constant current output 2100). That is why I was looking at the 11.1v battery pack. Like I said, I am LED 'green' so if you have any input on how/what I am trying to do, that would be great especially if I could drive that 10w Cree w/ a lower voltage battery pack. Also, I looking for around a 3hr burn time.

Thanks,

Matt

Peter - I don't think my 'line diagram' is showing up right if you are using a mobile device - sorry, but thanks for the link.

DIW: any help or suggestions would be greatly appreciated. Here is the components I was looking at: cree HD LED (3.2v, 10w, 3000 max.) and a buck driver (10-30v input and 2100 output constant current). If I could use a lower voltage battery pack that would be great. Also, I would like around a 3hr burn time.

Thanks,

Matt

If the LED is really only 3V at 2.1A (tell us exactly which LED you are talking about and we can verify that), then any of the AMC7135 based drivers can run the LED quite well on a single LiIon cell. Each AMC7135 chip is 350 mA. Drivers are available with anywhere from 1 to 8 chips. Six chips would give you 2100 mA, while three would give you 1050 mA. You could hack a board to switch three chips on and off to give you the brightness control you want, or get two three-chip boards and use the switch to turn one of them on and off. Chips can be added or removed as necessary.

To get a 3 hour burn, you'd need 2100mA * 3 Hrs = 6300 mAH. You'd get at from three 18650 cells in parallel, with some to spare if you get good cells.

Cree XP-L one up HD star

The data sheet for that LED shows it has a typical Vf of 3.15V at 2.1A. The AMC7135 has a dropout voltage around 0.15V. Add those together and you 3.3V. This is the battery voltage needed to get the LED to run at 2.1A.

Most LiIon battery discharge curves show the cells holding above 3.3V until they are nearly completely discharged.

The discharge curve is the voltage of the cell vs. time as it is being discharged. It depends a lot on how much current you are drawing. This is often measured in factors of "C", with C being the current that would discharge the cell in one hour. So for a 2.6AH cell, C is 2.6A. Often you'll see multiple curves on a single graph, representing different c-rates. Since you are looking for 3 hour burn time, you want a discharge rate of C/3 or a little less. This is good, as the cell voltage is higher for longer at lower rates.

What all this means is that a Cree XP-L, a '7135-based driver, and a "3.7V" battery pack are a pretty good combination. You'd get respectable efficiency, constant light output until the battery is nearly discharged, followed by a period of diminishing light as the battery approaches full discharge. Since diminishing light means diminishing current, this period can last a remarkably long time, if you can live with seriously diminished light.

Typically.

That word is the amateur's friend and the professional's nightmare. It means you can build a light, and it will probably work about like you predicted. Maybe a little better, maybe a little worse. That's fine if you are building one, and if it disappoints you it's no big deal. But there's some chance it will perform far worse than expected. As a hobbyist, I can accept this (one of my lights was pretty disappointing both mechanically and electrically; I set it aside and built another that I'm very happy with), but as a professional this is not acceptable. When I deliver a product to my customer, it HAS to work. If it doesn't, not only do I HAVE to fix it (usually for free), but it compromises my reputation as well. Professionally, I pay a lot more attention to worst case values than typical ones. As a hobbyist, it's the opposite.

Just wanted to warn you. Caveat Emptor.

DIW - thank you very much. I now have a much better understanding of basic LED circuity and I know and understand a little more about LiON what they are talking about with discharge rates and 'C'. I am going to build my light w/ your input and see how it works. I will keep you posted - again thanks for the help.

Matt