Poormans 7135 Driver Setup
I'm looking at drivers for a 2x xml build. I know the taskleds are mentioned often, but I'm looking for a cheaper alternative.
The Poormans 7135 setup at cpf looks interesting, but someone mentioned a possible problem when using the multimode driver. I've searched a good bit and haven't been able to find any specific info. With a full 8.4v charge, it looks like the Vd across the following led @200mA would be around 3v. 5.4v across the driver is below its max - 6v.
Anyone using this setup or know of the problem?
Added - I'm not concerned with any inefficiency at low current. It will be used at med. and high.
I'm kinda confused about your configuration. You said 2x XMLs but then your calculations imply the Vf of a single LED with the rest of the voltage dropping across the regulator.
It's not the inefficiency of the driver at low current you should be concerned about, it's the inefficiency at high current that's going to be the issue. At 2 amps with 2 LEDs and an 8.4V battery pack you're going to be burning up 4 watts of power as heat in the driver. You're going to need a serious heatsink for the driver itself if you use a linear regulator.
But, describe your configuration in more detail and we can provide better feedback.
I was thinking 2x xml with 2 18650 in series - as shown in the first cpf diagram (except 2x. eliminate bottom led and bottom bat.). The driver drives 1 led. The second led is placed between the battery and the driver. This reduces the voltage seen by the driver so that it is within its input range.
This chart shows the initial battery voltage on a fresh charge will be closer to 8v under load and quickly drop to a more efficient range. The initial power across the driver would still be 1v*2.8a = 2.8w. Is that too much?
http://www.candlepowerforums.com/vb/...mah-(protected) (Post 29)
Would direct driving with a couple resistors on a two-mode switch be a better option? Know of any other cheap drivers that will work?
Last edited by rlouder; 12-10-2010 at 04:35 PM.
It makes absolutely no difference where the 2 LEDs are in a series circuit like that.
Vf of an XM-L at 2.8 amps is a maximum of 3.3volts. Keep in mind that the graph in the datasheet is for a junction temp of 25c. That's only realistic in a lab environment. You're actual junction temp will be higher and Vf drops as the junction temp goes up. So realistically, you're probably going to see a Vf of 3.1 or 3.2v. For calculations, let's use 3.2v. Things will be even worse if the Vf ends up closer to 3.1v.
8V battery - 2 x 3.2 Vf = 1.6v drop across the regulator
1.6v * 2.8a = 4.5 watts dissipated by the regulator. In my opinion that's too much.
Using a resistor has all the same efficiency drawbacks as a linear regulator except it doesn't adjust as the battery voltage changes.
He's essentially be spreading that across 8 resistors though as he would have to run 8 AMC7135 chips to get 2.8 amps.
I don't think it's any problem.
I'm pretty sure I know what the OP is trying to do, and so does MTBMcGuyver, just the OP's way of expressing it is either all wrong, he understands it wrong, or I understand him wrong.
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Thanks for the info. I was guessing 3.5v Vf per led and didn't realize the Vf would drop as temps increased.
When you posted, "It makes absolutely no difference where the 2 LEDs are...", I'm guessing you were talking about when making the calculations. Others might not realize that one of the leds will have to be placed between the driver and battery with this design.
Sorry about not expressing it better. I edited the post to try to make it clearer. Good thing I posted a link to the diagram.
You're right - It would be a 8*7135.
I'm hoping to set up something similar. Except I'll be using 2 XPG emitters with a 3-mode Shining Beam driver (7135 based). Also I'm planning to use a 6-cell NiMH battery instead of LiIon. That would give me a maximum voltage of around 6x1.4V=8.4V also. (Could go as high as 9V, but the NiMH self discharges so fast that in practice this will never occur)
Originally Posted by rlouder
My understanding of how the "Poorman" circuit works is you figure the Vf across the LED that is wired between the driver and battery, and subtract that from the Vbatt. In the worst case, the circuit will be regulated so that the current (has to be the same everywhere in a series circuit) is the lowest setting the driver offers. In my case, it's 60 mA.
