Will non PWM LEDs work?

So, I have been playing with LEDs and resistors and was skeptical of posting any thing here for fear of being burned and letting the smoke out so to speak, but here I go with flame suit on for the excitement of experimental aviation. There are so many folks here that are way smarter than me so please play nice. LED lighting is the future for sure but, it is so expensive and I give my full respect to all the lighting venders on this site, and they will probably be laughing at me but that's ok, I like to play with stuff and so do you. So, while playing and learning, I have found that driving the LEDs with PWM drivers creates noise on the com, so I was thinking how to do it without the PWM drivers, resistors, but they create heat, then there I s the complicated formula of amps, parallel series, forward voltage and so on. And by the way, there are online calculators for all this, so me being a simple guy and thinking KISS, I did my experiment, and let me stop here and give credit to the folks here that did the trustfire and LED position lighting threads. Ok this is going on to long. Ok, here is a test vid. http://youtu.be/vbrbvaIVL3Q

So, the light I am using is a Trustfire 13,000 Lm flashlight, ok, so to get 13k, you will need to run 36A and an active water cooling radiator heat sink, not feasible, but that is how they rate these, but......you can run these Cree LEDs at a fraction of advertised LM and get amazing light output. The test I did was 30 min. Temps increased and stabilized after 10 min. The lamp heat sink went to 190F. 1 ohm resistor went to 230F and the 2 ohm resistor went to 192 F. Amps into resistors went to 3.4 and out of resistors to lamp 3.0 Amps. I do not know how long any of the components tested would last in real life, but this is where I am at so far............they seam to live at these conditions......to be continued....

Ok let me try to help. First of all LEDs must always be driven by a constant current to ensure that they are not over driven. Even if they are driven by a PWM driver that driver will be a constant current source that is simply turned on and off by the PWM control logic. I personally think it would be better to add a filter to your power lines for the PWM driver than using a simple resistor but I'll try to address your question about using a resistor.

If you are using multiple LEDs with a resistor at high currents then you want to minimize the voltage drop across the resistor. At a constant current a lower voltage drop equates to a lower power dissipation. P(power) = I(current) * V(voltage). With at single LED you are stuck with the voltage across the resistor as V(battery) - V(LED) = V(resistor). However with multiple LEDs you can put a certain number in series so the equation becomes V(battery) - n*V(LED) = V(resistor). So now you can lower the voltage across the resistor and keep the same current through the LEDs. I(current) = V(resistor) / R(resistance). The limit in this approach is that the sum of the LED voltages must be lower than the battery voltage or the LED will not be properly biased. Also you still need some voltage across the resistor since there is tolerance on the resistor value and you don't want this to effect the actual LED current too much. I think its probably about 3 or 4 LEDs in series depending on the forward voltage of the LEDs you have selected. You can build multiple strings of these LED series chains if you need more light.

Depending on the design a PWM LED driver might be a boost type so that more LEDs can be placed in series therefore eliminating the need for multiple chains. The boost configuration creates a voltage higher than the battery voltage, say +25V so that the LEDs will still be forward biased.

I hope this helps.

so, match the LEDs in an array parallel series close to the available voltage but not go over the available forward voltage then drive the array at the amperage that will allow the LEDs not to burn within the heat sink design parameters. Got it. I did go to a California school so I have no faith in my ability to figure this out, I am just going to go as far as I can and then back off a little, at least this is cheaper than re doing a control surface, and more challenging.

Bret,

The resistors will work, but there's a good chance of letting the smoke out of those expensive LEDs, and besides that why turn all that juice into heat...

After trying many different drivers I ended up using these for my Nav lights and cabin lights. Zero com or rf noise.

http://www.meanwell.com/webapp/product/search.aspx?prod=LDD-L

You can parallel two of the 1500s together to give you 3 amps, and then put 3 or even 4 LEDs in series on that line.

They are less than $6 from Mouser.

Lenny

I've modified and ripped off the Pete Howell method (resistors) three times now and it works great. No noise, less to fail. As for wasted juice, you still draw less than an incandescent and still get the lifespan. Who cares if you're minimizing current, it doesn't add up to spit. Why fight it?

I'd agree with Lenny. The problem isn't with PWM; it's with poorly designed/implemented PWM (in the products that are 'noisy'). If you read between the lines, you can see that from past threads where different variations of the Trustfire lights had different drivers, and some drivers were noisy and some were not.

If you use resistors (or linear regulators, which are basically transistors used as resistors), then the power efficiency of the LED light might not be much better than conventional lights.

PWM can be implemented without causing noise problems. Odds are quite high that you're already flying with multiple PWM power supplies in the plane already, since almost everything that needs less than 12V but is connected to aircraft/car power will have a PWM regulator inside.

Charlie

Lenny Iszak said:

Bret,

The resistors will work, but there's a good chance of letting the smoke out of those expensive LEDs, and besides that why turn all that juice into heat...

After trying many different drivers I ended up using these for my Nav lights and cabin lights. Zero com or rf noise.

http://www.meanwell.com/webapp/product/search.aspx?prod=LDD-L

You can parallel two of the 1500s together to give you 3 amps, and then put 3 or even 4 LEDs in series on that line.

