5 watt LEDs for fpv?

[quinn]

Recently I've taken an interest in 250 fpv racing and I've decided to try some night flying. I ordered 2 5 watt LEDs to mount on the front pointing the same direction as the camera. Do you guys think this will give me some decent lighting? Usually I'm flying within 10 feet of the ground and zipping around between trees. I wanted to ask before I ordered them, but the site has been down. Also I wanted to ask about powering them. The specs say the forward voltage is 7v. That means I want to run them at that voltage right? Or am I misunderstanding? Do I need resistors to limit current or anything? I'm planning on using a spare 10 amp castle bec I have and just set it to 7 volts if that will work.

[gstoneberg]

I would purchase an LED driver circuit to run your power LEDs. Something like this should work and run off the 3s lipo. It's only the size of a postage stamp. It will pull nearly an amp off your pack though (which makes sense, 10w/12v = .833a + loss).
http://www.ebay.com/itm/10-Watt-12V-...item4d1920237f

10watts of white LED with the right lenses is a powerful floodlight. I made a 9 watt red/green night light to hog hunt with and it was painful to look at from 50 yards. Up close it might hurt your eyes. I'd try one first, you might find it more than sufficient, but I've never done FPV so I might be all wet.

EDIT: I remembered watching a YT video where someone pulled the IR filter out of a mobius keychain cam to do night fpv. I went and looked and here it is (not sure what light source they used):

Night Ops FPV - Mobius vs FPV Cam (1 min 58 sec)

Good luck.

[quinn]

Awesome thanks! But im not sure that driver has the correct balance of voltage and amperage. It has the correct wattage, but my LEDs are labeled as a forward voltage of 7v at 800 ma each so 1.6 amps total at 7v. I guess I'll just search for something similar at the correct voltage.

[gstoneberg]

Your LEDs will be in series so the voltage is additive but not the current. I do find the 7v forward voltage a little concerning since this driver is a 12v output. That's your concern and not the current. In my case I ran 3, 3 watt LEDs off 1 driver. They got a little warm so you may need a heat sink.

[quinn]

OK so in series they would be running around 6v at slightly higher than their rated amperage? That might be alright. I'm not really sure how the driver works. Does it basically force the led to take the extra amps at the lower voltage? Or will the LEDs just draw less amps and not put out the full 10 watts?

Also what would happen if I just hooked the leds in parallel to a bec putting out 7 volts without any current restriction? Would they pull too many/not enough amps? I'm just used to supplying electronics with the voltage they need (escs, servos, cameras, etc) and they just draw the amps they need no matter what the power source is capable of putting out. Are LEDs just different?

[gstoneberg]

Crud, I had typed out a long reply and hit the wrong key and lost it.

I'm not sure how that driver is designed, but my guess would be that it will limit the current to its rated max. I think it's reasonable that one or both LEDs might not light up if they're both on the same driver. I have to say that a 7v forward voltage is very odd. I've only heard of forward voltages in the 2,2v and 3.5v ranges. It makes me think that the LED you purchased has two 3.5v forward voltage LEDs in the same package to get the 5watt output. That limits you from using a BEC at 7v to run your LEDs with a simple parallel circuit because you must have a current limiting resistor. In my experience, running an LED without a limiting resistor makes it a flashbulb (I'm dating myself). If you're not old enough to remember flashbults they made a very bright short duration light which burned them up. OTOH, if you ran your light circuit off the 12v 3s pack voltage you could use a parallel circuit where each leg had a 4watt 6.8ohm resistor in it. In that case, the currents would add and you'd be pulling 1.5amps off the 3s pack. Google "led resistor calculator" to check how to build circuits, I used ledcalc.com.

You might find it a lot easier to purchase a different power LED with a lower forward voltage to use with your BEC. But honestly, for as cheap as those drivers are, I'd buy one for each of your 5watt LEDs and be done with it.

[quinn]

Well here's the ones I got:
https://www.superbrightleds.com/more...Specifications

They do appear to be 7v each at 800 ma. If I could run them off my 3s pack with resistors, that would be the best option. When you said run them in parralel off the 3s, did you mean series so it halfves the voltage for each one? Or do the resistors limit both current AND voltage? I'll check out the led calculator and see if I can figure it out. Its not a big deal to order whatever I need to make this work, I would just rather not wait for something from China.

[quinn]

OK, I looked at a calculator and got the same results you did, a 4.4 watt 6.8 ohm on each positive leg running them in parralel. It also told me that my forward voltage was suspiciously high. I guess they are just odd LEDs. I didn't really find many options for 5watt LEDs. Basically I just wanted the most light possible without significantly lowering my flight time. My quad uses an average of 200 watts, so 10 watts is only 5% of that and should only reduce my flight time by 15 seconds. I'll order a couple of those resistors.

Funny you mentioned the Möbius because I just ordered one a few days ago, however I won't be using it as the fpv cam because of the latency. It will be a secondary camera for HD recording. I'm using a pz0420 for my video feed to the goggles. I'm thinking 10 watts of light should do well. I'm sure it will also turn some heads when people see a blinding ball of light floating through the dark sky. Should be a lot of fun. Wanted to setup my 7hv for night, but its just too expensive for the blades, and also to loud to be flying at night.

[quinn]

OK, I'm having a hard time finding the right resistor. Will a 5w 6.8ohm be OK even though the calculator is telling me 4.4 watts? 5 watts makes more sense anyway

Never mind lol, I figured it out. Just needs to be at least 4.4 watt capable so 5 watt is fine. Ordered the resistors, also ordered a couple 3 watt LEDs and resistors for those in case the 5 watters are just too ridiculous.

BTW, thanks for all the help! Let me know if the way I'm doing it doesn't sound right.

[gstoneberg]

Yes, going up in wattage is absolutely fine, going down would not be. I don't see an issue with your plan, let us know how it goes.

I've been doing all my flight recording with a mobius and it does fine in daylight. It also recorded my night flights on my micro heli with night blades. It is a better day cam than night though. I think that's why they pulled the IR filter. However, that would render it pretty useless in daylight.

[badstraw]

You guys have any pics for the wiring?

[gstoneberg]

Go here and plug in your source voltage, number of LEDs, forward voltage and target LED current and it'll create a little wiring diagram for you.

ledcalc.com

Note: this doesn't work for 12v LED strings, which only need to be connected to positive and negative 12v (3s) at one end of the string. Subsequent strings can be daisy-chained.