The lowest Vf that Cree shows for the XP-G emitter is about 2.6V ( at something around 50mA). That would mean the driver would see about:
8.4V-2.6V = 5.8V
if I understand how the circuit works. That's still within the rated 6V max for the driver. In all other modes, less voltage will be dropped over the driver, so it will be OK.
I'm not certain I understand this circuit well enough to predict whether the 3-modes will work correctly.I'm hoping that the problem you're referring to is that it's possible for the low output mode to cause the voltage drop over the driver to exceed the maximum acceptable voltage for the 7135 chip.
If it doesn't work I'll post here. Should have the parts to try it in another week or so. Cutter shipped my emitters 9 days ago. I already have the regulators and batteries.
Well, I actually was thinking in terms of wiring as well. But on closer inspection of the driver datasheet, you're right. It does matter, from a wiring perspective, because of the separate supply pin on the chip that's limited to 6v. It doesn't matter for the calculations.
Originally Posted by rlouder
For a Lithium battery the voltage is only at 8.4 volts at the very start and only with a freshly charged battery. It drops down to 7.4 volts for the main part of the discharge. At 7.4 volts and a LED voltage of 3.2 times two for the LEDs in series gives only 1 volt across the regulator. At 2.8 amps that is 2.8 watts in the regulator and 17.9 watts in the LEDs. This results in 86% efficiency. That is actually not too bad. It is certainly in the range of efficiency of the less expensive regulators. Only a few switching regs are over 95%, but most are in the 85% to 95% range.
Now you do have to plan for the extra heat when the battery is freshly charged, but that will only last for a few minutes (10?) before the voltage drops down to the more nominal 7.4 volts.
One nice thing about the 7135's is that they auto shutdown before they fry. It makes them pretty robust.
For a six cell Nimh the situation is similar except the efficiency is slightly better since the main discharge is at about 7.2 volts. This reduction in voltage boosts the efficiency to 88%.
Let us know how it goes Walt. While searching, I think I ran across a post on cpf where someone used this with xpg's (4*7135).
If I go this route, I plan on wiring one led with a one 18650 and measure the Vf on low first. As long as the Vf is greater than 2.2v, I'm guessing it won't go poof when I add a second battery and led. Knowing the driver has built-in thermal protection will give me a little more confidence.
Got the XP-G emitters in today. I'm used to the MCEs, these are tiny! Especially on the 10mm stars.
Originally Posted by rlouder
I wired them up with the Shining Beam 7135 driver, one across the driver output, one in series with the driver. I had to use 2xLiIon batteries in series (my NiMH battery isn't wired up yet).
The unloaded Vbatt was 8.2V.
The circuit works. I got light on my first try. Not only that, but the power mode switching works too. I measured current from the battery to the driver of (approximately) 0.6A, 0.38A, and 1.3A. I put my finger on the regulator and didn't feel any heat.
I'd rather have a more efficient driver with more features like the B2Flex, but this one fits in a 1" square aluminum tube.
A thought on B2Flex fit. If you don't mind the extra depth of tubing and weight, and could use the radiative surface, my crude calculations say a 1" B2Flex will fit on the diagonal of 1" outside square tube with 1/8" walls (3/4" square inside) (1.06") with a little massaging for the contact points in the corners where the tubes are generally radiused. Since 1/16 walls are more likely, the fit won't be as tight. Now that they have a thermal pad, sinking might be a challenge, but it seems doable. YMMV
Brian, check out my build further down (1st LED lights, lots of Qs) - a potted b2flex in 1in.sq. tubing is perfectly doable.
Originally Posted by BrianMc
I'm aware that there is space on the diagonal.
If the graphic I attempted to upload works, you can see why there isn't space in my design. The switch is in the center, between the driver + optic assemblies. It extends just over 1/2 inch into the case. (It's constructed from a flashlight clicky switch and seal).
The driver is going to sit on the bottom of the switch + seal assembly.
Good to hear the driver worked.
That looks like a neat design. If I understand it, you're using just a ~3" long, 1" tube and cutting one side to make it a U-channel? Should be small and light. Are you adding any mass to the rear for cooling? Post some pics.
