Cree Q5, no driver, just resistors

[MMACH 5]

I've got my Cree Q5 and am ready to replace my 15w halogen headlight.

However, I decided not to run a modulated driver with it. I'm going to use an on/off/on switch, wiring different resistors from each of the "on" positions.

It will be running from a 4.0 Ah-6 Volt NiMH battery.

Do I still have to install some type of driver/circuit board or can I simply run the juice through resistors to attain a "High" and "Low" setting?

I've been to several electrical supply stores (RS & Fry's), looking at resistors and the sales reps seem to have no idea how to convert the information I'm giving them into which resistors to purchase.
I'd like to run at a 500ma and 1000ma, as I think this will give me ample light and run time. So which resistors do I need to attain this?

Sorry for the noob-ish questions. This is my first DIY light.

Thanks.

 

[Gunner12]

I'd suggest a driver for the high mode and maybe a resistor for the low mode(or a multimode driver). A resistor setup would be cheaper though and probably a bit easier then a driver setup. But a driver will give you a stable output regardless of the the LED's Vf.

 

[snaps]

I've just built a triple Cree Q5 light using a 6 ohm & 15 ohm resistor for high & low off a 14.4 volt battery & it works fine but dims after about 2.5 hours. This program is useful to work out values http://www.jtice.com/led_pro/led_pro.htm

 

[Hack On Wheels]

The LED Pro program is great, or you can calculate it yourself.
(Voltage of the battery/ies) - (Vf of LED(s)) / (desired current) = (Resistance needed)

Things to keep in mind: The voltage of the battery/ies will be higher than the nominal rating when fresh. This means that you could end up pumping more current into the LED than you plan if you are using the light on High when the battery/ies are fresh. For example, I am planning to do a single R2 light run off 4AA NiMH (~4.8v) with a 1ohm resistor option and a 2 ohm resistor option (On-Off-On switch). At nominal voltage the light will be running at 1100mA and ~600mA on low. However, if the batteries are at their peak voltage and I use the High setting, I will potentially have 1900mA running through the LED and could fry it (if run like that too long/much). As a result, I will have to start every ride with the Low option and once that has dropped a bit I can switch it into the High setting as needed.

Nothing wrong with this setup, though a driver will require less thought to use once the light is made. Also, remember to have resistors with appropriate watt ratings (LED Pro will calculate the wattage as well)!

Good luck with the light.

 

[waiwak]

I've got my Cree Q5 and am ready to replace my 15w halogen headlight.

However, I decided not to run a modulated driver with it. I'm going to use an on/off/on switch, wiring different resistors from each of the "on" positions.

It will be running from a 4.0 Ah-6 Volt NiMH battery.

Do I still have to install some type of driver/circuit board or can I simply run the juice through resistors to attain a "High" and "Low" setting?

I've been to several electrical supply stores (RS & Fry's), looking at resistors and the sales reps seem to have no idea how to convert the information I'm giving them into which resistors to purchase.
I'd like to run at a 500ma and 1000ma, as I think this will give me ample light and run time. So which resistors do I need to attain this?

Sorry for the noob-ish questions. This is my first DIY light.

Thanks.

The math is fairly straight forward (Ohm's law), and I'll walk you through it.

You find the Vf chart or table for your LED, such as here.

For the Cree XR-E, that table lists Vf for 1000ma at 3.7 volts. You're going to use a 6 volt battery. At 1000ma, you need a 2.3 volt drop (6 volts of your battery - 3.7 volt Vf of the LED at 1000ma) across your high beam resistor.

R = V / I = 2.3 volts / 1 amp = 2.3 ohms for your high beam.

The power rating of the high beam resistor will need to be at the very least:
P = I * E = 1 amp * 2.3 volts = 2.3 watts.

For safety reasons, that 2.3 ohm resistor should be rated for at least twice that number of watts.

For the Cree XR-E, that table does not list a Vf for 500ma, but it lists 3.3 volts at 350ma, and 3.5 volts at 700ma, so we'll guestimate that it will have a Vf near 3.4 volts at 500ma. You're going to use a 6 volt battery. At 500ma, you need a 2.6 volt drop (6 volts of your battery - 3.4 volt Vf of the LED at 500ma) across your low beam resistor.

R = V / I = 2.6 volts / 0.5 amp = 5.2 ohms for your low beam.

The power rating of the low beam resistor will need to be at the very least:
P = I * E = 0.5 amp * 2.6 volts = 1.3 watts.

For safety reasons, that 5.2 ohm resistor should be rated for at least twice that number of watts.

It's likely that you won't find a 2.3 ohm 5 watt resistor, nor a 5.2 ohm 2.5 watt resistor, in which case use the next higher resistance values, and the next higher watt ratings, or connect resistors in series and/or in parallel to get the values you need.