They are less than $6 from Mouser.

Lenny

Click to expand...

Interesting, thanks for the link, I see max output 700 milliamp, these look like they will work for a position light setup, but the trustfire setup I am building is going to draw three amps each side, I have made heat sink mounts for my one and two 25 watt resistors that I am using in parallel for the landing light. The voltage dropped from 14.2 to 12.0 at three amps, right where I want it. Only losing 400 milliamp through the resistors. LEDs are happy so far. I have a set of PAR 46 450 watt 28V landing - driving lights on my truck and this little led is going to be close as far as throwing light out.

Scroll down one page in that pdf doc, the 1500 will give you 1.5A. Two of them in parallel will get you 3A. Each of those 12 LEDs can soak up 3 amps though if you want to drive them all the way to max performance.

Anyway, not trying to derail your project. One thing I relearned many times during my build is that perfection is the enemy of getting it done. Just wanted you to be aware that LED drivers can work without noise.

With these crazy flashlights available these days I'm tempted to rip out my HIDs too. My HIDs put out somewhere around 3-4000 lumens. When they are on it looks like an airliner is coming in Imagine 13,000 lumens on each wing. Hahhh.

Brett, just a heads-up when doing work with electrons. The current is the same everywhere in the circuit. A quick & dirty way to think about it is water through a pipe. Current is the quantity of water (number of electrons). Voltage is the head pressure. As you measure water pressure along a pipe, with various constrictions (resistors) along the pipe (wire), the pressure will drop after each constriction, but the flow (current) remains the same anywhere along the pipe (wire). If 1 gallon per minute comes out of the pipe, one gallon went in, and one gallon passes every measurement point in the pipe.

Perhaps you are measuring 400 milli*volts* of pressure drop across the resistor?

Charlie

rv7charlie said:

Brett, just a heads-up when doing work with electrons. The current is the same everywhere in the circuit. A quick & dirty way to think about it is water through a pipe. Current is the quantity of water (number of electrons). Voltage is the head pressure. As you measure water pressure along a pipe, with various constrictions (resistors) along the pipe (wire), the pressure will drop after each constriction, but the flow (current) remains the same anywhere along the pipe (wire). If 1 gallon per minute comes out of the pipe, one gallon went in, and one gallon passes every measurement point in the pipe.

Perhaps you are measuring 400 milli*volts* of pressure drop across the resistor?

Charlie

Click to expand...

Not sure. What I measured was into resistors...14.2 V 3.4 Amp. Out of 1 and 2 ohm 25 watt each parallel resistors 12 Volt and 3 amp to the lamp. These are the numbers that I was using to test using my adjustable buck converter. I have spent hours at this setting making sure this lamp will survive. Weeks and weeks with the same trustfire flashlight. All this is so rediculas but intriguing about a stupid flashlight but when you fire it up and see what it is aluminating, you have to look at this thing and say, what the he??! Is that coming from that tinny $60.00 flashlight? No respons yet about the PAR 46 lamps I described in an above post, those are used on the F18s and stuff.......

Bret,

Are you working on the landing/taxi lights or the nav lights? I made a PCB for the nav lights if you are interested. Its just the LEDs, 5 of the LEDs that I picked is all that is required. I haven't designed the driver circuit yet though. I plan to design a PWM driver board that will drive both my taxi lights and the nav lights. Its on my todo list but it may be a little while.

One other comment. You can use more LEDs at a lower current to distribute the heat and therefore you don't need as good of a heat sink.

The position lights are easy, I forget his name but he did the 3 watt star LEDs with a 5 watt resistor, I give credit to him and that is what I am doing. Now then, the landing light with the trustfire, forgive me again, he did a long thread with the 11,000 Lm, trustfire and I give him credit for getting me started on this adventure. When I started I had RF with my handheld and wanted to see IF! I could do the landing light without PWM. So far, so good!

Hi Bret,

I took a look at digikey and here's a LED driver for up to 4A:

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00079085.pdf

Here's a 30$ eval board with that part stuffed, which might get you most of the way towards your goal:

http://www.digikey.com/product-detail/en/STEVAL-ILL051V2/497-14132-ND/4503637

Sorry for multiple post, the iPad is not playing well tonight, the above post is with the phone, anyway, here is my final setup, it works, no smoke, and....well....there ya go, no noise, and happy light.

Bret,

Yeah I agree about the nav lights. I could have done the star too but I was trying to create the eyebrow look you see on some cars. I ended up with the LEDs just is a straight row though so I could use the same board on both sides. I'm still trying to figure out the mounting for my taxi light.

I also found some heatsink strips on ebay. Do a search on heatsink and strip and you will find lots. Here is just one that I found. http://www.ebay.com/itm/Heat-Sink-150x16x10mm-Heat-Sink-Strip-Aluminum-150-16-10MM-Cooling-Fin-/121398200184?pt=LH_DefaultDomain_0&hash=item1c43e58778