Sometimes I get boxed into a plan then the aha! happens. Yeah, that is tight. An on/off switch could be put in line with a small lead, but a momentary feeding input to the B2Flex would be a very tight fit. Short of adding a box section on the backside to accommodate the switch or driver, or a power bulge for the switch, you have no option but a smaller driver. The upside is a compact, and likely light weight, light.
Originally Posted by Walt Dizzy
Apologies for the thread hijack. You are the OP though!
Originally Posted by rlouder
I will post complete plans when I get the bugs worked out: I'm not satisfied that I have a good way to locate the optics accurately over the emitters yet.
The plan is to use 1" square tube with 0.75" diameter holes cut into the side, directly over the optics. The rectangle between the optic and the side of the tube is Lexan sheet, it will be glued to the inside of the tube. More parts will be added to seal the tube ends, and provide for a mount.
In the mean time, the 7135 based drivers do work correctly in the Poorman circuit. The issue with switching, I believe, is that the Vf must drop enough from Vbatt to keep the remaining voltage from exceeding the 6V max for the 7135 devices.
Using the lowest driver power mode makes the current through the circuit very low, ~50mA. At the lower current, the voltage drop across the emitter between the battery and driver (call it the parasite emitter?) must also drop.
Let's say that the Vf for the emitter is 3.5V at 1.5A, but only 2.4V at 0.05A. If the battery is supplying 8.4V under load ( an exceptional battery, however the load is rather low in this case), the voltage across the parasite emitter will only be 2.4V. Therefore the voltage across the driver has to be 6.0V, right at its limit. If the Vf for the emitters is even lower, bad stuff could happen.
I lack the experience to say this would be unlikely: I'm not sure what the Vf for various emitters are, especlially at their lowest current. Look at the current vs Vf chart for your emitter if you are in doubt.
I'm not quite sure where you get a 50ma number from. The 7135 is a 350ma current sink. The only way to get 50 ma is to turn the 7135 off by removing source power the chip. The problem here is with the 7135 off the voltage drop in the LEDs get very small and is determined by the leakage currents. You will almost for sure get too much voltage on the 7135 output pin. The only way I can see this working is where the lowest mode is where only one 7135 is on and that will be 350ma.
Here's a link to the distributor's web page:
Originally Posted by vroom9
They claim it has an output of 60 mA on low setting, I measured ~50 mA.
They don't claim it uses 7135 chips. I saw it mentioned as part of another thread. I think it was the one the OP links to. Perhaps the statement is incorrect. I'm not confident I can figure out what devices are on the driver board, my knowledge of electronics is basic.
The important point is that this driver is rated for 6V max, but by the use of the Poorman circuit, it may be used to control multiple LEDs whose series total Vfs are above the max voltage rating of the driver.
I'm only slightly interested in the internal workings of the driver circuit. My interest is in finding a way to put together a simple, powerful, compact, light weight, inexpensive bike light, powered by a NiMH battery.
The battery part is so I can loan or give away or sell a light without worrying the user will burn his house down! LiIon batteries are a fantastic technology, but using them with a cheap charger scares me. I only use a balancing charger with my LiIon batteries.
The specs for the 8*7135 I'm looking at shows 140ma for low. The If/Vf chart on cree's xm-l data sheet shows ~2.7v @ 140ma. The line stops at 100ma. Should be good.
vroom9 The driver doesn't open-circuit the 7135 chips for lower modes. It uses PWM.
Build results using the 8*7135 driver with 2 xml's.
The three mode driver works great as far as I can tell. All the modes work fine and the driver did not get warm with freshly charged batteries. It's rated at 2.8A, so it under drives a little compared to the more expensive 3A drivers. That turned out to be a plus for me because 2 xml's produce a good amount of heat for the small housing I used.
I couldn't measure the current - placing either of my dmm's in the circuit caused the amperage to drop. Vf of both led's was ~3.1v, with a difference of 0.01v between them.
Here's my build post: Another First Build - XML
Bumping an old thread.
How has the Poorman's set-up been working?
What is the run time expectations on 2x XM-L's powered by a 2x 18650 2400mah battery pack.
all my dual XML lights are running great.
less than an hour on full tilt. (2800 mA)