 

[znomit]

I ran a dual Cree P3 using 8NiMH. 3.6ohm resistor and a 10ohm pot for dimming. Worked great.

You want to run the calculations for the full battery range. A 6v NiMH if running 5 cells will go from 6.5v to 5.5v during use.
So a 2.3 ohm resistor will actually give approx 1.2A current when fresh and 800mA when empty.
3 ohm will go from 1A down to 700mA.
Be conservative with resistor values(choose next size up).

Check your actual battery voltage under load with fresh batteries to select the resistor value.

 

[MMACH 5]

Lots of great information here! Thanks.

So, here are the resistors I picked up tonight:
5w 2.2 ohm
5w 2.7 ohm
(one of the above for high)

2w .47 ohm
.5w 4.7ohm
(these two in a series for low)

Is this right or am I going to cook the LED?

Just FYI, I'm building the light into a copper cap with an R/C helicopter heat sink around it.

 

[Hack On Wheels]

I ran a dual Cree P3 using 8NiMH. 3.6ohm resistor and a 10ohm pot for dimming. Worked great.

You want to run the calculations for the full battery range. A 6v NiMH if running 5 cells will go from 6.5v to 5.5v during use.
So a 2.3 ohm resistor will actually give approx 1.2A current when fresh and 800mA when empty.
3 ohm will go from 1A down to 700mA.
Be conservative with resistor values(choose next size up).

Check your actual battery voltage under load with fresh batteries to select the resistor value.

Just a moment, does that mean that 4.8v NiMH setup will peak at 5.2v? I would have thought that each cell would have a bit bigger range than just +/- 0.1v, but it works out much nicer with the smaller range!
Also, how easy is it to find a 10ohm pot? I thought they tended to be higher ohm values than that?
Now the question is whether or not 1500mA will fry an R2.... hmmm...

 

[waiwak]

Lots of great information here! Thanks.

So, here are the resistors I picked up tonight:
5w 2.2 ohm
5w 2.7 ohm
(one of the above for high)

2w .47 ohm
.5w 4.7ohm
(these two in a series for low)

Is this right or am I going to cook the LED?

Just FYI, I'm building the light into a copper cap with an R/C helicopter heat sink around it.

If that 4.7 ohm resistor is really rated for only half a watt, you'll be cooking the resistor. Not sure if the resistor will cook open or closed, but if it cooks closed, you'll cook your LED too.

I did all of the calculations by hand. Before frying a $10 LED, you might want to run the numbers through that free software, just to double check.

 

[MMACH 5]

I put a meter on the battery and got something closer to 7v, if I'm using the meter correctly, (it's a multimeter and I set it to DC 10).

Then I started taking the light head apart...

The ends of the heat sink's "wings" will need to be Dremeled down just a bit:

And the Cree's star is just 1mm too big to fit down into the copper cap, so I'll Dremel down the corners:

It will all fit into what used to be the MR11 halogen bulb space.
However, there is no way for me to fit the resistors and switch into the housing.

So... I guess I'm going to fall back on my original plan and put in a modulated driver. The switch that is built into this light head is nice and compact, so I don't think I'll have too hard a time getting the driver in there.

Thanks again for all the input. I'm going to bookmark this thread, because I'm sure I can use this information in the future.

 

[MMACH 5]

If that 4.7 ohm resistor is really rated for only half a watt, you'll be cooking the resistor. Not sure if the resistor will cook open or closed, but if it cooks closed, you'll cook your LED too.

I did all of the calculations by hand. Before frying a $10 LED, you might want to run the numbers through that free software, just to double check.

Hmmmm....

I'm just going by what is on the package.

It's probably a good thing I'll be going with a modulated driver.

 

[znomit]

Just a moment, does that mean that 4.8v NiMH setup will peak at 5.2v? I would have thought that each cell would have a bit bigger range than just +/- 0.1v, but it works out much nicer with the smaller range!
Also, how easy is it to find a 10ohm pot? I thought they tended to be higher ohm values than that?
Now the question is whether or not 1500mA will fry an R2.... hmmm...

Just looked up my reference...Brodys CPF AA battery test with graphs showing voltage and runtime for different loads. NiMH AAs hot off the charger are actually 1.4v(so a 5 cell is 7v). They drop pretty fast to 1.3 and then very slowly drain down to 1.1, after which they drain very fast. 1.2v +/- 0.1 is a good nominal voltage but you need to design for the initial burst at 1.4v.

The thing is resistive drive works well if the battery voltage is much higher than the LED voltage, but then you are wasting a lot of power...that 2.3w you are burning in a resistor is 200lumen that could be lighting up the road. Cheap drivers are a few bucks from DX and offer constant brightness and much better efficiency.

Might have found the pot in a parts bin at work, but theres a 5ohm here so Im sure you can find them locally too.

I wouldn't be torturing my precious R2s with 1.5A